Archives: Study Notes

Vascular Anatomy

🩸 Anterior Circulation – Internal Carotid & Branches

Internal Carotid Artery (ICA) – Segments & Key Branches ▼

ICA Segments (high-yield)

Segment Location Key Points
Cervical Carotid bifurcation → skull base No major branches; common site for atherosclerosis
Petrous Carotid canal (temporal bone) Protected; small branches to petrous bone & middle ear
Cavernous Within cavernous sinus Close to CN III, IV, V1, V2, VI → cavernous sinus pathology
Supraclinoid (Terminal) After exiting cavernous sinus Gives off: Ophthalmic, PCom, Anterior Choroidal, ACA, MCA

ICA Stroke – Clinical Pattern

  • Often = MCA ± ACA territory (if poor collateral flow via ACom/PCom)
  • Carotid T occlusion: Severe deficit – dense contralateral hemiplegia, hemisensory loss, gaze deviation, aphasia (dominant) or neglect (non-dominant), homonymous hemianopia
  • Retinal ischemia: Amaurosis fugax or monocular vision loss (ophthalmic artery)
  • Clue: Ipsilateral monocular blindness + contralateral hemiparesis → think ICA
💎 Board Pearl

“Carotid T” = terminal ICA occlusion at the carotid bifurcation into ACA & MCA → massive MCA ± ACA syndrome + often poor collaterals.

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🧠 Middle Cerebral Artery (MCA) & Divisions

MCA Anatomy & Territories ▼

MCA Segments

  • M1: Horizontal/sphenoidal segment – from origin to bifurcation/trifurcation
  • M2: Insular segments (superior & inferior divisions)
  • M3/M4: Opercular & cortical branches over convexity

Key branches:

  • Lenticulostriate arteries (from M1): Supply basal ganglia & posterior limb of internal capsule (“arteries of stroke”)
  • Cortical branches: Lateral frontal, parietal, temporal lobes

Typical MCA Territory Deficit

  • Contralateral face & arm weakness > leg
  • Contralateral hemisensory loss (face/arm > leg)
  • Contralateral homonymous hemianopia (optic radiations)
  • Dominant hemisphere: Aphasia (Broca/Wernicke/global)
  • Non-dominant hemisphere: Hemispatial neglect, anosognosia, constructional apraxia
  • Eyes often deviate toward the lesion in acute phase

MCA Syndromes – M1 Proximal vs Distal vs Divisions

Pattern Clinical Features Key Clues / Localization
M1 Proximal
(before lenticulostriates)		 • Dense contralateral hemiplegia (face, arm, leg)
• Contralateral hemisensory loss
• Gaze deviation toward lesion
• Aphasia (dominant) or neglect (non-dominant)
• Often early edema, mass effect 		Cortical + deep signs.
Internal capsule + cortex involved.
Very severe deficit at onset (“devastating MCA stroke”).
M1 Distal
(after lenticulostriates)		 • Cortical signs prominent (aphasia/neglect, field cut)
• Weakness typically less dense (internal capsule spared)
• May have mild–moderate face/arm > leg weakness 		Cortical signs without profound dense hemiplegia.
Think more distal M1 or large M2 branch occlusion.
Superior Division (M2) • Face & arm weakness prominent
• Little or no visual field deficit
• Dominant: Broca’s aphasia (nonfluent, good comprehension)
• Non-dominant: Mild neglect, motor apraxia 		Motor + Broca’s = superior division (dominant).
Visual field often spared or mild.
Inferior Division (M2) • Prominent visual field deficit (HH or quadrantanopia)
• Little or no weakness
• Dominant: Wernicke’s aphasia (fluent, poor comprehension)
• Non-dominant: Severe neglect, anosognosia 		Fluent aphasia + HH with minimal weakness = inferior division (dominant).
Non-dominant: “classic neglect” pattern.
💎 Board Pearl

Dense hemiplegia + aphasia/neglect → think proximal M1 (cortex + internal capsule).
Fluent aphasia + HH but NO weakness → inferior division MCA.

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🧍 Anterior Cerebral, Anterior Choroidal & Ophthalmic Arteries

Anterior Cerebral Artery (ACA) ▼

Course: From ICA → A1 segment → ACom → A2 pericallosal/callosomarginal branches.

Territory:

  • Medial frontal & parietal lobes
  • Leg area of motor & sensory cortex
  • Anterior corpus callosum & cingulate

ACA Stroke Syndrome:

  • Contralateral leg weakness > arm/face
  • Contralateral leg sensory loss
  • Urinary incontinence
  • Abulia, akinetic mutism (medial frontal/anterior cingulate)
  • Frontal release signs (grasp reflex)
  • Alien limb phenomena (medial frontal/callosal)
Anterior Choroidal Artery (AChA) ▼

Origin: Supraclinoid ICA (classically) – small but high-yield vessel.

Structures supplied:

  • Posterior limb of internal capsule
  • Optic tract & lateral geniculate body
  • Medial temporal lobe (hippocampus)
  • Globus pallidus

Classic AChA Stroke Triad:

  • Contralateral hemiparesis (posterior limb IC)
  • Contralateral hemisensory loss
  • Contralateral homonymous hemianopia (optic tract/LGN)

Often incomplete in real-life, but exam loves the triad.

Ophthalmic Artery ▼

Origin: First major branch of supraclinoid ICA; enters optic canal with optic nerve.

Supplies: Retina, optic nerve head, orbit.

Clinical:

  • Amaurosis fugax: Transient monocular vision loss (“curtain coming down”) from retinal ischemia due to carotid disease.
  • Retinal artery occlusion: Sudden painless monocular blindness; cherry red spot.
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🔄 Posterior Circulation – Vertebrobasilar & PCA

Vertebral & Basilar Arteries – Overview ▼

Vertebral arteries: Join to form basilar at pontomedullary junction.

Key branches:

  • PICA – posterior inferior cerebellar artery
  • Anterior spinal artery
  • AICA – anterior inferior cerebellar artery
  • SCA – superior cerebellar artery
  • Paramedian & circumferential branches to brainstem
Posterior Cerebral Artery (PCA) ▼

Origin: Terminal branches of basilar artery.

Territory:

  • Occipital lobe (primary visual cortex)
  • Inferomedial temporal lobes
  • Posterior thalamus
  • Splenium of corpus callosum

PCA Stroke Syndrome:

  • Contralateral homonymous hemianopia ± macular sparing
  • Alexia without agraphia (left PCA + splenium)
  • Visual agnosia, prosopagnosia (ventral occipitotemporal)
  • Thalamic pain syndrome (Dejerine–Roussy) if thalamus involved
  • Bilateral PCA → cortical blindness ± Anton syndrome
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🧲 Brainstem & Cerebellar Stroke Syndromes (Pattern Recognition)

Syndrome Artery Localization & Key Features
Lateral Medullary (Wallenberg) PICA (usually vertebral/PICA) • Vertigo, nystagmus, nausea
• Ipsilateral facial pain/temp loss (trigeminal nucleus)
• Contralateral body pain/temp loss (spinothalamic)
• Dysphagia, hoarseness, diminished gag (nucleus ambiguus) – “Don’t PICA horse”
• Ipsilateral Horner’s, ataxia
Lateral Pontine AICA (anterior inferior cerebellar) • Similar to PICA but more facial nucleus involvement
• Ipsilateral facial paralysis, ↓ lacrimation, salivation, taste (ant 2/3)
• Vertigo, nystagmus
• Ipsilateral ataxia
• “Facial droop means AICA’s pooped”
Medial Medullary Anterior spinal (branch of vertebral) Triad:
• Contralateral hemiparesis (corticospinal)
• Contralateral dorsal column loss (proprioception/vibration)
• Ipsilateral tongue weakness (CN XII) – tongue deviates toward lesion
Locked-in Syndrome Basilar ventral pons • Quadriplegia, anarthria
• Preserved consciousness & vertical eye movements
• Result of bilateral corticospinal & corticobulbar tract involvement
Weber Syndrome Paramedian midbrain (PCA branches) • Ipsilateral CN III palsy
• Contralateral hemiparesis
• Classic midbrain “alternating” hemiplegia
Superior Cerebellar Artery (SCA) stroke SCA • Ipsilateral limb ataxia, dysmetria
• Nausea, vomiting, nystagmus
• Contralateral pain/temp loss (body)
• Facial pain/temp may be spared (vs PICA/AICA)
💎 Board Pearl

Posterior circulation strokes often give “crossed findings” – ipsilateral cranial nerve signs with contralateral motor/sensory deficits.

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🩻 Cerebral Venous System – Superficial, Deep & Dural Sinuses

Superficial & Deep Cerebral Veins ▼

Superficial Veins

  • Drain cerebral cortex and subcortical white matter
  • Empty mainly into superior sagittal sinus, transverse sinus
  • Bridging veins traverse subdural space → rupture → subdural hematoma, especially with atrophy/trauma

Deep Venous System

  • Internal cerebral veins: Drain deep structures (thalamus, basal ganglia, deep white matter)
  • Join to form the vein of Galen
  • Vein of Galen → straight sinus → transverse sinus
Dural Venous Sinuses & Clinical Correlates ▼

Major Dural Sinuses

  • Superior sagittal sinus: Along falx; drains superficial hemispheric veins
  • Inferior sagittal sinus → straight sinus: Deep midline structures
  • Transverse & sigmoid sinuses: Exit skull via jugular foramen → internal jugular vein
  • Cavernous sinus: On either side of sella; ICA + CN III, IV, V1, V2, VI inside/along walls

Cerebral Venous Sinus Thrombosis (CVST)

  • Risk factors: Hypercoagulable states, pregnancy/postpartum, OCPs, infection
  • Symptoms: Headache, papilledema, seizures, focal deficits
  • Superior sagittal sinus thrombosis: Bilateral parasagittal weakness, seizures, ↑ ICP
  • Lateral (transverse/sigmoid) sinus: Headache, cerebellar signs, raised ICP

Cavernous Sinus Thrombosis

  • Etiology: Often from facial/sinus infections
  • Clinical:
    • Painful ophthalmoplegia (CN III, IV, VI involvement)
    • Decreased corneal reflex (V1)
    • Periorbital edema, proptosis
    • Often bilateral due to intercavernous connections
💎 Board Pearl

Key venous patterns: • Elderly fall + gradual confusion = subdural (bridging veins).
• Young woman + headache + papilledema + seizure = suspect venous sinus thrombosis.

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📊 Vascular Anatomy & Stroke – Quick Reference

Clinical Finding Most Likely Vessel Localization Clue
Leg > arm weakness, abulia ACA Medial frontal/parietal, anterior cingulate
Face/arm > leg weakness, aphasia Dominant MCA Lateral frontal/parietal, perisylvian
Neglect, left-sided inattention Right MCA (inferior division) Non-dominant parietal/temporal
HH with macular sparing PCA Occipital cortex (dual supply)
HH + dense hemiparesis (face, arm, leg) Proximal M1 or ICA Cortex + internal capsule
HH + hemiparesis + hemisensory loss Anterior choroidal Posterior limb IC + optic tract
Vertigo + ipsilateral face pain/T loss + contralateral body pain/T loss + dysphagia PICA Lateral medulla (Wallenberg)
Painful ophthalmoplegia + proptosis Cavernous sinus (venous) CN III, IV, V1, V2, VI involvement
💎 Board Pearl

Think artery = pattern of deficit; think vein/sinus = headache, ↑ICP, seizures, multifocal deficits.

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Limbic System

🧠 Limbic System – Overview & Major Components

Core idea: The limbic system links emotion, memory, motivation, and autonomic responses. It is central for learning, fear, and reward – all very testable on boards.

Major Limbic Structures

Structure Location Key Functions
Hippocampus Medial temporal lobe (floor of temporal horn) Declarative memory (episodic & semantic); memory consolidation
Amygdala Anterior medial temporal lobe Fear, threat detection, emotional learning, aggression
Cingulate Gyrus Medial surface of frontal/parietal lobes above corpus callosum Emotion, pain affect, motivation, attention (ACC)
Parahippocampal Gyrus Medial temporal surface, surrounding hippocampus Contextual memory, navigation, gateway into hippocampus
Mammillary Bodies Inferior hypothalamus (posterior) Relay for memory circuits (Papez); damaged in Wernicke-Korsakoff
Septal Nuclei Basal forebrain Reward, pleasure, cholinergic projections to hippocampus
Nucleus Accumbens Ventral striatum (at junction of caudate & putamen) Reward, motivation, addiction (mesolimbic dopamine)
Orbitofrontal & Medial PFC Ventral & medial frontal lobes Emotion regulation, social behavior, decision-making
💎 Board Pearl

Limbic “big 3” for boards = hippocampus (memory), amygdala (fear/emotion), cingulate gyrus (motivation/pain affect).

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🔁 Limbic Circuits (Papez, Reward, Fear)

Papez Circuit – Declarative Memory Loop ▼

Core role: Consolidation of declarative (explicit) memory.

Pathway (classic board sequence):

  • Hippocampus → Fornix → Mammillary bodies
  • Mammillary bodies → Mammillothalamic tract → Anterior thalamic nucleus
  • Anterior thalamus → Cingulate gyrus
  • Cingulate gyrus → Cingulum → Parahippocampal gyrus → Back to hippocampus

Lesion sites & consequences:

  • Bilateral hippocampal lesion: Severe anterograde amnesia (HM)
  • Mammillary bodies/anterior thalamus: Wernicke–Korsakoff (chronic thiamine deficiency)
Mesolimbic Reward Circuit – Dopamine & Addiction ▼

Key pathway:

  • Ventral tegmental area (VTA) → Dopaminergic projections via medial forebrain bundle →
  • Nucleus accumbens, amygdala, hippocampus, orbitofrontal/medial prefrontal cortex

Functions:

  • Reward, reinforcement learning
  • Motivation and salience of stimuli
  • Central pathway in substance use disorders

Clinical relevance:

  • Addiction (drugs of abuse ↑ dopamine in nucleus accumbens)
  • Psychosis (mesolimbic hyperactivity – positive symptoms)
Amygdala Fear Circuit – Threat & Autonomic Response ▼

Inputs:

  • Thalamus & sensory association cortices (visual, auditory, somatic)
  • Hippocampus (contextual information)
  • Prefrontal cortex (evaluation, regulation)

Outputs:

  • Hypothalamus: Autonomic/endocrine responses (HR, BP, HPA axis)
  • Brainstem (PAG, parabrachial nucleus): Freezing, startle, respiratory changes
  • Basal forebrain & cortex: Emotional experience, attention bias to threat

Clinical: Central in anxiety disorders, PTSD, and emotional memory of traumatic events.

💎 Board Pearl

Think “Papez = memory loop”, “mesolimbic = reward/addiction”, “amygdala loop = fear/autonomic”. Many vignette stems are just dressed-up versions of these three circuits.

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📚 Hippocampus & Memory Systems

Types of Memory – What the Hippocampus Does (and Doesn’t) Handle

Memory Type Description Main Structures
Declarative (Explicit) Conscious memory for facts & events (episodic, semantic) Hippocampus, medial temporal lobe, diencephalon (thalamus, mammillary bodies)
Non-declarative (Implicit) Skills, habits, priming, conditioning (unconscious) Basal ganglia, cerebellum, neocortex, amygdala (fear conditioning)
Working Memory Short-term holding & manipulation (seconds) Dorsolateral prefrontal cortex
Hippocampal Anatomy & Vulnerabilities ▼

Internal structure:

  • Dentate gyrus: Granule cells; receives entorhinal input
  • CA fields (CA1–CA4): Pyramidal neurons (CA1 = Sommer’s sector)
  • Subiculum: Output region back to cortex

Connections:

  • Inputs via entorhinal cortex (perforant pathway)
  • Outputs via fornix to mammillary bodies & septal nuclei

Highly vulnerable to:

  • Hypoxia/ischemia: CA1 (Sommer’s sector) – early injury in cardiac arrest
  • HSV encephalitis: Predilection for medial temporal lobes (hippocampus & amygdala)
  • Mesial temporal sclerosis: Hippocampal atrophy → temporal lobe epilepsy
Clinical Patterns of Memory Loss ▼

Anterograde vs Retrograde:

  • Anterograde amnesia: Inability to form new memories after insult (hippocampus)
  • Retrograde amnesia: Loss of memories before insult (wider network involvement)

Key clinical scenarios:

  • H.M. (classic case): Bilateral medial temporal lobectomy → profound anterograde amnesia, preserved procedural memory
  • Transient global amnesia (TGA): Sudden anterograde amnesia, repetitive questioning, lasts hours, often hippocampal diffusion changes
  • Wernicke–Korsakoff: Thiamine deficiency → mammillary bodies & medial thalamus → anterograde amnesia, confabulation
💎 Board Pearl

Pure anterograde amnesia with preserved remote memory and normal language = usually bilateral hippocampal/medial temporal damage.

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😨 Amygdala, Emotion, and Behavior

Amygdala Functions & Connections ▼

Functions:

  • Rapid detection of threat (fear, anger)
  • Emotional coloring of memories (especially negative)
  • Fear conditioning (linking neutral stimuli to aversive events)
  • Social/emotional cue processing (facial expressions)

Key inputs: Sensory cortex, thalamus, hippocampus, prefrontal cortex

Key outputs: Hypothalamus (autonomic/endocrine), brainstem (freezing/startle), basal forebrain & cortex (emotional experience)

Side note: Amygdala activity is often increased in anxiety disorders and PTSD.

Classic Amygdala Syndromes ▼

Klüver–Bucy Syndrome (bilateral anterior temporal/amygdala):

  • Hyperorality (putting objects in mouth)
  • Hypersexuality
  • Placidity (reduced fear/aggression)
  • Visual agnosia (psychic blindness)
  • Hypermetamorphosis (compulsive exploration)

Urbach–Wiethe disease (rare):

  • Calcification of amygdala
  • Impaired fear recognition & reduced fear response

Temporal lobe epilepsy: Emotional auras (fear), autonomic changes, déjà vu, rising epigastric sensation often reflect amygdala/hippocampal involvement.

💎 Board Pearl

Behavioral triad “hyperorality + hypersexuality + placidity” in a temporal lesion vignette → think bilateral amygdala / Klüver–Bucy.

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🩺 Limbic Clinical Syndromes & Lesions

Key Limbic Syndromes for Boards

Syndrome Primary Lesion Site Clinical Features
Mesial Temporal Sclerosis Hippocampus (usually unilateral, medial temporal) Temporal lobe epilepsy with déjà vu, rising epigastric aura, automatisms, memory complaints
H.M.-type Amnesia Bilateral medial temporal lobes (hippocampi) Severe anterograde amnesia, some retrograde loss, preserved procedural memory & IQ
Wernicke–Korsakoff Syndrome Mammillary bodies, medial thalamus, periaqueductal gray Wernicke triad: confusion, ophthalmoplegia, ataxia (acute, reversible)
Korsakoff: profound anterograde amnesia, confabulation (chronic, often irreversible)
Klüver–Bucy Syndrome Bilateral anterior temporal lobes/amygdala Hyperorality, hypersexuality, placidity, visual agnosia, hypermetamorphosis
Limbic Encephalitis Medial temporal lobes (often bilateral) Subacute confusion, seizures, mood changes, prominent anterograde memory loss.
Autoimmune/paraneoplastic (LGI1, CASPR2, Hu, Ma2, etc.)
HSV Encephalitis Medial temporal & orbitofrontal cortex Fever, headache, focal seizures, behavioral changes, memory deficits; MRI temporal lobes
Anterior Cingulate Lesion Medial frontal/ACC Abulia, apathy, reduced motivation, akinetic mutism if severe
💎 Board Pearl

Limbic lesions often present with a triad of: new-onset seizures, memory impairment, and behavior/personality change. Think medial temporal/limbic process (tumor, limbic encephalitis, HSV).

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📊 Limbic System – Quick Localization Summary

Clinical Finding → Likely Limbic Localization

Clinical Finding Likely Structure
Anterograde amnesia after bilateral temporal injury Hippocampi (medial temporal lobes)
Confabulation + chronic memory loss in alcoholic patient Mammillary bodies & medial thalamus (Korsakoff)
Déjà vu, rising epigastric sensation, fear aura before seizure Mesial temporal (hippocampus + amygdala)
Hyperorality + hypersexuality + placidity Bilateral amygdala/anterior temporal (Klüver–Bucy)
Apathy/abulia with intact motor strength Anterior cingulate / medial frontal limbic cortex
Addiction, drug craving, reward-seeking Mesolimbic dopamine (VTA → nucleus accumbens)
💎 Board Pearl

If the vignette mentions: “medial temporal FLAIR signal + seizures + memory loss” – your first thought should be limbic encephalitis.

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NCS & EMG Basics

⚡ NCS Basics

Motor vs Sensory NCS

Feature Motor NCS (CMAP) Sensory NCS (SNAP)
Recording site Muscle Sensory nerve (digit or nerve trunk)
Amplitude reflects Number of motor axons + muscle fibers Number of sensory axons
Normal amplitude >4-5 mV (varies by nerve) >10-20 μV (varies by nerve)
Key clinical use Motor axon loss, NMJ, myopathy Distinguishes pre- vs postganglionic lesions

Key NCS Parameters

Parameter What It Measures Abnormal In
Amplitude Number of functioning axons Axonal loss, conduction block
Conduction velocity (CV) Speed of fastest fibers (myelination) Demyelination (<70-75% LLN)
Distal latency (DL) Time from distal stim to response Distal demyelination (>130% ULN)
Duration Synchrony of fiber conduction Temporal dispersion (demyelination)
💎 Board Pearl

SNAP preserved in radiculopathy because the lesion is preganglionic (DRG intact). SNAP lost in plexopathy and peripheral neuropathy (postganglionic). This is key for localization!

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🔍 Demyelinating vs Axonal Patterns

NCS Criteria

Feature Demyelinating Axonal
Conduction velocity <70-75% LLN Normal or mildly slow (>70% LLN)
Distal latency >130% ULN Normal or mildly prolonged
Amplitude May be preserved early; low late Low (proportional to axon loss)
Temporal dispersion Present (>30% duration increase) Absent
Conduction block Present (>50% amplitude drop) Absent
F-wave latency Prolonged or absent Normal or mildly prolonged
EMG fibrillations Less prominent (unless 2° axonal loss) Prominent

Clinical Examples

Demyelinating Axonal
GBS (AIDP) GBS (AMAN, AMSAN)
CIDP Diabetic polyneuropathy
CMT1 (hereditary) CMT2 (hereditary)
MMN Toxic neuropathies
Anti-MAG neuropathy Vasculitic neuropathy
💎 Board Pearl

Conduction block = weakness WITHOUT atrophy (axons intact but can’t conduct through demyelinated segment). Temporal dispersion indicates acquired (non-uniform) demyelination – not seen in hereditary demyelinating neuropathies like CMT1.

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📍 NCS Patterns by Location

Common Mononeuropathies – Entrapment Sites

Nerve Site NCS Findings Clinical
Median Carpal tunnel Prolonged DL; slow sensory CV across wrist; compare to ulnar Numbness digits 1-3; thenar weakness (severe)
Ulnar Elbow (cubital tunnel) CV slowing across elbow (>10 m/s drop); conduction block Numbness digits 4-5; intrinsic hand weakness
Ulnar Wrist (Guyon’s canal) Abnormal dorsal ulnar cutaneous spares (branches proximal) Similar to elbow but dorsum hand spared
Peroneal Fibular head Conduction block/slowing across fibular head; superficial peroneal SNAP normal Foot drop; lateral leg numbness
Radial Spiral groove Conduction block across spiral groove Wrist drop; Saturday night palsy
Tibial Tarsal tunnel Prolonged DL; low amplitude medial/lateral plantar Sole numbness/burning

Radiculopathy vs Plexopathy vs Neuropathy

Feature Radiculopathy Plexopathy Mononeuropathy
SNAP Normal (preganglionic) Abnormal Abnormal
CMAP Low (if severe axon loss) Low Low
EMG distribution Myotomal (multiple nerves, one root) Multiple nerves/roots, one plexus region Single nerve distribution
Paraspinals Abnormal Normal Normal
💎 Board Pearl

SNAP normal + paraspinals abnormal = radiculopathy. SNAP abnormal + paraspinals normal = plexopathy or peripheral nerve lesion. This is the most important distinction!

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🔬 Special NCS Studies

Late Responses

Study Pathway Clinical Use Abnormal In
F-wave Motor nerve → anterior horn → back (antidromic-orthodromic) Tests proximal nerve segments, roots GBS (early), radiculopathy, proximal neuropathy
H-reflex Ia afferent → spinal cord → motor neuron (monosynaptic) Electrical ankle jerk; tests S1 root S1 radiculopathy, polyneuropathy

Repetitive Nerve Stimulation (RNS)

Finding Low-Frequency (2-3 Hz) Post-Exercise/High-Frequency Diagnosis
Decrement >10% Yes Repair of decrement Myasthenia gravis
Low baseline + increment >100% May decrement Large increment Lambert-Eaton
Low baseline + small increment May decrement 20-40% increment Botulism

Blink Reflex

  • Tests: Trigeminal (V1 afferent) and facial (VII efferent) nerves; brainstem
  • R1: Ipsilateral, oligosynaptic (pons)
  • R2: Bilateral, polysynaptic (lateral medulla)
  • Uses: Facial nerve lesions, trigeminal neuropathy, brainstem lesions, GBS
💎 Board Pearl

F-waves test the entire motor nerve including proximal segments – prolonged or absent early in GBS when distal NCS still normal. H-reflex = S1; absent H-reflex with normal ankle jerk suggests early/mild S1 radiculopathy.

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📊 EMG Spontaneous Activity

Types of Spontaneous Activity

Finding Description Sound Associated Conditions
Fibrillation potentials Spontaneous single muscle fiber discharge; biphasic/triphasic; regular “Rain on a tin roof” Denervation, inflammatory myopathy, muscular dystrophy, NMJ disorders
Positive sharp waves (PSWs) Initial positive deflection then negative; regular Dull “thud” Same as fibrillations
Fasciculation potentials Spontaneous motor unit discharge; irregular “Popcorn” ALS, radiculopathy, benign fasciculations, cramp-fasciculation syndrome
Myotonic discharges Waxing and waning frequency AND amplitude; triggered by needle movement “Dive bomber” Myotonic dystrophy, myotonia congenita, paramyotonia, hyperkalemic PP
Myokymic discharges Grouped, repetitive bursts of same MUP; semi-rhythmic “Marching soldiers” Radiation plexopathy, GBS, MS, facial myokymia (brainstem glioma)
Neuromyotonic discharges Very high frequency (150-300 Hz); decrementing; “pinging” “Ping” or musical Isaac’s syndrome (anti-VGKC/CASPR2), thymoma, post-radiation
Complex repetitive discharges (CRDs) Polyphasic; regular; abrupt start/stop; no waxing/waning “Jackhammer” or “motorcycle” Chronic denervation, chronic myopathy, radiculopathy
Cramp potentials High-frequency MUP discharge; irregular; painful Rumbling Cramps (ALS, metabolic, benign)
💎 Board Pearl

Myotonic discharge = waxing/waning (dive bomber). CRD = NO waxing/waning (jackhammer). Myokymia on EMG = consider radiation injury or GBS. Neuromyotonia = Isaac’s syndrome (continuous muscle fiber activity).

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⏱️ EMG in Denervation & Reinnervation

Timeline of EMG Changes After Nerve Injury

Time After Injury NCS Findings EMG Findings
Immediate (0-7 days) Conduction block at injury site; distal responses NORMAL Reduced recruitment only; NO fibrillations yet
Acute (7-10 days) Distal CMAP/SNAP drop (Wallerian degeneration complete) Reduced recruitment; fibs/PSWs starting (proximal muscles first)
Subacute (2-3 weeks) Low/absent distal responses Fibs/PSWs prominent in proximal muscles
Subacute (4-6 weeks) Low/absent distal responses Fibs/PSWs in distal muscles; maximal denervation
Early reinnervation (2-4 months) May see nascent CMAPs Nascent MUPs (small, polyphasic); fibs persist
Chronic reinnervation (6+ months) CMAP may improve Large, polyphasic MUPs; reduced recruitment; fibs decrease
Chronic stable (years) May be normal or low amplitude Giant MUPs; reduced recruitment; NO fibs (stable)

Key Points

  • Fibs appear at different times depending on distance from lesion:
    • Paraspinals: ~10 days
    • Proximal limb: 2-3 weeks
    • Distal limb: 4-6 weeks
  • Nascent MUPs = first sign of reinnervation (small, polyphasic, unstable)
  • Chronic neurogenic MUPs = large amplitude, long duration, polyphasic (collateral sprouting)
  • Reduced recruitment = fewer MUPs firing fast (neurogenic pattern)
💎 Board Pearl

Wait 3 weeks for optimal EMG after nerve injury – fibs take time to appear. Fibs WITHOUT large MUPs = acute/ongoing denervation. Fibs WITH large MUPs = chronic with ongoing denervation. Large MUPs WITHOUT fibs = chronic stable.

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📈 MUP Analysis & Recruitment

Myopathic vs Neurogenic MUPs

Feature Myopathic Neurogenic
Amplitude Low (small) High (large/giant)
Duration Short Long
Phases Increased polyphasia Increased polyphasia
Recruitment Early (many small units for weak effort) Reduced (few units firing fast)
Reason Fewer muscle fibers per motor unit Collateral sprouting → larger motor units

Recruitment Patterns

Pattern Description Seen In
Normal Ratio ~5:1 (firing rate : number of MUPs) Normal
Reduced (neurogenic) Few MUPs firing rapidly (>15-20 Hz before next recruited) Neuropathy, radiculopathy, ALS
Early (myopathic) Many MUPs at low firing rates; full interference with weak effort Myopathy
Poor activation Few MUPs at low firing rates; normal MUP morphology UMN lesion, pain, poor effort
💎 Board Pearl

Myopathic = SNAP (Small, short, polyphasic with early recruitment). Neurogenic = LARP (Large amplitude, long duration, reduced recruitment, polyphasic). Remember: “Sick muscle = small units, sick nerve = big units.”

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🎯 Disease-Specific EMG Findings

Quick Recognition Table

Disease Key EMG/NCS Findings
ALS Widespread fibs + fasciculations + neurogenic MUPs; NORMAL sensory NCS; multiple regions (bulbar, cervical, thoracic, lumbar)
Myasthenia gravis Decrement on RNS; increased jitter on single-fiber EMG; normal routine NCS/EMG
Lambert-Eaton Low CMAP amplitude; >100% increment post-exercise; may have mild decrement at rest
GBS (AIDP) Prolonged/absent F-waves (early); conduction block; temporal dispersion; slow CV
CIDP Same as AIDP but symmetric and chronic; uniform demyelination
Multifocal motor neuropathy (MMN) Conduction block in motor nerves ONLY; normal sensory; asymmetric
Myotonic dystrophy Myotonic discharges (dive bomber); myopathic MUPs; fibs common
Polymyositis/Dermatomyositis Fibs/PSWs + myopathic MUPs (“irritable myopathy”); CRDs; normal NCS
Inclusion body myositis Mixed myopathic AND neurogenic features; fibs; long-duration MUPs
Steroid myopathy Myopathic MUPs; NO fibrillations; normal NCS
Critical illness myopathy Low CMAP with direct muscle stimulation; fibs; myopathic MUPs
Radiation plexopathy Myokymic discharges; axonal loss pattern
Isaac’s syndrome Neuromyotonic discharges; fasciculations; doublets/triplets
Carpal tunnel syndrome Prolonged median distal latency; slow sensory CV across wrist; compare to ulnar (normal)
💎 Board Pearl

ALS = motor only (normal sensory NCS is required). MMN = motor conduction block only. IBM = mixed pattern (unique!). Radiation plexopathy = myokymia (distinguishes from tumor infiltration which doesn’t have myokymia).

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📋 Summary Tables

Localization Quick Reference

Clinical Scenario SNAP Paraspinals Diagnosis
Weakness + sensory loss in one nerve Abnormal Normal Mononeuropathy
Weakness in myotome + sensory loss in dermatome Normal Abnormal Radiculopathy
Weakness/sensory loss spanning multiple nerves and roots Abnormal Normal Plexopathy
Length-dependent sensory > motor Abnormal (distal) Normal Polyneuropathy

Spontaneous Activity Quick Reference

Sound Finding Think Of
“Dive bomber” (waxing/waning) Myotonic discharge Myotonic dystrophy, myotonia congenita
“Marching soldiers” (grouped bursts) Myokymia Radiation injury, GBS
“Ping” (high-frequency, decrementing) Neuromyotonia Isaac’s syndrome
“Jackhammer” (no waxing/waning) CRD Chronic denervation/myopathy
“Rain on tin roof” Fibrillations Denervation, inflammatory myopathy

Key Clinical Pearls

🔍 High-Yield Points
  • SNAP normal = preganglionic (radiculopathy)
  • Conduction block = demyelinating
  • Low amplitude everywhere = axonal
  • Wait 3 weeks for EMG after nerve injury
  • Fibs appear proximal to distal (paraspinals first)
  • Myopathic = small, short, early recruitment
  • Neurogenic = large, long, reduced recruitment
  • F-waves abnormal early in GBS (before distal changes)
  • Myokymia = radiation plexopathy
  • ALS requires normal sensory NCS

Red Flags

⚠️ Important Considerations
  • Fibs in first week: Pre-existing denervation or myopathy (not new injury)
  • Abnormal sensory NCS in suspected ALS: Reconsider diagnosis
  • Conduction block in sensory AND motor: Think CIDP/GBS, not MMN
  • Rapidly progressive with normal EMG: Consider NMJ disorder, myopathy, or too early after injury
  • Myokymia without radiation history: Consider GBS, MS, or brainstem lesion
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↑

Sleep

🌙 Sleep Architecture

Sleep Stages

Stage EEG Pattern Key Features % of Sleep
Wake Alpha (8-13 Hz) relaxed; Beta alert Alpha = posterior, eyes closed —
N1 Theta (4-7 Hz); vertex sharp waves Light sleep; easily aroused; slow eye movements 5%
N2 Sleep spindles (12-14 Hz) + K-complexes Majority of sleep; memory consolidation 45-55%
N3 (Slow Wave) Delta (<4 Hz); >20% of epoch Deep sleep; restorative; GH release; hardest to arouse 15-20%
REM Low voltage, mixed frequency; sawtooth waves Rapid eye movements; muscle atonia; dreaming 20-25%

Sleep Cycle Organization

  • Cycle duration: 90-120 minutes
  • Cycles per night: 4-6
  • First half of night: More N3 (slow wave sleep)
  • Second half of night: More REM sleep
  • REM latency: ~90 minutes (shortened in narcolepsy, depression)
💎 Board Pearl

Sleep spindles + K-complexes = N2. Sleep spindles generated by thalamic reticular nucleus. K-complexes are cortical responses to stimuli. N2 is the most abundant stage (~50%).

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🧠 Neuroanatomy of Sleep-Wake Regulation

Wake-Promoting Systems

Structure Neurotransmitter Notes
Locus coeruleus Norepinephrine Off during REM
Raphe nuclei Serotonin Off during REM
Tuberomammillary nucleus Histamine Antihistamines cause sedation
Basal forebrain Acetylcholine Also active in REM
Lateral hypothalamus Orexin/Hypocretin Stabilizes wake; lost in narcolepsy type 1

Sleep-Promoting Systems

Structure Neurotransmitter Function
VLPO GABA, Galanin Inhibits wake centers; “sleep switch”
Adenosine — Builds during wake; caffeine blocks receptors

REM Sleep Regulation

REM-on: PPT/LDT (ACh), sublaterodorsal nucleus

REM-off: Locus coeruleus (NE), raphe (5-HT)

REM atonia: Sublaterodorsal nucleus inhibits spinal motor neurons. Loss → REM sleep behavior disorder.

💎 Board Pearl

Orexin/hypocretin from lateral hypothalamus stabilizes wakefulness. Loss = narcolepsy type 1. CSF orexin <90 pg/mL is diagnostic. Orexin receptor antagonists (suvorexant) treat insomnia.

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🕐 Circadian Rhythm

Suprachiasmatic Nucleus (SCN)

  • Location: Anterior hypothalamus, above optic chiasm
  • Function: Master circadian pacemaker
  • Input: Light via retinohypothalamic tract (melanopsin RGCs)
  • Output: Pineal gland (melatonin), temperature, cortisol
  • Intrinsic cycle: ~24.2 hours (needs daily light entrainment)

Melatonin

  • Source: Pineal gland
  • Regulation: Suppressed by light; peaks at night
  • Function: Signals darkness; promotes sleep onset
  • Clinical use: Circadian rhythm disorders, jet lag

Circadian Rhythm Disorders

Disorder Features Treatment
Delayed Sleep Phase Sleep onset 2+ hrs late; common in adolescents; “night owls” Morning bright light; evening melatonin
Advanced Sleep Phase Sleep onset/wake too early; common in elderly Evening bright light; morning melatonin
Non-24-Hour Free-running rhythm; common in blind Melatonin agonists (tasimelteon)
Shift Work Misalignment with work schedule Strategic light exposure; melatonin
💎 Board Pearl

Delayed = give melatonin in evening + morning light. Advanced = opposite. Non-24-hour rhythm is common in totally blind patients due to loss of light entrainment.

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💤 Sleep Disorders

Narcolepsy

Feature Type 1 (with Cataplexy) Type 2 (without Cataplexy)
Orexin/CSF Low (<110 pg/mL, usually <90) Normal
Cataplexy Present (emotion-triggered atonia) Absent
HLA association DQB1*06:02 (>90%) Less strong
MSLT criteria Mean sleep latency ≤8 min + ≥2 SOREMPs

Narcolepsy tetrad:

  1. Excessive daytime sleepiness (100%)
  2. Cataplexy (emotion-triggered weakness; type 1 only)
  3. Sleep paralysis (can’t move at sleep-wake transitions)
  4. Hypnagogic/hypnopompic hallucinations

Treatment:

  • EDS: Modafinil, armodafinil, solriamfetol, pitolisant; stimulants
  • Cataplexy: Sodium oxybate, antidepressants (SNRIs, TCAs)
💎 Board Pearl

Cataplexy is pathognomonic for narcolepsy type 1. Triggered by strong emotions (laughter, surprise). Consciousness preserved. CSF orexin <90 pg/mL is diagnostic even without MSLT.

Parasomnias

Feature NREM Parasomnias REM Parasomnias
Timing First third of night (N3 predominant) Last third of night (REM predominant)
Recall No memory of event Vivid dream recall
Eyes Open, glassy Closed
Examples Sleepwalking, sleep terrors, confusional arousals REM sleep behavior disorder, nightmare disorder
Age Children (usually outgrow) RBD: older adults (>50)

REM Sleep Behavior Disorder (RBD)

  • Pathophysiology: Loss of REM atonia → dream enactment
  • Clinical: Violent movements during dreams; may injure self/bed partner
  • PSG: REM without atonia (increased chin EMG tone during REM)
  • Association: Strongly linked to α-synucleinopathies (Parkinson’s, DLB, MSA)
  • Conversion: >80% develop parkinsonism within 10-15 years
  • Treatment: Melatonin (first line), clonazepam; bedroom safety
💎 Board Pearl

RBD is a prodrome of α-synucleinopathies. New RBD in elderly = high risk for PD, DLB, MSA. Can precede motor symptoms by years. Also seen with antidepressants (especially SSRIs, SNRIs).

Restless Legs Syndrome (RLS)

  • Criteria: Urge to move legs + worse at rest + better with movement + worse at night
  • Associations: Iron deficiency (check ferritin), uremia, pregnancy, neuropathy
  • Pathophysiology: Dopaminergic dysfunction; low brain iron
  • Treatment:
    • Iron supplementation if ferritin <75 ng/mL
    • Dopamine agonists (pramipexole, ropinirole) – watch for augmentation
    • Alpha-2-delta ligands (gabapentin, pregabalin) – now often first line
  • Augmentation: Symptoms occur earlier, spread to arms, worsen with dopamine agonists

Sleep Apnea

Feature Obstructive (OSA) Central (CSA)
Mechanism Upper airway collapse Loss of respiratory drive
Respiratory effort Present (paradoxical breathing) Absent
Associations Obesity, large neck, retrognathia Heart failure, opioids, brainstem lesions
Treatment CPAP, weight loss, oral appliance Treat underlying cause; ASV (not in HFrEF)

AHI (Apnea-Hypopnea Index):

  • 5-15: Mild
  • 15-30: Moderate
  • >30: Severe
—

📋 Summary Tables

Sleep Stage Quick Reference

EEG Finding Stage
Alpha waves (posterior) Relaxed wake
Theta + vertex sharp waves N1
Sleep spindles + K-complexes N2
Delta waves (>20%) N3
Low voltage + sawtooth waves REM

Sleep Disorder Quick Recognition

Clinical Clue Diagnosis
EDS + cataplexy + low CSF orexin Narcolepsy type 1
Elderly + dream enactment + later develops PD REM sleep behavior disorder
Child + first third of night + no recall + eyes open NREM parasomnia (sleepwalking/terror)
Urge to move legs + worse at rest + better with movement Restless legs syndrome
Snoring + witnessed apneas + EDS + obesity Obstructive sleep apnea
Adolescent can’t fall asleep until 2 AM Delayed sleep phase disorder
Blind patient with free-running rhythm Non-24-hour sleep-wake disorder

Key Clinical Pearls

🔍 High-Yield Points
  • N2 = most abundant stage (45-55%); spindles + K-complexes
  • First half of night = more N3; second half = more REM
  • Orexin stabilizes wake; loss = narcolepsy type 1
  • Cataplexy = pathognomonic for narcolepsy type 1
  • RBD predicts α-synucleinopathy (PD, DLB, MSA)
  • RLS: check ferritin – treat if <75 ng/mL
  • NREM parasomnias = first third; REM = last third
  • MSLT: ≤8 min mean latency + ≥2 SOREMPs = narcolepsy

Red Flags

⚠️ Important Considerations
  • New-onset RBD in elderly: Screen for parkinsonism; high conversion rate
  • RLS with low ferritin: Rule out GI blood loss
  • Severe OSA: Associated with HTN, arrhythmias, stroke risk
  • Sudden cataplexy onset in child: Consider secondary causes (hypothalamic lesion)
  • RLS augmentation: Consider switching from dopamine agonist to alpha-2-delta ligand
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↑

Peripheral Nerves and Muscles

🦴 Peripheral Nerves, Dermatomes & Key Muscles

Goal for boards: Be able to go from weak muscle → nerve → root → lesion site in both upper and lower limbs.

High-Yield Framework

  • Think in “motor lead sign”: which movement is weak? (e.g., wrist extension, ankle dorsiflexion)
  • Pair with:
    • Reflex (C5, C6, C7, L4, S1)
    • Dermatome (thumb, middle finger, little finger; big toe, lateral foot)
    • Pattern: peripheral nerve vs root vs plexus
💎 Board Pearl

For extremity localization, always think in triads: weak movement + reflex change + sensory map. If all line up in one root → radiculopathy. If they don’t → think nerve or plexus.

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💪 Upper Limb – Nerves & Key Muscles

Quick Root–Myotome Map (C5–T1) ▼
Root Key Movement Example Muscle Reflex
C5 Shoulder abduction Deltoid Biceps (C5–6)
C6 Elbow flexion, wrist extension Biceps, ECRL/B Brachioradialis (C5–6)
C7 Elbow extension, wrist flexion Triceps, FCR Triceps (C6–7)
C8 Finger flexion FDP, FDS None specific (helpful clinically)
T1 Finger abduction/adduction Interossei None specific

Quick memory: C5 = shoulder, C6 = wrist extension (“6-shooter”), C7 = triceps, C8 = finger flexion, T1 = finger abduction.

Major Upper Limb Nerves – “One Muscle, One Movement, One Area” ▼
Nerve Roots Key Muscle (Test) Sensory Area Classic Lesion
Axillary C5–6 Deltoid – shoulder abduction (15–90°) “Regimental badge” lateral shoulder Surgical neck humerus fracture; shoulder dislocation
Musculocutaneous C5–7 Biceps – elbow flexion, supination Lateral forearm Upper arm trauma, rarely isolated
Radial C5–T1 ECRL/B, EDC – wrist & finger extension Dorsal hand (radial side) “Saturday night palsy”, crutches, mid-shaft humerus fracture → wrist drop
Median C5–T1 Forearm: pronator teres (pronation)
Hand: APB, opponens pollicis (thumb opposition) 		Palmar 1–3½ digits Carpal tunnel; proximal lesion → “hand of benediction”
Ulnar C8–T1 Interossei – finger abduction/adduction, Froment sign Medial 1½ digits (palmar & dorsal) Cubital tunnel, Guyon canal – “claw hand”, weak grip

Simple memory: “RAD wrist” (radial – wrist extension), “MEDian = thumb opposition & sensation to 3½ fingers”, “ULNAR = interossei – PAD/DAB (adduct/abduct).”

💎 Board Pearl

If interossei are out → think ULNAR or T1, not median or C7. Test by asking patient to hold a card between fingers (palmar interossei) or spread fingers against resistance (dorsal interossei).

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🖐️ Upper Limb Dermatomes – Quick Map

Key Spots to Test (Boards & Bedside) ▼
Root Landmark Mnemonic / Tip
C5 Lateral upper arm (deltoid area) Same as axillary nerve sensory area
C6 Thumb & radial forearm “6-shooter thumb”
C7 Middle finger “7 = middle” (central digit)
C8 Little finger & ulnar border of hand Think “8 = pinky & ring”
T1 Medial forearm/arm Close to arm–chest junction

Board pattern: C6 thumb, C7 middle, C8 little finger. If numbness fits this AND reflex matches → radiculopathy; if only hand area, think peripheral nerve.

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🦵 Lower Limb – Nerves & Key Muscles

Quick Root–Myotome Map (L2–S1) ▼
Root Key Movement Example Muscle Reflex
L2 Hip flexion Iliopsoas None reliable
L3 Hip flexion, knee extension Quadriceps Patellar (L3–4)
L4 Ankle dorsiflexion Tibialis anterior Patellar (L3–4)
L5 Great toe extension, hip abduction EHL, gluteus medius None specific
S1 Plantarflexion, eversion Gastrocnemius, peroneus longus Achilles (S1)

Memory: “L4 to the floor” (dorsiflexion); “S1 = S-run” – push off, plantarflexion & Achilles reflex.

Major Lower Limb Nerves – Key Muscles & Patterns ▼
Nerve Roots Key Muscle (Test) Sensory Area Classic Lesion
Femoral L2–4 Quadriceps – knee extension Anterior thigh, medial leg (saphenous) Retroperitoneal bleed, pelvic surgery; ↓ knee jerk
Obturator L2–4 Adductors – hip adduction Medial thigh Pelvic surgery, obturator canal lesions – gait instability
Sciatic L4–S3 Hamstrings – knee flexion Back of thigh, splits into tibial & peroneal Hip dislocation, injections too medial → sciatic neuropathy
Tibial L4–S3 Gastrocnemius, soleus – plantarflexion; toe flexors Sole of foot (plantar) Tarsal tunnel, popliteal lesions – difficulty toe-walking
Common fibular (peroneal) L4–S2 Deep branch: tibialis anterior – dorsiflexion
Superficial branch: peronei – eversion 		Dorsum of foot, lateral leg Fibular head compression (“foot drop nerve”)
Superior gluteal L4–S1 Gluteus medius/minimus – hip abduction No major cutaneous branch Trendelenburg gait (pelvis drops opposite side)
Inferior gluteal L5–S2 Gluteus maximus – hip extension No major cutaneous branch Difficulty climbing stairs, rising from chair

Memory: “DEEP fibular = DEEP space” (1st web space sensory); “SUPERficial fibular = SUPERficial dorsum” of foot.

💎 Board Pearl

Common fibular nerve is the most commonly injured lower limb nerve. Foot drop with preserved plantarflexion and inversion = fibular; if inversion also weak, think L5 radiculopathy.

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🦶 Lower Limb Dermatomes – Quick Map

High-Yield Spots for Exams ▼
Root Landmark Tip
L1 Inguinal region “L1 = 1nguinal”
L2 Upper anterior thigh Proximal thigh
L3 Medial knee “3 on the knee”
L4 Medial leg & medial malleolus “L4 to the floor” – medial ankle
L5 Dorsum of foot, great toe Classic L5 radiculopathy spot
S1 Lateral foot, little toe “S1 = Small toe & Sole (lateral)”
S2 Posterior thigh & calf “S2 = back of leg too”
S3–5 Perianal (“saddle” area) Cauda equina / conus lesions

Board pattern: L4 → medial ankle, L5 → dorsum & big toe, S1 → lateral foot/little toe, S3–5 → saddle anesthesia.

💎 Board Pearl

Saddle anesthesia + urinary retention = RED FLAG Cauda Equina/Conus. Needs urgent MRI and neurosurgical evaluation.

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🩺 Clinical Patterns – Putting It Together

Upper Limb – Radiculopathy vs Peripheral Nerve ▼
Pattern Key Findings Localization
Wrist drop Weak wrist/finger extension; triceps may be spared; sensory loss dorsum of hand Radial neuropathy (spiral groove or PIN)
C7 radiculopathy Weak triceps + wrist/finger extensors, ↓ triceps reflex, pain/numbness to middle finger C7 root (disc at C6–7)
Carpal tunnel Numbness in thumb–middle fingers, worse at night, thenar weakness (late) Median neuropathy at wrist
Ulnar claw Weak finger ab-/adduction, clawing of 4th/5th digits, sensory loss ulnar 1½ fingers Ulnar neuropathy (elbow or wrist)
C8/T1 radiculopathy Weak interossei, finger flexors, sensory loss medial forearm/hand; often neck pain C8 or T1 roots (e.g., Pancoast tumor)

Key distinction: Root lesions usually involve multiple nerves + reflex change + neck pain. Single nerve lesions follow a named nerve territory and may spare reflexes.

Lower Limb – Foot Drop & Radiculopathy ▼
Pattern Key Findings Localization
Foot drop – peroneal neuropathy Weak dorsiflexion & eversion
Normal plantarflexion & inversion
Sensory loss dorsum of foot/lateral leg
Often from leg crossing, fibular head compression 		Common fibular nerve at fibular neck
Foot drop – L5 radiculopathy Weak dorsiflexion and inversion (tibialis anterior + posterior)
Sensory loss lateral leg + dorsum foot, great toe
May have back pain, positive straight-leg raise 		L5 root (e.g., L4–5 disc)
Femoral neuropathy Weak knee extension, ↓ patellar reflex
Sensory loss anterior thigh/medial leg 		Femoral nerve (e.g., retroperitoneal bleed, pelvic surgery)
S1 radiculopathy Weak plantarflexion, ↓ Achilles reflex
Sensory loss lateral foot/little toe 		S1 root (L5–S1 disc)

Memory: If plantarflexion & Achilles are normal and only dorsiflexion is weak → more likely fibular nerve than L5 root.

💎 Board Pearl

Foot drop localization:
• Fibular nerve: foot drop, normal inversion & reflexes, local compression risk.
• L5 root: foot drop + weak inversion, +/- back pain, dermatomal sensory loss, reflexes often normal.

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📊 Quick Reference Tables

One-Liner Localizations

Clinical Sign Likely Localization
Shoulder abduction weakness + lateral shoulder numbness Axillary nerve (C5–6)
Wrist/finger extension weakness (“wrist drop”) Radial nerve (vs C7 radiculopathy if triceps/reflex involved)
Night numbness in thumb–middle finger, thenar atrophy Median neuropathy at wrist (carpal tunnel)
Weak finger ab-/adduction, ulnar 1½ finger numbness Ulnar nerve (elbow/wrist)
Knee extension weakness + ↓ patellar reflex Femoral neuropathy or L3–4 root lesion
Positive Trendelenburg sign (pelvis drops opposite side) Superior gluteal nerve (L4–S1)
Foot drop with preserved plantarflexion & inversion Common fibular neuropathy
Saddle anesthesia + bladder dysfunction Cauda equina / conus medullaris
💎 Final Board Pearl

On RITE/boards, “pure motor + single nerve territory” = neuropathy; “motor + sensory + reflex + back/neck pain in dermatomal pattern” = radiculopathy. Use the myotome–dermatome–nerve triads above to localize fast.

↑

Neurotransmitters

🧪 Neurotransmitters – High-Yield Overview

Key idea: For boards, think in terms of: NT → major nucleus → main pathway → function → clinical + drugs.

Major Neurotransmitter Classes

Class Examples Key Features
Excitatory (fast) Glutamate, Aspartate Main CNS excitatory NT; ionotropic (AMPA, NMDA) & metabotropic receptors
Inhibitory (fast) GABA (CNS), Glycine (spinal cord) Main CNS & spinal inhibitory NTs; Cl⁻ or K⁺ mediated
Monoamines Dopamine, NE, Serotonin Slow modulators; small nuclei with diffuse projections; psychiatric & movement disorders
Cholinergic Acetylcholine (ACh) Cortex, hippocampus, neuromuscular junction, autonomic ganglia
Neuropeptides Substance P, Enkephalins, Endorphins, Orexin, CRH, etc. Co-transmitters; slow, modulatory; often G-protein coupled receptors

Receptor Types

  • Ionotropic (ligand-gated channels): Fast, millisecond scale
    • Glutamate: AMPA, NMDA, Kainate
    • GABAA, Glycine
    • Nicotinic ACh receptors
  • Metabotropic (G-protein coupled): Slower, modulatory
    • mGluRs, GABAB
    • Monoamine receptors (D, α, β, 5-HT)
    • Muscarinic ACh receptors
    • Most neuropeptide receptors
💎 Board Pearl

Most fast EPSPs = glutamate (AMPA); most fast IPSPs = GABAA (CNS) or glycine (spinal cord/brainstem). Monoamines & peptides modulate, but do not usually mediate the primary fast synaptic transmission.

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⚡ Synapse Physiology (High-Yield Steps)

Chemical Synapse – Steps & Targets of Drugs/Toxins ▼
Step Physiology Key Drugs/Toxins
1. AP arrival Action potential → depolarization of presynaptic terminal Na⁺ channel blockers (phenytoin, carbamazepine, lidocaine)
2. Ca²⁺ influx Voltage-gated Ca²⁺ channels open → Ca²⁺ entry Ca²⁺ channel blockers (gabapentin, pregabalin; some anesthetics)
3. Vesicle fusion SNARE complex mediates vesicle fusion and exocytosis Botulinum toxin: cleaves SNAREs (↓ ACh release)
4. NT binding NT diffuses across cleft → binds receptors Receptor agonists/antagonists (benzos @ GABAA, ketamine @ NMDA, etc.)
5. Termination Reuptake, enzymatic breakdown, diffusion Reuptake inhibitors: SSRIs, SNRIs, TCAs (SERT/NET)
Enzyme inhibitors: MAOIs, COMT inhibitors, AChE inhibitors

Electrical synapses: Gap junctions; fast, bidirectional; found in some brainstem nuclei, hypothalamus, and early development.

💎 Board Pearl

Most clinically used CNS drugs act at the synapse, NOT at the axon: receptors, transporters, enzymes, or vesicle release machinery.

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🔥 Glutamate – Main Excitatory Neurotransmitter

Receptors & Functions ▼
Receptor Type Key Features Clinical/Drugs
AMPA Ionotropic (Na⁺/K⁺) Fast EPSPs; majority of excitatory transmission Targeted indirectly by many anticonvulsants
NMDA Ionotropic (Ca²⁺/Na⁺) Voltage & ligand dependent (Mg²⁺ block); central to plasticity & excitotoxicity Antagonists: ketamine, PCP, memantine
Implicated in stroke, TBI, epilepsy
Kainate Ionotropic Less prominent; modulates excitability Experimental agonists used to induce seizures in models
mGluRs Metabotropic Pre- and postsynaptic modulation, slow effects Targets for experimental epilepsy/psychiatric drugs

Metabolism: Glutamate–glutamine cycle between neurons and astrocytes (astrocytes clear glutamate via EAAT transporters).

Clinical – Excitotoxicity & Disease ▼
  • Excitotoxicity: Excess glutamate → prolonged NMDA activation → Ca²⁺ overload → neuronal death
    • Seen in ischemic stroke, hypoxia, TBI, status epilepticus
  • Riluzole (ALS): ↓ glutamate release, modest survival benefit
  • Memantine (Alzheimer’s): NMDA antagonist, protects against excitotoxicity
  • Ketamine: NMDA antagonist; anesthetic & rapid-acting antidepressant
💎 Board Pearl

Ischemia → ↓ ATP → failed Na⁺/K⁺ pump → depolarization → massive glutamate release → NMDA-mediated Ca²⁺ influx → neuronal death. This cascade underlies many neuroprotective strategies.

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🧊 GABA – Main Inhibitory Neurotransmitter (CNS)

Receptors, Metabolism & Drugs ▼
Receptor Type Effect Key Drugs
GABAA Ionotropic (Cl⁻ channel) Fast inhibition (hyperpolarization via Cl⁻ influx) Benzodiazepines, barbiturates, zolpidem
General anesthetics, alcohol (potentiation)
GABAB Metabotropic (G-protein) Opens K⁺ channels, ↓ Ca²⁺ → slow inhibition Baclofen: GABAB agonist (spasticity)

Metabolism: Synthesized from glutamate by glutamic acid decarboxylase (GAD); broken down by GABA transaminase.

Clinical: Vigabatrin irreversibly inhibits GABA transaminase → ↑ GABA (used in refractory epilepsy, infantile spasms).

Glycine & Spinal Inhibition ▼
  • Glycine: Main inhibitory NT in spinal cord and lower brainstem
  • Strychnine: Competitive glycine antagonist → severe muscle spasms, seizures
  • Tetanus toxin: Blocks release of glycine and GABA from inhibitory interneurons → disinhibition & spasticity
💎 Board Pearl

Tetanus = loss of inhibitory glycinergic & GABAergic interneurons → disinhibited motor neurons → generalized rigidity & spasms.

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🎯 Monoamines – Dopamine, Norepinephrine, Serotonin

Major Monoamine Systems (Nucleus → Projection → Function → Disorders) ▼
NT Nucleus / Origin Main Pathways & Functions Clinical / Drugs
Dopamine Substantia nigra pars compacta (SNc)
VTA (mesolimbic/mesocortical)
Tuberoinfundibular (hypothalamus) 		Nigrostriatal: Movement
Mesolimbic: Reward, psychosis
Mesocortical: Motivation, cognition
Tuberoinfundibular: ↓ Prolactin 		↓ Nigrostriatal: Parkinson’s disease
↑ Mesolimbic: Schizophrenia (positive symptoms)
Antipsychotics = D2 antagonists; L-dopa, agonists for PD
Norepinephrine Locus coeruleus (pons) Diffuse projections to cortex, limbic system, spinal cord
Arousal, attention, stress response 		Implicated in depression, anxiety, ADHD
SNRIs, TCAs, stimulants ↑ NE (and DA)
Serotonin (5-HT) Raphe nuclei (midbrain → medulla) Mood, anxiety, sleep, pain modulation
Descending pain inhibition in spinal cord 		SSRIs, SNRIs, TCAs ↑ 5-HT
Triptans = 5-HT1B/1D agonists (migraine)
Risk of serotonin syndrome with polypharmacy

Metabolism: Monoamines broken down by MAO (A/B) and COMT. Metabolites include HVA (DA), VMA (NE/Epi), 5-HIAA (5-HT).

💎 Board Pearl

Dopamine pathways: Nigrostriatal (movement), Mesolimbic (reward/psychosis), Mesocortical (negative symptoms), Tuberoinfundibular (prolactin). Side effect patterns of antipsychotics mirror these pathways (EPS, hyperprolactinemia, negative/cognitive symptoms).

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🔑 Acetylcholine – Cortex, NMJ & Autonomics

CNS Cholinergic Systems ▼
Nucleus Projection Function Clinical
Nucleus basalis of Meynert Diffuse to neocortex Attention, arousal, cortical activation Degenerates in Alzheimer’s disease → basis for AChE inhibitors
Medial septal nucleus Hippocampus Memory, hippocampal theta rhythms Memory impairment with cholinergic dysfunction
Pontine cholinergic nuclei Thalamus, cortex REM sleep, arousal Sleep regulation; REM-related phenomena
Neuromuscular Junction & Autonomics (Quick Neuro Review) ▼
  • NMJ: ACh at nicotinic receptors → muscle contraction
    • Myasthenia gravis: Antibodies to postsynaptic nicotinic AChR
    • Lambert–Eaton: Antibodies to presynaptic Ca²⁺ channels → ↓ ACh release
  • Autonomics:
    • All preganglionic (symp + parasymp) use ACh (nicotinic)
    • Most parasympathetic postganglionic: ACh (muscarinic)
  • AChE inhibitors: Donepezil, rivastigmine (Alzheimer’s); pyridostigmine (MG)
  • Organophosphates: Irreversible AChE inhibitors → cholinergic crisis
💎 Board Pearl

Nucleus basalis of Meynert is the key cholinergic nucleus affected early in Alzheimer’s disease. AChE inhibitors (donepezil, rivastigmine) are designed to compensate for this loss.

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🧬 Neuropeptides & Modulators

Key Neuropeptides – High Yield Only ▼
Peptide Function Clinical Relevance
Substance P Pain transmission (esp. in dorsal horn); neurogenic inflammation NK1 receptor antagonists (aprepitant) used as antiemetics
Endorphins/Enkephalins Endogenous opioids; pain modulation, reward Opioid receptors (μ, κ, δ) targeted by analgesics (morphine, fentanyl)
Orexin (hypocretin) Wakefulness, appetite Loss in narcolepsy type 1; orexin antagonists (suvorexant) for insomnia
CRH, ACTH, etc. Stress axis (hypothalamic–pituitary) Interact with mood, anxiety, neuroendocrine disorders
💎 Board Pearl

Narcolepsy type 1 = loss of orexin-producing neurons in lateral hypothalamus. This is a favorite board association.

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📊 Summary Tables & Quick Reference

Neurotransmitter Localization – Quick Board Table

Neurotransmitter Major Nucleus Main Targets Key Disorders
Glutamate Ubiquitous (most excitatory neurons) Entire CNS Stroke, epilepsy, neurodegeneration (excitotoxicity)
GABA Interneurons, cerebellum, basal ganglia CNS inhibition Epilepsy, anxiety, spasticity
Dopamine SNc, VTA Striatum, limbic system, cortex Parkinson’s, schizophrenia, addiction
Norepinephrine Locus coeruleus Diffuse cortical & spinal projections Depression, anxiety, ADHD
Serotonin (5-HT) Raphe nuclei Cortex, limbic, spinal cord Depression, anxiety, migraine, pain
ACh Nucleus basalis, septal nuclei, brainstem Cortex, hippocampus, NMJ Alzheimer’s, myasthenia gravis, organophosphate poisoning
💎 Board Pearl – One-Liners
  • Parkinson’s: ↓ dopamine (SNc → striatum)
  • Alzheimer’s: ↓ ACh (nucleus basalis)
  • Depression: ↓ NE and 5-HT
  • Schizophrenia: ↑ mesolimbic dopamine, ↓ mesocortical dopamine
  • Huntington’s: ↓ GABA & ACh in striatum, relative ↑ dopamine
↑

Physiology of Muscles

🔬 Muscle Fiber Types

Type I vs Type II Fibers

Feature Type I (Slow Twitch) Type II (Fast Twitch)
Color Red (high myoglobin) White (low myoglobin)
Metabolism Oxidative (aerobic) Glycolytic (anaerobic)
Mitochondria Many Few
Fatigue resistance High (endurance) Low (quick fatigue)
Function Sustained activity, posture Rapid, powerful movements
ATPase staining Light at pH 9.4 Dark at pH 9.4

Clinical Relevance of Fiber Type

Fiber Type Affected Conditions
Type I atrophy Myotonic dystrophy type 1, congenital myopathies
Type II atrophy Disuse, steroids, cachexia, aging (sarcopenia)
Type I predominance Endurance athletes, central core disease
💎 Board Pearl

Type II fiber atrophy = steroid myopathy, disuse. These are the “expendable” fibers lost first in catabolic states. Type I fibers are preserved because they’re needed for posture and breathing.

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⚡ Energy Metabolism & Metabolic Myopathies

ATP Sources in Muscle

Source Duration Clinical Defect
Phosphocreatine Seconds (immediate) Rare
Glycolysis/Glycogenolysis Minutes (short burst) Glycogen storage diseases
Fatty acid oxidation Hours (prolonged) Lipid storage myopathies
Oxidative phosphorylation Sustained Mitochondrial myopathies

Glycogen Storage Diseases

Disease Enzyme Defect Key Features Hallmark
McArdle’s (GSD V) Myophosphorylase Exercise intolerance, cramps, myoglobinuria “Second wind” phenomenon; no lactate rise on forearm test
Pompe’s (GSD II) Acid maltase (α-glucosidase) Proximal weakness, respiratory failure, cardiomyopathy (infantile) Diaphragm weakness out of proportion to limbs; ERT available
Tarui’s (GSD VII) Phosphofructokinase Similar to McArdle’s “Out of wind” phenomenon (worse with glucose); hemolysis
💎 Board Pearl

McArdle’s = second wind (feels better after 10-15 min as fatty acids kick in). Tarui’s = out of wind (glucose makes it worse by blocking fatty acid use). Both have no lactate rise on forearm exercise test.

Lipid Storage Myopathies

Disease Defect Key Features Hallmark
CPT II Deficiency Carnitine palmitoyltransferase II Recurrent myoglobinuria with prolonged exercise, fasting, cold, infection Most common cause of recurrent myoglobinuria in adults; normal strength between attacks
Primary Carnitine Deficiency Carnitine transporter (OCTN2) Cardiomyopathy, weakness, hypoglycemia Low serum carnitine; responds to carnitine supplementation

Mitochondrial Myopathies

Syndrome Key Features Hallmark
CPEO (Chronic Progressive External Ophthalmoplegia) Ptosis, ophthalmoplegia (no diplopia), proximal weakness Ptosis + ophthalmoplegia WITHOUT diplopia
KSS (Kearns-Sayre) CPEO + retinitis pigmentosa + cardiac conduction defects; onset <20 years Heart block – needs monitoring/pacemaker
MELAS Stroke-like episodes, seizures, lactic acidosis, myopathy Strokes not following vascular territories; often occipital
MERRF Myoclonus, epilepsy, ataxia, ragged red fibers Myoclonic epilepsy
💎 Board Pearl

Ragged red fibers on Gomori trichrome = mitochondrial myopathy. CPEO has no diplopia because both eyes move together (symmetric). Always screen KSS for heart block. MELAS strokes are non-vascular distribution.

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🧬 Muscular Dystrophies

Major Muscular Dystrophies

Dystrophy Gene/Protein Inheritance Key Features Hallmark
Duchenne (DMD) Dystrophin (absent) X-linked Onset 2-5 yrs; calf pseudohypertrophy; cardiomyopathy; wheelchair by 12 Gowers’ sign; CK >10,000
Becker (BMD) Dystrophin (reduced/abnormal) X-linked Later onset; milder; ambulation into adulthood; cardiomyopathy can be severe Cardiomyopathy out of proportion to skeletal weakness
Myotonic Dystrophy Type 1 (DM1) DMPK (CTG repeat) AD Distal weakness, myotonia, cataracts, cardiac conduction defects, frontal balding, testicular atrophy Grip myotonia; “hatchet face”
Myotonic Dystrophy Type 2 (DM2/PROMM) CNBP (CCTG repeat) AD Proximal weakness, myotonia (milder), muscle pain, no congenital form Proximal > distal (opposite of DM1); muscle pain prominent
FSHD D4Z4 contraction (chr 4) AD Face, scapular, humeral weakness; scapular winging; asymmetric Can’t whistle, smile, or close eyes tightly; scapular winging
LGMD Multiple genes (>30 types) AD or AR Proximal limb-girdle weakness; variable age of onset Heterogeneous group; need genetic testing
Emery-Dreifuss Emerin or Lamin A/C X-linked or AD Humeroperoneal weakness; early contractures (elbow, Achilles, neck) Cardiac conduction defects (sudden death risk); contractures before weakness
Oculopharyngeal (OPMD) PABPN1 (GCG repeat) AD Onset >40 yrs; ptosis, dysphagia, proximal weakness Late-onset ptosis + dysphagia; French-Canadian or Hispanic ancestry
💎 Board Pearl

DM1 = distal, DM2 = proximal. DM1 has anticipation (worse in successive generations); congenital form has profound hypotonia. Always screen DM1 and Emery-Dreifuss for cardiac conduction disease. FSHD is very asymmetric.

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🔥 Inflammatory Myopathies

Comparison of Inflammatory Myopathies

Feature Dermatomyositis (DM) Polymyositis (PM) Inclusion Body Myositis (IBM)
Age Any (children or adults) Adults >18 >50 years
Weakness pattern Proximal, symmetric Proximal, symmetric Distal (finger flexors) + proximal (quads); asymmetric
Skin findings Heliotrope rash, Gottron’s papules, shawl sign, mechanic’s hands None None
CK Elevated (10-50x) Elevated (10-50x) Normal or mildly elevated
Pathology Perifascicular atrophy; perivascular inflammation (B cells, CD4) Endomysial inflammation (CD8 T cells invading non-necrotic fibers) Rimmed vacuoles; CD8 T cells; congophilic inclusions
Cancer association Yes (screen!) Possible (less than DM) No
Treatment response Good (steroids, IVIG) Good (steroids) Poor (no effective treatment)
Dysphagia Can occur Can occur Common (60%)

Key Antibodies

Antibody Association
Anti-Jo-1 (and other anti-synthetases) Antisynthetase syndrome: myositis + ILD + arthritis + mechanic’s hands + Raynaud’s
Anti-Mi-2 Classic DM with good prognosis
Anti-MDA5 Amyopathic DM with rapidly progressive ILD
Anti-TIF1-γ (p155/140) DM with high cancer risk
Anti-NXP2 DM with calcinosis (children) or cancer (adults)
Anti-SRP Necrotizing myopathy; severe, cardiac involvement
Anti-HMGCR Statin-associated necrotizing myopathy (persists after stopping statin)
Anti-cN1A (Mup44) IBM (not specific but supportive)
💎 Board Pearl

IBM = elderly male + finger flexor + quad weakness + doesn’t respond to steroids. Pathology shows rimmed vacuoles. DM has perifascicular atrophy; PM has endomysial CD8 invasion. Anti-TIF1-γ = screen hard for cancer.

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💊 Toxic & Drug-Induced Myopathies

Common Drug-Induced Myopathies

Drug/Toxin Mechanism Clinical Features Key Points
Statins Toxic myopathy; or immune-mediated (anti-HMGCR) Myalgias, weakness, elevated CK, rhabdomyolysis (rare) Most resolve with discontinuation; anti-HMGCR requires immunotherapy
Corticosteroids Type II fiber atrophy Proximal weakness; normal CK; no myalgias Fluorinated steroids worse (dexamethasone, triamcinolone)
Colchicine Microtubule disruption Proximal weakness; may have neuropathy Risk increases with renal failure, CYP3A4 inhibitors
Chloroquine/Hydroxychloroquine Lysosomal dysfunction Proximal weakness; cardiomyopathy Curvilinear bodies on EM; may have neuropathy
Alcohol Direct toxicity Acute: rhabdomyolysis; Chronic: proximal weakness Most common toxic myopathy; Type II fiber atrophy
Zidovudine (AZT) Mitochondrial toxicity Proximal weakness; ragged red fibers Reversible with discontinuation
Immune checkpoint inhibitors Autoimmune Myositis, myasthenia, myocarditis Can be severe; may overlap with MG
💎 Board Pearl

Steroid myopathy = normal CK. Distinguish from underlying inflammatory myopathy flare (elevated CK). Statin myopathy usually resolves, but anti-HMGCR necrotizing myopathy persists and needs immunosuppression.

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📊 EMG Patterns in Muscle Disease

Myopathic vs Neurogenic Patterns

Feature Myopathic Neurogenic
MUP amplitude Low (small) High (large)
MUP duration Short Long
Recruitment Early (many small units for weak effort) Reduced (few units firing fast)
Polyphasia Increased Increased
Fibrillations May be present (inflammatory, necrotic myopathies) Present (denervation)

Specific EMG Findings by Disease

Disease Characteristic EMG Finding
Myotonic dystrophy Myotonic discharges (“dive bomber” sound); waxing-waning frequency and amplitude
Inflammatory myopathies (DM, PM) Fibrillations, PSWs + myopathic MUPs; “irritable myopathy”
IBM Mixed myopathic AND neurogenic features
Muscular dystrophies Myopathic MUPs; fibs/PSWs in actively necrotic dystrophies
Steroid myopathy Myopathic MUPs; NO fibrillations (no membrane irritability)
Critical illness myopathy Low CMAP amplitudes; fibs; myopathic MUPs; reduced muscle membrane excitability
💎 Board Pearl

Myopathic = small, short, polyphasic MUPs with early recruitment. Fibrillations in myopathy indicate membrane instability (inflammation, necrosis, DM1). IBM is unique: mixed pattern due to its dual pathology (inflammation + degeneration).

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📋 Summary Tables & Quick Reference

Metabolic Myopathy Presentation Patterns

Presentation Think Of
Exercise intolerance with “second wind” McArdle’s disease
Recurrent myoglobinuria with prolonged exercise/fasting CPT II deficiency
Proximal weakness + respiratory failure (adult) Pompe’s disease (late-onset)
Ptosis + ophthalmoplegia without diplopia Mitochondrial myopathy (CPEO)
Stroke-like episodes + seizures + lactic acidosis MELAS

Dystrophy Quick Recognition

Clinical Clue Diagnosis
Boy + calf pseudohypertrophy + Gowers’ sign DMD
Distal weakness + grip myotonia + cataracts + frontal balding DM1
Can’t whistle + scapular winging + asymmetric FSHD
Early contractures + cardiac conduction defects Emery-Dreifuss
Late-onset ptosis + dysphagia OPMD

Inflammatory Myopathy Quick Recognition

Clinical Clue Diagnosis
Heliotrope rash + Gottron’s papules + proximal weakness Dermatomyositis
Elderly + finger flexor weakness + quad weakness + doesn’t respond to steroids IBM
Myositis + ILD + mechanic’s hands + arthritis Antisynthetase syndrome (anti-Jo-1)
Persistent weakness after stopping statin Anti-HMGCR necrotizing myopathy

Red Flags

⚠️ Urgent Situations
  • Rapidly progressive weakness + respiratory decline: Check FVC urgently; may need ICU
  • Myoglobinuria (dark urine): Rhabdomyolysis risk; aggressive hydration, monitor renal function
  • New dermatomyositis in adult: Screen for malignancy (especially with anti-TIF1-γ)
  • Cardiac symptoms in muscular dystrophy: DMD/BMD, DM1, Emery-Dreifuss all have cardiac risk
  • Dysphagia in myopathy: Aspiration risk; may need modified diet or feeding tube
  • Late-onset Pompe’s with respiratory symptoms: Diaphragm weakness; start ERT

Key Clinical Pearls

🔍 High-Yield Points
  • Type II fiber atrophy = steroid, disuse, cachexia
  • McArdle’s = second wind; Tarui’s = out of wind
  • CPT II = most common cause of recurrent rhabdomyolysis in adults
  • DM1 = distal; DM2 = proximal
  • IBM = mixed EMG pattern + doesn’t respond to immunotherapy
  • Perifascicular atrophy = dermatomyositis; rimmed vacuoles = IBM
  • Normal CK in weakness = consider steroid myopathy, endocrine, or non-organic
  • Always screen DM for malignancy; KSS/Emery-Dreifuss/DM1 for cardiac conduction
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↑

Nerves & Neuromuscular Junction

🔬 Nerve Structure & Organization

Peripheral Nerve Anatomy

Layer Description Clinical Significance
Endoneurium Surrounds individual nerve fibers Must be intact for regeneration; contains blood-nerve barrier
Perineurium Surrounds fascicles (bundles of fibers) Main component of blood-nerve barrier; determines nerve tensile strength
Epineurium Surrounds entire nerve Contains vasa nervorum; surgical repair target

Myelination

Feature PNS (Schwann Cells) CNS (Oligodendrocytes)
Cell:Axon ratio 1 Schwann cell : 1 internode 1 oligodendrocyte : up to 50 internodes
Regeneration Good (Schwann cells guide regrowth) Poor (inhibitory environment)
Diseases GBS, CIDP, CMT MS, leukodystrophies

Nodes of Ranvier

  • Location: Gaps between myelin segments
  • Function: Site of saltatory conduction; high concentration of voltage-gated Na+ channels
  • Paranodal region: Contains K+ channels; exposed in demyelination → conduction block
💎 Board Pearl

Saltatory conduction allows action potentials to “jump” between nodes of Ranvier, greatly increasing conduction velocity. Demyelination exposes paranodal K+ channels → hyperpolarization → conduction failure.

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⚡ Nerve Fiber Classification

Erlanger-Gasser Classification (Sensory & Motor)

Fiber Type Diameter (μm) Velocity (m/s) Myelination Function
Aα 12-20 70-120 Heavy Motor to skeletal muscle; proprioception (Ia, Ib)
Aβ 5-12 30-70 Heavy Touch, pressure (type II)
Aγ 3-6 15-30 Medium Motor to muscle spindle (intrafusal)
Aδ 2-5 12-30 Light Fast pain, temperature, touch (type III)
B 1-3 3-15 Light Preganglionic autonomic
C 0.5-1.5 0.5-2 Unmyelinated Slow pain, temperature, postganglionic autonomic (type IV)
💎 Board Pearl

Conduction velocity ≈ 6 × diameter (in μm). Large, myelinated fibers (Aα) are affected first by compression/ischemia. Small unmyelinated fibers (C) are affected first by metabolic/toxic neuropathies (diabetes).

Clinical Correlates of Fiber Type Involvement

Fiber Type Affected Clinical Features Example Conditions
Large fiber Loss of proprioception, vibration; sensory ataxia; areflexia GBS, CIDP, B12 deficiency, Friedreich ataxia
Small fiber Burning pain, loss of pinprick/temperature; autonomic dysfunction; preserved reflexes Diabetic neuropathy (early), amyloid, Fabry disease
Motor fiber Weakness, atrophy, fasciculations ALS, MMN, lead toxicity
Autonomic fiber Orthostatic hypotension, anhidrosis, GI/GU dysfunction Diabetes, amyloid, autoimmune autonomic ganglionopathy
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🔗 Neuromuscular Junction

NMJ Anatomy

  • Presynaptic terminal: Contains ACh vesicles, voltage-gated Ca2+ channels (P/Q type)
  • Synaptic cleft: Contains acetylcholinesterase (AChE)
  • Postsynaptic membrane: Contains nicotinic ACh receptors (nAChR) on junctional folds

Neuromuscular Transmission Steps

  1. Action potential arrives at nerve terminal
  2. Ca2+ influx through P/Q-type voltage-gated calcium channels
  3. ACh vesicle fusion with presynaptic membrane (SNARE proteins)
  4. ACh release into synaptic cleft (quantal release)
  5. ACh binds to postsynaptic nicotinic receptors
  6. Na+ influx → end-plate potential (EPP)
  7. If EPP exceeds threshold → muscle action potential → contraction
  8. ACh hydrolysis by acetylcholinesterase

Safety Factor

Definition: EPP amplitude is normally 3-4x greater than threshold needed for muscle AP

Clinical significance:

  • Ensures reliable transmission even with some receptor loss
  • In myasthenia gravis: reduced AChR → decreased safety factor → transmission failure with repetitive use
  • In Lambert-Eaton: reduced ACh release → facilitation with exercise (increased Ca2+ accumulation)
💎 Board Pearl

P/Q-type Ca2+ channels are the target in Lambert-Eaton myasthenic syndrome. SNARE proteins (synaptobrevin, SNAP-25, syntaxin) are targeted by botulinum toxin and tetanus toxin.

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⚠️ Neuromuscular Junction Disorders

Comparison of Major NMJ Disorders

Feature Myasthenia Gravis Lambert-Eaton Botulism
Target Postsynaptic AChR Presynaptic P/Q Ca2+ channels Presynaptic SNARE proteins
Mechanism Antibody blocks/destroys AChR Antibody reduces Ca2+ influx → less ACh release Toxin cleaves SNAREs → blocks ACh release
Weakness pattern Ocular, bulbar, proximal; fatigable Proximal legs > arms; improves with exercise Descending: cranial → limbs → respiratory
Reflexes Normal Reduced/absent (improve post-exercise) Reduced/absent
Autonomic Usually spared Dry mouth, constipation, impotence Prominent (dilated pupils, dry mouth, ileus)
RNS pattern Decrement at low-frequency (2-3 Hz) Low baseline CMAP; increment >100% post-exercise Low baseline CMAP; small increment post-exercise
Association Thymoma (10-15%); thymic hyperplasia Small cell lung cancer (50-60%) Contaminated food, wound, infant (honey)
Myasthenia Gravis – Details ▼

Antibodies:

  • AChR antibodies: 85% of generalized MG
  • MuSK antibodies: 5-8%; more bulbar, muscle atrophy
  • LRP4 antibodies: Rare; milder phenotype
  • Seronegative: ~10%

Clinical features:

  • Fatigable weakness (worse with activity, better with rest)
  • Ptosis, diplopia (ocular MG in 50% at onset)
  • Bulbar: dysarthria, dysphagia, facial weakness
  • Limb weakness (proximal > distal)

Myasthenic crisis: Respiratory failure requiring intubation; triggered by infection, surgery, medications

Drugs that worsen MG: Aminoglycosides, fluoroquinolones, beta-blockers, magnesium, neuromuscular blockers

Lambert-Eaton Myasthenic Syndrome – Details ▼

Key features:

  • Proximal leg weakness → arms → bulbar (opposite of MG)
  • Facilitation: Strength improves briefly after sustained contraction
  • Autonomic symptoms prominent (dry mouth in >80%)
  • Reflexes absent but may appear after exercise

Cancer association:

  • 50-60% have small cell lung cancer (SCLC)
  • Cancer may present years after LEMS diagnosis
  • Screen with CT chest; repeat if initially negative

Treatment: 3,4-diaminopyridine (blocks K+ channels → prolongs depolarization → more Ca2+ entry)

💎 Board Pearl

MG = fatigable (gets worse with use). LEMS = facilitates (gets better with use). Both have proximal weakness. LEMS has autonomic symptoms; MG does not. Always screen LEMS for SCLC!

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🔧 Nerve Injury & Regeneration

Seddon Classification

Type Pathology Recovery EMG/NCS
Neurapraxia Local demyelination; axon intact Complete; weeks to 3 months Conduction block; no denervation
Axonotmesis Axon disrupted; endoneurium intact Good; 1 mm/day (1 inch/month) Wallerian degeneration; fibs/PSWs; reinnervation potentials
Neurotmesis Complete nerve transection Poor; requires surgery Complete denervation; no recovery without repair

Sunderland Classification (More Detailed)

Grade Seddon Equivalent Injury Prognosis
I Neurapraxia Myelin only Excellent
II Axonotmesis Axon (endoneurium intact) Good
III Axonotmesis Axon + endoneurium Variable
IV Axonotmesis Axon + endo + perineurium Poor
V Neurotmesis Complete transection None without surgery

Wallerian Degeneration

  • Definition: Degeneration of axon and myelin distal to site of injury
  • Timeline:
    • Begins within 24-48 hours
    • Complete by 7-10 days
    • NCS shows absent/reduced responses distally by day 7-10
    • EMG shows fibrillations/PSWs by 2-3 weeks (proximal) to 4-5 weeks (distal)
  • Schwann cells: Proliferate, phagocytose debris, form “bands of Büngner” to guide regeneration

Nerve Regeneration

  • Rate: ~1 mm/day or ~1 inch/month
  • Factors affecting recovery:
    • Age (younger = better)
    • Distance from target muscle
    • Time since injury (motor end plates degenerate after 12-18 months)
    • Accuracy of reinnervation
  • Signs of reinnervation: Tinel’s sign (advancing), nascent motor unit potentials on EMG
💎 Board Pearl

Neurapraxia = conduction block without Wallerian degeneration. No fibrillations on EMG. Full recovery expected. In axonotmesis, fibs/PSWs appear in 2-5 weeks (earliest in proximal muscles).

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📊 Electrodiagnostic Correlates

Nerve Conduction Study (NCS) Basics

Parameter What It Measures Abnormal In
Amplitude Number of functioning axons Axonal loss, conduction block
Conduction velocity Speed of fastest fibers (myelination) Demyelination
Distal latency Time from distal stim to response Distal demyelination

Demyelinating vs Axonal Patterns

Feature Demyelinating Axonal
Conduction velocity Markedly slow (<70% LLN) Normal or mildly slow
Distal latency Prolonged (>130% ULN) Normal or mildly prolonged
Amplitude May be preserved (early) or low Low (proportional to axon loss)
Temporal dispersion Present Absent
Conduction block Present Absent
F-wave latency Prolonged or absent Normal or mildly prolonged
EMG fibrillations Less prominent (unless secondary axonal loss) Prominent
Examples GBS (AIDP), CIDP, CMT1 Diabetic neuropathy, AMAN, CMT2

Key EDX Findings

Conduction Block ▼

Definition: >50% reduction in proximal vs distal CMAP amplitude (excluding common entrapment sites)

Significance:

  • Indicates focal demyelination
  • Axon is intact but impulse cannot pass through demyelinated segment
  • Causes weakness WITHOUT atrophy (no Wallerian degeneration)
  • Potentially reversible with remyelination

Classic conditions:

  • GBS (acute)
  • CIDP (chronic)
  • Multifocal motor neuropathy (MMN)
  • Hereditary neuropathy with liability to pressure palsies (HNPP)
Temporal Dispersion ▼

Definition: Increased CMAP duration with proximal stimulation (>30% increase)

Mechanism: Different degrees of demyelination cause different conduction velocities → desynchronization of impulses

Significance: Feature of acquired demyelinating neuropathies (not seen in uniform hereditary demyelination like CMT1A)

F-Waves and H-Reflex ▼

F-Wave

  • Pathway: Motor nerve → anterior horn → same motor nerve back (antidromic → orthodromic)
  • Tests: Entire motor nerve including proximal segments and roots
  • Abnormal in: Proximal demyelination (GBS), radiculopathy
  • Features: Variable latency and morphology; small amplitude

H-Reflex

  • Pathway: Ia afferent → spinal cord → alpha motor neuron → muscle (monosynaptic reflex)
  • Essentially: Electrical equivalent of ankle jerk (S1 root)
  • Tests: S1 nerve root function; only reliably obtained in tibial nerve/soleus
  • Abnormal in: S1 radiculopathy, polyneuropathy
💎 Board Pearl

Conduction block = demyelinating. Low amplitudes everywhere = axonal. Temporal dispersion indicates acquired (non-uniform) demyelination. F-waves test proximal nerve segments not accessible to routine NCS.

—

⚡ Channelopathies

Sodium Channelopathies

Disorder Gene/Channel Mechanism Clinical Features
Hyperkalemic Periodic Paralysis SCN4A (Nav1.4) Gain of function → prolonged depolarization Attacks with high K+, fasting, rest after exercise; myotonia common
Paramyotonia Congenita SCN4A (Nav1.4) Impaired fast inactivation Cold-induced myotonia; “paradoxical” myotonia (worsens with activity)
Sodium Channel Myotonia SCN4A (Nav1.4) Delayed inactivation Myotonia without weakness; K+-aggravated

Calcium Channelopathies

Disorder Gene/Channel Mechanism Clinical Features
Hypokalemic Periodic Paralysis CACNA1S (Cav1.1) – 70%
SCN4A – 10%		 Loss of function → reduced excitability Attacks with low K+, carbs, rest after exercise; NO myotonia
Malignant Hyperthermia RYR1 (ryanodine receptor) Uncontrolled Ca2+ release from SR Triggered by volatile anesthetics, succinylcholine; rigidity, hyperthermia, rhabdomyolysis

Chloride Channelopathies

Disorder Gene/Channel Clinical Features
Myotonia Congenita (Thomsen/Becker) CLCN1 (ClC-1) Myotonia (muscle stiffness); improves with activity (“warm-up”); muscle hypertrophy; NO weakness

Periodic Paralysis Comparison

Feature Hypokalemic PP Hyperkalemic PP
K+ during attack Low (<3.5) High or normal
Triggers Carbs, rest after exercise, stress Fasting, rest after exercise, cold, K+
Myotonia Absent Often present
Attack duration Hours to days Minutes to hours
Treatment K+ replacement; acetazolamide prophylaxis Carbs, inhaled β-agonist; acetazolamide or dichlorphenamide prophylaxis
💎 Board Pearl

HypoKPP: no myotonia. HyperKPP: myotonia common. Both worsen with rest after exercise. Acetazolamide works for both (metabolic acidosis → reduced attack frequency). Malignant hyperthermia = RYR1 mutation; treat with dantrolene.

—

📊 Summary Tables & Quick Reference

Nerve Fiber Types Quick Reference

Fiber Function Lost First In
Aα (large, myelinated) Motor, proprioception Compression, ischemia
Aβ (large, myelinated) Touch, pressure Compression, ischemia
Aδ (small, myelinated) Fast pain, temperature Metabolic (later)
C (small, unmyelinated) Slow pain, autonomic Metabolic (diabetes), toxic

NMJ Disorders – RNS Patterns

Disorder Low-Frequency RNS (2-3 Hz) Post-Exercise/High-Frequency
Myasthenia Gravis Decrement >10% Repair of decrement (post-activation facilitation)
Lambert-Eaton Low baseline; may decrement Increment >100%
Botulism Low baseline; may decrement Small increment (20-40%)

Nerve Injury – Timing of EDX Findings

Finding Timing After Injury
Reduced recruitment Immediately
Reduced SNAP/CMAP distal to lesion 7-10 days (Wallerian degeneration complete)
Fibrillations in proximal muscles 2-3 weeks
Fibrillations in distal muscles 4-5 weeks
Nascent MUPs (reinnervation) 2-4 months (depends on distance)

Key Clinical Pearls

🔍 High-Yield Points
  • Velocity ≈ 6 × diameter: Large fibers conduct faster
  • Demyelinating = slow velocity, conduction block, temporal dispersion
  • Axonal = low amplitude, fibrillations on EMG
  • MG fatigues; LEMS facilitates
  • Neurapraxia: Best prognosis, conduction block, no fibs
  • Regeneration rate: 1 mm/day (1 inch/month)
  • F-waves: Test proximal nerve; prolonged in GBS
  • Always screen LEMS for small cell lung cancer

Red Flags

⚠️ Urgent Situations
  • Respiratory muscle weakness in MG: Check FVC; <15-20 mL/kg = intubate
  • Rapidly progressive weakness with areflexia: GBS – monitor respiratory function
  • Descending paralysis with autonomic symptoms: Botulism – antitoxin urgently
  • New-onset LEMS: Search for SCLC (may precede cancer by years)
  • Malignant hyperthermia: Stop anesthesia, give dantrolene, cool patient
—
↑

Subcortical Nuclei (BG, Thalamus, Hypothalamus)

🎯 Basal Ganglia – Anatomy

Overview

Definition: Group of subcortical nuclei involved in motor control, learning, emotions, and executive functions

Location: Deep to cerebral cortex, lateral to thalamus

Component Structures

Structure Components Function/Notes
Striatum Caudate + Putamen INPUT nucleus – receives from cortex; connected by cell bridges across internal capsule
Lentiform Nucleus Putamen + Globus Pallidus Anatomical grouping (lens-shaped); lateral to internal capsule
Globus Pallidus (GP) GPe (external) + GPi (internal) OUTPUT nucleus (GPi); GPe is part of indirect pathway
Subthalamic Nucleus (STN) — Only EXCITATORY nucleus in BG; target for DBS in Parkinson’s
Substantia Nigra SNc (pars compacta) + SNr (pars reticulata) SNc = dopamine source; SNr = output (like GPi)
💎 Board Pearl

Striatum = INPUT, GPi/SNr = OUTPUT. The internal capsule separates caudate (medial) from lentiform nucleus (lateral). Striatal cell bridges give it “striped” appearance.

Anatomical Relationships

Spatial Organization & Blood Supply ▼

Spatial relationships:

  • Caudate: C-shaped, follows lateral ventricle (head, body, tail)
  • Putamen: Lateral to globus pallidus
  • Internal capsule: Between caudate/thalamus (medial) and lentiform nucleus (lateral)
  • External capsule: Lateral to putamen
  • Extreme capsule: Between claustrum and insula

Blood supply:

  • Lenticulostriate arteries (from MCA) – putamen, globus pallidus, caudate head, internal capsule
  • Anterior choroidal artery – globus pallidus, posterior limb internal capsule
  • Recurrent artery of Heubner (from ACA) – caudate head, anterior limb internal capsule

Clinical: Lenticulostriate arteries are common site of hypertensive hemorrhage → “putaminal hemorrhage”

Neurotransmitters in Basal Ganglia

Neurotransmitter Source Effect
Dopamine SNc → Striatum (nigrostriatal pathway) D1 receptors = excitatory (direct pathway)
D2 receptors = inhibitory (indirect pathway)
GABA Striatum, GPe, GPi, SNr Inhibitory; main neurotransmitter of BG output
Glutamate Cortex → Striatum; STN → GPi/SNr Excitatory; STN is only excitatory BG nucleus
Acetylcholine Striatal interneurons Opposes dopamine; increased in Parkinson’s
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🔄 Basal Ganglia Circuits

Direct vs Indirect Pathways

Direct Pathway (GO Pathway) ▼

Function: FACILITATES movement

Pathway:

  1. Cortex → excites Striatum (glutamate)
  2. Striatum → inhibits GPi/SNr (GABA)
  3. GPi/SNr → releases inhibition on Thalamus (less GABA)
  4. Thalamus → excites Cortex (glutamate)

Net effect: Disinhibition of thalamus → increased cortical activity → movement facilitated

Dopamine effect: D1 receptors on direct pathway neurons → dopamine EXCITES direct pathway → facilitates movement

Mnemonic: “D1 = Direct = Do it”

Indirect Pathway (STOP Pathway) ▼

Function: INHIBITS movement

Pathway:

  1. Cortex → excites Striatum (glutamate)
  2. Striatum → inhibits GPe (GABA)
  3. GPe → releases inhibition on STN (less GABA)
  4. STN → excites GPi/SNr (glutamate)
  5. GPi/SNr → inhibits Thalamus (GABA)
  6. Thalamus → less excitation to Cortex

Net effect: Increased inhibition of thalamus → decreased cortical activity → movement suppressed

Dopamine effect: D2 receptors on indirect pathway neurons → dopamine INHIBITS indirect pathway → reduces suppression → facilitates movement

Mnemonic: “D2 = inDirect = Don’t do it (inhibits inhibition)”

Circuit Summary

Feature Direct Pathway Indirect Pathway
Function Facilitates movement (GO) Inhibits movement (STOP)
Dopamine receptor D1 (excitatory) D2 (inhibitory)
Effect of dopamine Activates pathway → more movement Inhibits pathway → less suppression → more movement
In Parkinson’s (low DA) Underactive → less movement Overactive → more suppression
In Huntington’s Preserved initially Lost early → less suppression → chorea
💎 Board Pearl

Both pathways have same end goal for dopamine: Dopamine from SNc facilitates movement by BOTH activating direct pathway (D1) AND inhibiting indirect pathway (D2). Loss of dopamine → bradykinesia.

Hyperdirect Pathway

Route: Cortex → STN → GPi (bypasses striatum)

Function: Rapid suppression of movement; “emergency brake”

Clinical: Important for impulse control; may be involved in OCD

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⚡ Movement Disorders

Hypokinetic vs Hyperkinetic Disorders

Type Mechanism Examples
Hypokinetic Increased GPi/SNr output → excessive thalamic inhibition Parkinson’s disease, parkinsonism
Hyperkinetic Decreased GPi/SNr output → reduced thalamic inhibition Huntington’s, hemiballismus, dystonia, chorea

Specific Movement Disorders

Parkinson’s Disease ▼

Pathology: Loss of dopaminergic neurons in SNc; Lewy bodies (α-synuclein)

Mechanism:

  • Loss of dopamine → underactive direct pathway + overactive indirect pathway
  • Net: Increased GPi output → increased thalamic inhibition → bradykinesia

Cardinal features (TRAP):

  • Tremor (resting, “pill-rolling,” 4-6 Hz)
  • Rigidity (cogwheel)
  • Akinesia/bradykinesia
  • Postural instability

Other features: Masked facies, micrographia, shuffling gait, reduced arm swing, hypophonia

Treatment targets:

  • Levodopa – dopamine precursor
  • Dopamine agonists (pramipexole, ropinirole)
  • MAO-B inhibitors (rasagiline, selegiline)
  • DBS of STN or GPi
Huntington’s Disease ▼

Genetics: CAG repeat expansion in huntingtin gene (chromosome 4); autosomal dominant

Pathology: Loss of striatal neurons (especially indirect pathway medium spiny neurons)

Mechanism:

  • Early: Loss of indirect pathway → decreased GPi output → chorea
  • Late: Loss of direct pathway → rigidity, bradykinesia

Clinical features:

  • Chorea – irregular, random, flowing movements (early)
  • Psychiatric – depression, irritability, psychosis (often precede motor)
  • Cognitive decline – subcortical dementia
  • Oculomotor – impaired saccades

Imaging: Caudate atrophy → “box-car” ventricles

Treatment: Tetrabenazine, deutetrabenazine (VMAT2 inhibitors) for chorea

Hemiballismus ▼

Definition: Violent, flinging movements of proximal limb (unilateral)

Lesion: Contralateral subthalamic nucleus (STN)

Mechanism:

  • STN normally excites GPi (glutamate)
  • STN lesion → reduced GPi activity → reduced thalamic inhibition → excessive movement

Etiology: Usually lacunar stroke; also hyperglycemia (nonketotic), tumor, MS

Treatment: Often resolves; dopamine blockers if persistent

Other Hyperkinetic Disorders ▼

Dystonia

  • Sustained muscle contractions → twisting, repetitive movements, abnormal postures
  • Pathophysiology: Loss of surround inhibition in BG circuits
  • Types: Focal (cervical, blepharospasm), segmental, generalized
  • Treatment: Botulinum toxin (focal), DBS (generalized)

Chorea

  • Irregular, brief, random, flowing movements
  • Causes: Huntington’s, Sydenham’s (post-streptococcal), lupus, pregnancy, drugs

Athetosis

  • Slow, writhing movements (distal > proximal)
  • Often combined with chorea (choreoathetosis)
  • Causes: Cerebral palsy, kernicterus

Wilson’s Disease

  • Copper accumulation → BG degeneration
  • Movement disorder (tremor, dystonia, parkinsonism) + psychiatric + hepatic
  • MRI: “Face of the giant panda” sign in midbrain
💎 Board Pearl

Hemiballismus = STN lesion (usually lacunar stroke). Most dramatic movement disorder. Contralateral to lesion. Often improves spontaneously. DBS target for Parkinson’s = STN (increases its activity to reduce dyskinesias).

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🔷 Thalamus – Overview

General Organization

Location: Paired structures forming lateral walls of 3rd ventricle

Function: “Gateway to cortex” – relay and processing station for virtually all sensory, motor, and limbic information

Blood supply:

  • Tuberothalamic artery (from PComm) – anterior thalamus
  • Paramedian arteries (from PCA) – medial thalamus
  • Thalamogeniculate arteries (from PCA) – lateral thalamus
  • Posterior choroidal arteries (from PCA) – posterior thalamus

Internal Medullary Lamina

Y-shaped white matter that divides thalamus into nuclear groups:

  • Anterior group
  • Medial group
  • Lateral group (subdivided into dorsal and ventral tiers)

Intralaminar nuclei: Within the lamina (centromedian, parafascicular)

Reticular nucleus: Surrounds thalamus laterally; does NOT project to cortex

—

📍 Thalamic Nuclei & Connections

Relay Nuclei (Specific Nuclei)

Nucleus Input Output (Cortex) Function
VPL (Ventral Posterolateral) Medial lemniscus, spinothalamic tract (BODY) Primary somatosensory (S1) Body sensation
VPM (Ventral Posteromedial) Trigeminal pathway, taste (FACE) Primary somatosensory (S1) Face sensation, taste
VL (Ventral Lateral) Cerebellum (dentate), basal ganglia Motor cortex (M1) Motor coordination
VA (Ventral Anterior) Basal ganglia (GPi, SNr) Premotor, prefrontal cortex Motor planning
LGN (Lateral Geniculate) Optic tract Primary visual cortex (V1) Vision
MGN (Medial Geniculate) Inferior colliculus (auditory) Primary auditory cortex Hearing
💎 Board Pearl

VPL = body, VPM = face (M = Medial = face/Mouth). LGN = Light (vision), MGN = Music (hearing). VL receives cerebellar input; VA receives BG input.

Association & Limbic Nuclei

Nucleus Connections Function
Anterior Nucleus Mammillary bodies → cingulate gyrus Part of Papez circuit; memory, emotion
Mediodorsal (MD) Amygdala, prefrontal cortex Executive function, emotion, memory
Pulvinar Association cortices (parietal, temporal, occipital) Visual attention, language, multimodal integration
Lateral Dorsal (LD) Hippocampus → cingulate Spatial memory, emotion
Lateral Posterior (LP) Parietal cortex Sensory integration

Nonspecific Nuclei

Nucleus Function Notes
Intralaminar Nuclei (CM, PF) Arousal, attention, pain processing Project diffusely to cortex and striatum
Reticular Nucleus Gating thalamic relay (modulates what reaches cortex) Does NOT project to cortex; only inhibitory output

Thalamic Syndromes

Dejerine-Roussy Syndrome (Thalamic Pain Syndrome) ▼

Lesion: VPL/VPM (posterolateral thalamic stroke, usually thalamogeniculate artery)

Acute phase:

  • Contralateral sensory loss (all modalities)
  • Contralateral hemiparesis (if internal capsule involved)

Chronic phase (weeks-months later):

  • Central post-stroke pain – severe, burning, poorly localized
  • Allodynia – painful response to light touch
  • Hyperpathia – exaggerated pain response
  • Spontaneous pain episodes

Treatment: Tricyclics, gabapentin, pregabalin; often refractory

Other Thalamic Stroke Syndromes ▼
Territory Nuclei Involved Clinical Features
Anterior (Tuberothalamic) Anterior nucleus, VA, VL anterior Executive dysfunction, apathy, personality change, memory impairment
Paramedian MD, intralaminar nuclei Memory loss, decreased arousal, vertical gaze palsy; if bilateral → “top of basilar” syndrome
Inferolateral (Thalamogeniculate) VPL, VPM, VL Pure sensory stroke → Dejerine-Roussy; may have hemiataxia
Posterior (Posterior Choroidal) Pulvinar, LGN, MGN Visual field defects (quadrantanopia), hemisensory loss, aphasia (dominant)
💎 Board Pearl

Bilateral paramedian thalamic strokes (artery of Percheron variant) → vertical gaze palsy + memory loss + decreased arousal. Classic “top of basilar” finding.

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🌡️ Hypothalamus – Overview

General Organization

Location: Forms floor and lower lateral walls of 3rd ventricle; below thalamus

Boundaries:

  • Anterior: Lamina terminalis, optic chiasm
  • Posterior: Mammillary bodies
  • Superior: Hypothalamic sulcus (separates from thalamus)
  • Inferior: Pituitary stalk (infundibulum)

Function: Master regulator of homeostasis – temperature, hunger, thirst, circadian rhythm, autonomic function, pituitary control

Anatomical Divisions

Region Location Key Nuclei
Anterior (Supraoptic) Above optic chiasm Supraoptic, paraventricular, suprachiasmatic, preoptic
Middle (Tuberal) At level of tuber cinereum Arcuate, ventromedial, dorsomedial
Posterior (Mammillary) At mammillary bodies Mammillary bodies, posterior hypothalamic nucleus

Major Connections

  • Fornix: Hippocampus → mammillary bodies (memory)
  • Mammillothalamic tract: Mammillary bodies → anterior thalamus
  • Medial forebrain bundle: Connects limbic structures; reward pathway
  • Hypothalamohypophyseal tract: To posterior pituitary (oxytocin, ADH)
  • Tuberoinfundibular tract: To median eminence (releasing hormones)
  • Dorsal longitudinal fasciculus: To brainstem autonomic centers
—

📍 Hypothalamic Nuclei & Functions

Key Nuclei and Functions

Nucleus Location Function Lesion Effect
Suprachiasmatic (SCN) Anterior Circadian rhythm (receives retinal input) Loss of circadian rhythm
Supraoptic Anterior Produces ADH (vasopressin) Diabetes insipidus
Paraventricular Anterior Produces oxytocin and ADH; CRH release Diabetes insipidus
Preoptic/Anterior Anterior Cooling center (heat dissipation); GnRH release Hyperthermia
Lateral Hypothalamus Lateral Hunger center; orexin production Anorexia, weight loss
Ventromedial Middle Satiety center Hyperphagia, obesity, savage behavior
Arcuate Middle Releases dopamine (inhibits prolactin); GHRH Hyperprolactinemia
Posterior Hypothalamus Posterior Heating center (heat conservation); sympathetic Poikilothermia (inability to regulate temp)
Mammillary Bodies Posterior Memory (Papez circuit) Wernicke-Korsakoff syndrome
💎 Board Pearl

Lateral = hunger (destroy Lateral → Lean). Ventromedial = satiety (destroy VM → Very Much eating). Anterior hypothalamus = cooling (A/C = Air Conditioning). Posterior = heating.

Hypothalamic Functions Summary

Temperature Regulation ▼
  • Anterior/Preoptic: Cooling (parasympathetic) – vasodilation, sweating
  • Posterior: Heating (sympathetic) – vasoconstriction, shivering

Clinical:

  • Anterior lesion → hyperthermia
  • Posterior lesion → poikilothermia (body temp matches environment)
Autonomic Control ▼
  • Anterior/medial: Parasympathetic (rest and digest)
  • Posterior/lateral: Sympathetic (fight or flight)

Connections: Via dorsal longitudinal fasciculus and descending autonomic pathways to brainstem and spinal cord

Pituitary Control ▼

Posterior Pituitary (Neurohypophysis)

  • Direct neural connection via hypothalamohypophyseal tract
  • ADH: From supraoptic and paraventricular nuclei
  • Oxytocin: From paraventricular nucleus

Anterior Pituitary (Adenohypophysis)

  • Controlled via hypophyseal portal system
  • Releasing hormones: CRH, TRH, GnRH, GHRH
  • Inhibiting hormones: Dopamine (inhibits prolactin), somatostatin (inhibits GH)

Hypothalamic Clinical Syndromes

Syndrome Cause/Lesion Features
Diabetes Insipidus Supraoptic/paraventricular or stalk lesion Polyuria, polydipsia, dilute urine, hypernatremia
SIADH Inappropriate ADH secretion Hyponatremia, concentrated urine, fluid retention
Narcolepsy Loss of orexin neurons (lateral hypothalamus) Excessive daytime sleepiness, cataplexy, sleep paralysis
Hypothalamic Obesity Ventromedial hypothalamus lesion (craniopharyngioma) Hyperphagia, rapid weight gain
Wernicke-Korsakoff Mammillary body damage (thiamine deficiency) Confabulation, anterograde amnesia, ataxia, ophthalmoplegia
Kallmann Syndrome GnRH neuron migration failure Hypogonadotropic hypogonadism + anosmia
💎 Board Pearl

Craniopharyngioma (Rathke’s pouch remnant) compresses hypothalamus → bitemporal hemianopia, hypopituitarism, diabetes insipidus, hypothalamic obesity. Calcified suprasellar mass on imaging.

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📊 Summary Tables & Quick Reference

Movement Disorder Localization

Disorder Structure Mechanism
Parkinson’s SNc (dopamine loss) Increased GPi output → bradykinesia
Huntington’s Striatum (caudate) Indirect pathway loss → chorea
Hemiballismus Subthalamic nucleus Decreased GPi output → violent flinging
Wilson’s disease Putamen, globus pallidus Copper deposition → mixed movement disorder

Thalamic Nuclei Quick Reference

Mnemonic Nucleus Function
VPL = body Ventral Posterolateral Body somatosensory
VPM = face (M=Mouth) Ventral Posteromedial Face sensation, taste
LGN = Light Lateral Geniculate Vision
MGN = Music Medial Geniculate Hearing
VL = cerebelLum Ventral Lateral Motor (cerebellar input)
VA = basal gAnglia Ventral Anterior Motor planning (BG input)

Hypothalamic Functions Quick Reference

Function Nucleus Mnemonic
Hunger Lateral Lateral = Lean when destroyed
Satiety Ventromedial VM = Very Much eating when destroyed
Cooling Anterior/Preoptic A/C = Air Conditioning
Heating Posterior Posterior = furnace in back
Circadian rhythm Suprachiasmatic SCN = Clock
ADH Supraoptic, Paraventricular SON + PVN = water balance

Red Flags – Subcortical Lesions

⚠️ Urgent/Emergent Features
  • Acute hemiballismus: Usually STN stroke – may need urgent neuroimaging
  • Rapidly progressive parkinsonism: Consider atypical parkinsonism (MSA, PSP, CBD)
  • Bilateral thalamic lesions + decreased arousal: Top of basilar syndrome
  • Hypothalamic syndrome + visual field defect: Craniopharyngioma, pituitary apoplexy
  • Confusion + ophthalmoplegia + ataxia: Wernicke’s encephalopathy – give thiamine!
  • Young patient with chorea: Consider Wilson’s disease (treatable!)
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↑

Spinal Cord

🧵 Spinal Cord – Anatomy & Organization

Extent: Foramen magnum → ~L1–L2 vertebral level (adult)

  • Conus medullaris: Terminal cord → gives rise to cauda equina
  • Enlargements: Cervical (C5–T1, upper limb), Lumbar (L2–S3, lower limb)
  • Segments vs vertebrae: Cord segments end higher than same-numbered vertebrae (important for localizing lesions)
Region Key Features Clinical Relevance
Cervical Large white matter, obvious anterior horns (C5–T1) Common site for myelopathy (spondylosis)
Thoracic Small anterior horns, intermediolateral cell column (T1–L2) Horner syndrome with T1 involvement
Lumbar Less white matter, large anterior horns Polio, ALS, radiculopathies affect LMNs here
Sacral Mostly gray matter, S2–S4 parasympathetic Bladder, bowel, sexual dysfunction with conus/cauda lesions

📡 White Matter Tracts (High Yield)

Dorsal Columns (DCML) – Vibration & Proprioception ▼
  • Modality: Vibration, joint position, fine touch
  • Somatotopy (below T6): Gracilis (legs, medial); above T6 adds Cuneatus (arms, lateral)
  • Pathway: Dorsal root → dorsal columns → synapse in medulla (nuclei gracilis/cuneatus) → decussate in medulla → medial lemniscus → thalamus → cortex
  • Lesion in spinal cord: Ipsilateral loss of vibration/position sense below level

Clinical: B12 deficiency, tabes dorsalis, nitrous oxide toxicity → sensory ataxia, positive Romberg.

Spinothalamic Tract – Pain & Temperature ▼
  • Modality: Pain, temperature, crude touch
  • Pathway: Dorsal root → Lissauer’s tract → dorsal horn → decussate in anterior white commissure over 1–2 segments → ascend contralaterally
  • Lesion in cord: Contralateral loss of pain/temp starting ~1–2 levels below lesion

Clinical: Central cord/syrinx → bilateral cape-like loss of pain/temp (spinothalamic crossing fibers).

Corticospinal Tract (CST) – Voluntary Motor ▼
  • Origin: Primary motor cortex (area 4), premotor, SMA
  • Decussation: Pyramidal decussation in caudal medulla → lateral CST (contralateral)
  • Spinal lesion: Ipsilateral UMN signs below level (weakness, spasticity, hyperreflexia, Babinski)
  • At lesion level: LMN signs if anterior horn/root involved

Clinical: Myelopathy = UMN below (↑reflexes) + possible LMN at level (atrophy, fasciculations).

🌑 Gray Matter & Autonomic Nuclei

Horns & Columns ▼
  • Dorsal horn: Sensory processing
  • Ventral horn: LMNs to skeletal muscle
  • Intermediate zone: Autonomics & interneurons

Key Nuclei

  • Intermediolateral cell column (T1–L2): Sympathetic preganglionic neurons
  • S2–S4: Parasympathetic to bladder, bowel, sexual function
  • Clarke’s nucleus (T1–L2): Dorsal spinocerebellar tract (ipsilateral leg proprioception)

Clinical:

  • Horner syndrome: Lesion of T1 sympathetic outflow (Pancoast tumor, syrinx)
  • Conus medullaris: Early bladder/bowel/sexual dysfunction, saddle anesthesia

🩸 Blood Supply of the Spinal Cord

Anterior Spinal Artery (ASA) – 2/3 of Cord ▼
  • Supplies anterior 2/3 of cord: corticospinal tracts, spinothalamic tracts, ventral horns
  • Spares dorsal columns

ASA Syndrome:

  • Bilateral motor weakness below lesion
  • Bilateral pain & temperature loss
  • Preserved vibration & proprioception
  • Autonomic dysfunction (bladder, bowel)
Posterior Spinal Arteries (PSA) – Dorsal Columns ▼
  • Supply dorsal columns and posterior horns

PSA Syndrome:

  • Loss of vibration and position sense
  • Sensory ataxia, positive Romberg
  • Motor and pain/temp often preserved
Radicular Arteries & Adamkiewicz ▼
  • Radicular arteries: Segmental reinforcement of ASA/PSA
  • Artery of Adamkiewicz: Usually T9–L2, supplies lower thoracic/lumbosacral cord
  • Clinical: Aortic surgery or hypotension → infarct of lower cord, flaccid paraplegia → then spasticity

⚠️ Spinal Cord Syndromes

Brown-Séquard Syndrome – Hemicord Lesion ▼
  • Ipsilateral below lesion: UMN weakness (CST), loss of vibration/proprioception (DCML)
  • Contralateral below (starting ~1–2 levels down): Loss of pain & temperature (STT)
  • At lesion level: LMN signs, segmental sensory loss

Etiologies: Trauma, tumor, MS, penetrating injury.

Central Cord Syndrome – “Hands > Legs” ▼
  • Hyperextension injury in cervical spondylosis (elderly) or syringomyelia
  • Weakness: Arms > legs (cervical CST fibers for arms more central)
  • Sensation: Often bilateral cape-like pain/temp loss
  • Variable bladder involvement
Posterior Cord Syndrome ▼
  • Loss of vibration & proprioception, sensory ataxia, positive Romberg
  • Motor strength, pain & temperature largely preserved
  • Etiologies: B12 deficiency, tabes dorsalis, nitrous oxide, posterior spinal artery infarct
Anterior Cord Syndrome – ASA Infarct ▼
  • Bilateral motor paralysis below lesion (CST)
  • Bilateral pain/temp loss (STT)
  • Vibration/proprioception spared (dorsal columns)
Conus Medullaris vs Cauda Equina ▼
Feature Conus Medullaris Cauda Equina
Location L1–L2 cord segment Lumbar & sacral roots
Onset Sudden More gradual
Weakness Symmetric; proximal & distal Asymmetric, radicular, distal
Sensation Saddle anesthesia Asymmetric dermatomal loss
Bladder/Bowel Early, prominent sphincter dysfunction Late, less prominent early on
Reflexes Ankle jerk ↓, bulbocavernosus ↓ Hyporeflexia in affected roots

💎 Spinal Cord – Board Pearls

  • Spinothalamic decussation occurs 1–2 levels above entry → explains contralateral pain/temp loss starting slightly below lesion.
  • Dorsal columns decussate in the medulla, not in the cord → spinal lesions give ipsilateral position/vibration loss.
  • Anterior spinal artery syndrome: motor + pain/temp loss, preserved vibration/proprioception.
  • Syringomyelia: bilateral cape-like pain/temp loss with preserved dorsal column function; think Chiari I.
  • B12 deficiency: combined degeneration of DCML + CST → sensory ataxia + UMN signs.
  • Horner syndrome with arm weakness = think lesion at C8–T2 (Pancoast, syrinx, lateral medullary/cord lesions).
💎 Quick Localization Trick

UMN signs below + LMN at the level = cord lesion. If legs are worse than arms with a sensory level, it’s almost never purely brain — think spinal cord.