Mechanoreceptors

Meissner Corpuscles

• Location: dermal papillae of glabrous (hairless) skin — fingertips, lips, palms, soles
• Adaptation: rapidly adapting (respond to onset/offset of stimulus)
• Modality: light touch, texture changes, low-frequency vibration (~30–50 Hz)
• Fiber type: A-beta (large, myelinated)
• Clinical: most dense in fingertips → critical for fine tactile discrimination

Merkel Discs

• Location: basal epidermis of glabrous skin; highest density in fingertips
• Adaptation: slowly adapting (sustained response throughout stimulus)
• Modality: sustained pressure, edges, fine spatial detail → two-point discrimination
• Fiber type: A-beta
• Clinical: responsible for reading Braille; tested clinically with two-point discrimination calipers

Pacinian Corpuscles

• Location: deep dermis, subcutaneous tissue, periosteum, joint capsules, mesentery
• Adaptation: rapidly adapting (very fast — responds only to changes)
• Modality: deep pressure, high-frequency vibration (~100–300 Hz)
• Fiber type: A-beta
• Clinical: tested with 128 Hz tuning fork on bony prominences; lost early in peripheral neuropathy and B12 deficiency

Ruffini Endings

• Location: deep dermis, joint capsules, ligaments
• Adaptation: slowly adapting
• Modality: skin stretch, sustained pressure; contribute to proprioception and joint position sense
• Fiber type: A-beta
• Clinical: important for detecting direction of stretch across skin surface

Thermoreceptors

• Cold receptors: free nerve endings; A-delta fibers; respond to temperatures ~10–35°C; peak at ~25°C
• Warm receptors: free nerve endings; C fibers; respond to temperatures ~30–45°C; peak at ~45°C
• Extreme heat (>45°C) and extreme cold (<10°C) activate nociceptors, not thermoreceptors
• TRP channels: TRPV1 (capsaicin/heat), TRPM8 (menthol/cold) — important pharmacological targets

Nociceptors

• A-delta nociceptors: thinly myelinated → sharp, well-localized "first pain"; activated by mechanical and thermal stimuli
• C-fiber nociceptors: unmyelinated → dull, burning, poorly localized "second pain"; most numerous nociceptor type; polymodal
• Silent (sleeping) nociceptors: normally inactive; become sensitized after tissue injury → contribute to inflammatory hyperalgesia
• Nociceptors do not adapt — clinically important for persistent pain signaling

Proprioceptors

Muscle Spindles

• Location: within skeletal muscle belly, parallel to extrafusal fibers
• Function: detect muscle stretch (length and rate of change)
• Afferents: Ia (primary — dynamic stretch, annulospiral endings) and II (secondary — static stretch, flower-spray endings)
• Efferents: gamma motor neurons adjust spindle sensitivity
• Reflex: monosynaptic stretch reflex (myotatic reflex) — Ia afferent → alpha motor neuron

Golgi Tendon Organs

• Location: musculotendinous junction, in series with muscle fibers
• Function: detect muscle tension/force (not length)
• Afferents: Ib fibers (large, myelinated)
• Reflex: inverse myotatic reflex — inhibits agonist, facilitates antagonist → protective against excessive force

Joint Receptors

• Ruffini-like endings (slowly adapting) — joint position at extremes of range
• Pacinian-like corpuscles (rapidly adapting) — joint movement
• Free nerve endings — pain from joint capsule

Comprehensive Receptor Table

Sensory and Motor Fiber Types

• General principle: larger diameter + thicker myelin = faster conduction
• Sensory fibers classified by Erlanger-Gasser (A, B, C) and Lloyd-Hunt (I–IV for muscle afferents) systems
• Clinical relevance: large myelinated fibers (A-alpha, A-beta) affected first in demyelinating neuropathies; small fibers (A-delta, C) affected first in small fiber neuropathy

Modalities Carried

• Fine (discriminative) touch
• Vibration
• Conscious proprioception (joint position sense)
• Two-point discrimination

Three-Neuron Pathway

First-Order Neuron

• Receptor (mechanoreceptor, proprioceptor) → dorsal root ganglion (DRG) (cell body)
• Peripheral process = receptor; central process enters spinal cord via medial division of dorsal root
• Large-diameter, heavily myelinated A-beta fibers
• Central axon ascends ipsilaterally in dorsal columns without synapsing in the spinal cord

Dorsal Columns (Ipsilateral Spinal Cord)

• Fasciculus gracilis (medial) — lower body: lower limbs + trunk below T6
  • Present at all spinal levels
  • Terminates in nucleus gracilis (caudal medulla)
• Fasciculus cuneatus (lateral) — upper body: upper limbs + trunk above T6
  • Present only above T6
  • Terminates in nucleus cuneatus (caudal medulla)
• Somatotopy: sacral fibers most medial; cervical fibers most lateral (new fibers added laterally as tract ascends)

Second-Order Neuron

• Cell bodies in nucleus gracilis / nucleus cuneatus (caudal medulla)
• Axons sweep anteriorly and medially as internal arcuate fibers
• DECUSSATION at level of caudal medulla (sensory decussation)
• After crossing, fibers form the medial lemniscus
• Medial lemniscus ascends contralaterally through medulla → pons → midbrain

Thalamic Relay (Third-Order Neuron)

• Medial lemniscus terminates in ventral posterolateral (VPL) nucleus of thalamus
• Third-order neurons project via posterior limb of internal capsule
• Terminate in primary somatosensory cortex (S1) — postcentral gyrus (Brodmann areas 3, 1, 2)

Modalities Carried

• Lateral spinothalamic tract: pain and temperature (the clinically more important division)
• Anterior spinothalamic tract: crude (non-discriminative) touch and pressure
• Together = anterolateral system

Three-Neuron Pathway

First-Order Neuron

• Receptor (nociceptor, thermoreceptor) → DRG (cell body)
• Small-diameter fibers: A-delta (pain, cold) and C fibers (pain, warmth)
• Central process enters spinal cord via lateral division of dorsal root
• Fibers may ascend or descend 1–2 segments in Lissauer's tract (posterolateral tract) before synapsing

Second-Order Neuron (Dorsal Horn Synapse)

• Synapse in the dorsal horn:
  • Substantia gelatinosa (lamina II) — modulation of pain input; site of gate control
  • Nucleus proprius (laminae III–V) — origin of most spinothalamic projections
  • Marginal zone (lamina I) — nociceptive-specific neurons
• Second-order axons cross midline via anterior white commissure
• DECUSSATION occurs within 1–2 spinal segments above the level of entry
• After crossing, fibers ascend contralaterally in the anterolateral funiculus

Somatotopy in the Spinal Cord

• Sacral fibers = most lateral (outermost) — added first, pushed outward
• Cervical fibers = most medial (innermost) — added last
• New fibers are added medially as the tract ascends
• Clinical significance:
  • Extramedullary compression (e.g., tumor) → sacral fibers affected first (outermost) → sacral involvement early
  • Intramedullary lesion (e.g., syringomyelia) → cervical fibers affected first (innermost) → sacral sparing

Thalamic Relay (Third-Order Neuron)

• Lateral STT → VPL nucleus of thalamus (discriminative pain/temperature)
• Some fibers also project to:
  • Intralaminar nuclei (emotional/affective component of pain)
  • Reticular formation (arousal, autonomic responses to pain)
  • Periaqueductal gray (PAG) (descending pain modulation)
• Third-order neurons → posterior limb of internal capsule → primary somatosensory cortex (S1)

Trigeminal Nerve Divisions (V1, V2, V3)

• V1 (ophthalmic): forehead, upper eyelid, nose bridge, cornea → exits via superior orbital fissure
• V2 (maxillary): cheek, upper lip, upper teeth, palate, nasal cavity → exits via foramen rotundum
• V3 (mandibular): lower face, chin, lower teeth, anterior 2/3 tongue (sensation), ear (partly) → exits via foramen ovale
  • V3 is the only division with motor fibers (muscles of mastication)
• All three divisions converge on the trigeminal (semilunar/Gasserian) ganglion in Meckel's cave

Trigeminal Sensory Nuclei

Main (Principal/Chief) Sensory Nucleus

• Location: lateral pons
• Modality: fine touch and pressure from the face
• Analogous to: dorsal column nuclei (gracilis/cuneatus) for the body
• Projection: dorsal trigeminothalamic tract (ventral trigeminal lemniscus) → ipsilateral and contralateral VPM thalamus

Spinal Trigeminal Nucleus

• Location: extends from caudal pons through entire medulla to upper cervical cord (C2–C3)
• Modality: pain and temperature from the face
• Analogous to: dorsal horn (substantia gelatinosa) for the body
• Trigeminal fibers descend in the spinal trigeminal tract before synapsing
• Onion-skin pattern: perioral region (V2/V3 boundary) represented rostrally; peripheral face represented caudally
• Projection: ventral trigeminothalamic tract → contralateral VPM thalamus
• Clinical: lesion of spinal nucleus/tract → ipsilateral facial pain/temperature loss (e.g., lateral medullary syndrome)

Mesencephalic Nucleus

• Location: midbrain (lateral periaqueductal gray)
• Modality: proprioception from muscles of mastication, temporomandibular joint, periodontal ligament
• Unique: contains primary sensory neuron cell bodies within the CNS (the only such example) — functionally equivalent to a DRG inside the brainstem
• Role: mediates the jaw jerk reflex (afferent limb)

Trigeminothalamic Projections

• Dorsal trigeminothalamic tract: from main sensory nucleus (touch) → bilateral VPM thalamus (some ipsilateral)
• Ventral trigeminothalamic tract: from spinal trigeminal nucleus (pain/temperature) → contralateral VPM thalamus
• VPM thalamus → posterior limb of internal capsule → face area of somatosensory cortex (lateral postcentral gyrus)

Primary Somatosensory Cortex (S1)

Location and Brodmann Areas

• Postcentral gyrus — Brodmann areas 3a, 3b, 1, 2
• Blood supply: MCA (face, arm, hand) and ACA (leg, foot — medial surface)

Cytoarchitectonic Subdivisions

• Area 3a: proprioception from muscle spindles; receives input from VPL thalamus
• Area 3b: primary tactile processing — receives the densest thalamocortical projection; most critical for basic somatosensation
• Area 1: texture discrimination
• Area 2: size and shape discrimination; integrates tactile + proprioceptive input
• Information flows: 3a/3b → 1 → 2 → posterior parietal cortex (hierarchical processing)

Sensory Homunculus

• Inverted somatotopic map on the postcentral gyrus
• Medial (interhemispheric fissure): foot, leg, genitalia — ACA territory
• Lateral convexity (superior): trunk, arm, hand
• Lateral convexity (inferior, near Sylvian fissure): face, lips, tongue — MCA territory
• Cortical area proportional to receptor density, not body part size → hand, lips, tongue have disproportionately large representation

Sensory Association Cortex

Superior Parietal Lobule (Areas 5 and 7)

• Area 5: higher-order somatosensory integration; integrates input from S1
• Area 7: multimodal integration (somatosensory + visual); spatial awareness
• Functions:
  • Sensorimotor integration for reaching and grasping
  • Body schema / spatial body representation
  • Directed attention to contralateral space
• Lesions: contralateral neglect (especially right parietal → left hemispatial neglect), tactile agnosia, constructional apraxia

Secondary Somatosensory Cortex (S2)

• Location: parietal operculum (superior bank of Sylvian fissure)
• Functions: bilateral body representation, tactile learning/memory, pain perception
• Receives input from both ipsilateral and contralateral S1

Cortical Sensory Modalities

Gate Control Theory (Melzack & Wall, 1965)

• Concept: pain transmission in the dorsal horn is modulated by a "gate" in the substantia gelatinosa (lamina II)
• Gate closed (pain inhibited):
  • Large-diameter A-beta fiber activity (touch, vibration) activates inhibitory interneurons → suppresses pain transmission
  • Explains why rubbing a painful area reduces pain perception
• Gate open (pain facilitated):
  • Small-diameter C-fiber activity inhibits the inhibitory interneurons → allows pain signals to pass
• Clinical application: TENS (transcutaneous electrical nerve stimulation), dorsal column stimulators activate large fibers → close the gate

Descending Pain Modulation

• Periaqueductal gray (PAG) in midbrain — major integration center for descending analgesia
• PAG → nucleus raphe magnus (NRM) (serotonergic, rostral ventromedial medulla)
• PAG → locus coeruleus (noradrenergic, dorsolateral pons)
• NRM and locus coeruleus project to dorsal horn (laminae I, II, V) via dorsolateral funiculus
• Mechanism: release of serotonin (5-HT) and norepinephrine activates enkephalinergic interneurons in dorsal horn → presynaptic and postsynaptic inhibition of pain transmission
• Pharmacological relevance:
  • Opioids: activate PAG neurons (disinhibition) → enhance descending inhibition
  • TCAs/SNRIs (duloxetine, venlafaxine): enhance serotonin/norepinephrine in descending pathway → used for neuropathic pain

Endogenous Opioid System

• Endorphins: derived from pro-opiomelanocortin (POMC); act on mu receptors; released from hypothalamus and pituitary
• Enkephalins: derived from proenkephalin; act on delta receptors; found in dorsal horn interneurons, PAG
• Dynorphins: derived from prodynorphin; act on kappa receptors; widespread CNS distribution
• All three families inhibit pain transmission at multiple levels: periphery, dorsal horn, brainstem, thalamus
• Mechanism: presynaptic inhibition of neurotransmitter release (reduce substance P and glutamate) + postsynaptic hyperpolarization of projection neurons

Referred Pain

• Definition: pain perceived at a site distant from its actual source, typically visceral pain referred to somatic structures
• Mechanism: convergence-projection theory — visceral and somatic afferents converge on the same second-order neurons in the dorsal horn; brain misattributes the signal to the somatic dermatome
• Classic examples:
  • Myocardial ischemia → left arm, jaw, epigastrium (T1–T5 dermatomes)
  • Diaphragmatic irritation → shoulder pain (C3–C5, phrenic nerve)
  • Appendicitis → periumbilical pain (T10) before localizing to RLQ
  • Gallbladder → right shoulder (C3–C5 via phrenic nerve irritation of diaphragm)

Central Sensitization

• Definition: increased excitability of central pain neurons after sustained nociceptive input → amplified pain response
• Mechanisms:
  • Wind-up: progressive increase in C-fiber-evoked responses from repeated stimulation; mediated by NMDA receptor activation
  • Long-term potentiation (LTP) in dorsal horn synapses
  • Upregulation of AMPA/NMDA receptors on projection neurons
  • Loss of inhibitory interneuron function (disinhibition)
• Clinical manifestations:
  • Allodynia: pain from normally non-painful stimulus
  • Hyperalgesia: exaggerated pain response to noxious stimulus
  • Expansion of receptive fields: pain beyond original injury territory
• Conditions: fibromyalgia, complex regional pain syndrome (CRPS), post-stroke central pain (Dejerine-Roussy), neuropathic pain

Bedside Sensory Examination

Patterns of Sensory Loss

Dermatomal Distribution

• Follows a single nerve root territory
• Indicates radiculopathy (disc herniation, foraminal stenosis, herpes zoster)
• Often associated with pain in the same dermatome + motor/reflex findings in the corresponding myotome
• Key landmarks: C5 = lateral arm; C6 = thumb/lateral forearm; C7 = middle finger; C8 = ring/small finger; T4 = nipple line; T10 = umbilicus; L4 = medial leg; L5 = dorsum foot; S1 = lateral foot

Peripheral Nerve Distribution

• Follows the territory of a specific named nerve (not a single dermatome)
• Often involves multiple dermatomes in the distribution of one nerve
• Examples: median nerve (palmar thumb, index, middle finger); ulnar nerve (small finger, medial hand); lateral femoral cutaneous nerve (lateral thigh — meralgia paresthetica)

Stocking-Glove (Length-Dependent) Pattern

• Distal-to-proximal gradient of sensory loss, symmetric bilaterally
• Longest fibers affected first → feet before hands
• Classic for peripheral polyneuropathy (diabetes, alcohol, B12 deficiency, uremia)
• All modalities may be affected, but vibration and proprioception (large fiber) or pain and temperature (small fiber) may predominate depending on fiber type involvement

Dissociated Sensory Loss

• Definition: selective loss of one sensory modality group with preservation of another at the same body region
• Reflects separate anatomical pathways for DCML vs. STT modalities
• Occurs in: syringomyelia, anterior spinal artery syndrome, Brown-Séquard, lateral medullary syndrome

Lesion Localization Table

Brown-Séquard Syndrome (Spinal Cord Hemisection)

Pathophysiology

• Hemisection of the spinal cord (trauma, tumor, demyelination, infarction)
• Explains the critical importance of knowing where each pathway decussates

Clinical Features

• Ipsilateral, below lesion:
  • Dorsal column loss — vibration, proprioception, fine touch (these fibers ascend ipsilaterally and cross in medulla)
  • Corticospinal tract (UMN) weakness — spastic paresis (has already crossed at pyramidal decussation)
• Contralateral, 1–2 levels below lesion:
  • Spinothalamic tract loss — pain and temperature (these fibers cross at the spinal cord level)
• At the level of the lesion (ipsilateral):
  • LMN signs in the affected myotome (anterior horn cell damage)
  • Band of dermatomal pain/temperature loss (dorsal horn damage)

Syringomyelia

Pathophysiology

• Fluid-filled cavity (syrinx) expanding within the central spinal cord, most commonly cervical
• Destroys the anterior white commissure first → disrupts crossing spinothalamic fibers
• Commonly associated with Chiari I malformation; also post-traumatic, tumor-associated

Clinical Features

• "Cape-like" (suspended) dissociated sensory loss:
  • Loss of pain and temperature bilaterally over shoulders, arms, and upper trunk (crossing fibers disrupted)
  • Preserved fine touch, vibration, proprioception (dorsal columns intact)
• Sacral sparing: sacral spinothalamic fibers are most lateral and may be unaffected by central lesion
• As syrinx expands:
  • Anterior horn involvement → LMN weakness, atrophy, fasciculations in hands/arms
  • Lateral corticospinal tract → UMN signs in legs
  • Lateral horn (C8–T2) → Horner syndrome (ptosis, miosis, anhidrosis)
• Charcot joints: painless joint destruction (especially shoulders) from loss of protective pain sensation

Thalamic Pain Syndrome (Dejerine-Roussy)

Pathophysiology

• Infarction or hemorrhage involving VPL/VPM nucleus of the thalamus
• Most commonly from thalamogeniculate artery (branch of PCA) occlusion

Clinical Features

• Acute phase: contralateral hemisensory loss affecting all modalities (face + body)
• Delayed phase (weeks to months):
  • Severe, intractable burning/lancinating contralateral pain (central post-stroke pain)
  • Allodynia — pain from light touch
  • Hyperpathia — exaggerated, prolonged response to noxious stimuli
• May be accompanied by hemiataxia (if ventral lateral nucleus involved)
• Treatment: amitriptyline, gabapentin/pregabalin, lamotrigine; often refractory

Cortical Sensory Loss

• Primary S1 lesion: contralateral loss of discriminative sensation — stereognosis, graphesthesia, two-point discrimination
• Primary sensation (pain, temperature, vibration) may be relatively preserved (some input reaches cortex via thalamic projections to other cortical areas)
• Posterior parietal lesion: contralateral hemispatial neglect (especially right parietal), anosognosia, constructional apraxia, sensory extinction
• Key distinguishing feature from thalamic lesion: cortical = discriminative loss with preserved primary sensation; thalamic = all modalities impaired + delayed pain

Peripheral Neuropathy Patterns

Length-Dependent (Stocking-Glove) Neuropathy

• Most common pattern — distal symmetric polyneuropathy
• Feet affected before hands; sensory loss ascends proximally as disease progresses
• Causes: diabetes (#1), alcohol, B12 deficiency, uremia, chemotherapy (cisplatin, vincristine, taxanes), hereditary (CMT)
• May be large fiber (vibration/proprioception loss, sensory ataxia), small fiber (burning pain, temperature loss), or mixed

Mononeuropathy Multiplex

• Asymmetric involvement of individual named nerves (e.g., radial + peroneal + ulnar)
• Causes: vasculitis (PAN, ANCA-associated), diabetes, sarcoidosis, leprosy, multifocal motor neuropathy, hereditary neuropathy with liability to pressure palsies (HNPP)
• When extensive, may mimic polyneuropathy ("confluent mononeuropathy multiplex")

Sensory Neuronopathy (Ganglionopathy)

• Affects the dorsal root ganglion cell body → non-length-dependent, asymmetric, may affect arms and face as well as legs
• Causes: paraneoplastic (anti-Hu/ANNA-1 antibodies with small cell lung cancer), Sjögren syndrome, cisplatin toxicity, pyridoxine (B6) toxicity
• Clinical: severe sensory ataxia, pseudoathetosis, asymmetric multifocal sensory loss, widespread absent sensory nerve action potentials (SNAPs) on NCS

Spinal Cord Syndrome Summary