Sensory System

What You'll Learn

Receptor types — know the four mechanoreceptors (Meissner, Merkel, Pacinian, Ruffini), their adaptation rates, and clinical relevance (two-point discrimination, vibration testing)
Fiber classification — A-alpha through C fibers; which carry what modality, myelination status, conduction velocities
Dorsal column–medial lemniscus (DCML) pathway — full 3-neuron arc, site of decussation (medulla), modalities carried (fine touch, vibration, proprioception)
Spinothalamic tract (STT) — full pathway, decussation site (anterior white commissure, 1–2 levels above entry), lateral vs. anterior divisions
Trigeminal sensory system — three nuclei (main sensory, spinal, mesencephalic), V1/V2/V3 distributions, VPM thalamus relay
Sensory cortex — Brodmann areas 3, 1, 2 (S1); sensory homunculus; cortical sensory modalities (stereognosis, graphesthesia, two-point discrimination)
Pain pathways — gate control theory, descending modulation (PAG → raphe nuclei → dorsal horn), referred pain, central sensitization
Localization patterns — dermatomal vs. peripheral nerve vs. cord level vs. brainstem vs. thalamic vs. cortical sensory loss
Clinical syndromes — Brown-Séquard, syringomyelia, Dejerine-Roussy, cortical sensory loss, stocking-glove neuropathy

HighYield Pearls

DCML decussates in caudal medulla: as internal arcuate fibers → contralateral medial lemniscus → VPL thalamus; carries fine touch, vibration, proprioception. Cord lesion → ipsilateral loss below.
Spinothalamic decussates at spinal entry level: via anterior white commissure (1–2 levels above entry); carries pain, temp, crude touch. Cord lesion → contralateral loss 1–2 levels below.
Brown-Séquard: ipsi motor (CST) + ipsi DCML below + contra spinothalamic — classic dissociated hemicord syndrome.
Syringomyelia: cape distribution bilateral UE pain/temp loss (decussating spinothalamic fibers near central canal) with spared DCML; LMN UE + spastic LE if expansion.
Pure posterior column syndrome (tabes dorsalis, posterior spinal artery infarct): isolated DCML loss → sensory ataxia + positive Romberg. Dorsolateral column disease (B12, copper, HIV vacuolar myelopathy, some inflammatory/demyelinating myelopathies): dorsal columns + lateral corticospinal tracts → sensory ataxia + UMN signs.
Dejerine-Roussy (thalamic pain): delayed contralateral burning dysesthesia after VPL/VPM infarct; notoriously refractory.
Cortical (parietal) sensory loss: primary modalities preserved; lose agraphesthesia, astereognosis, 2-point discrimination, extinction on double simultaneous stimulation.
Trigeminal nuclei: main sensory (pons, fine touch) + spinal trigeminal (caudal medulla, pain/temp — analog of spinothalamic) + mesencephalic (proprioception, only PNS-like nucleus in CNS).
Sensory neuronopathy (DRG attack): anti-Hu, Sjögren, cisplatin/oxaliplatin → non-length-dependent loss of all modalities + pseudoathetosis + areflexia.
Small fiber neuropathy: painful burning feet + autonomic, normal NCS; diagnose by skin biopsy (↓ IENFD); DM is #1 cause.
Key dermatomes: C6 thumb, C7 middle finger, C8 little finger, T4 nipple, T10 umbilicus, L1 inguinal, L4 medial leg, L5 great toe dorsum, S1 lateral foot, S2–S5 perianal/saddle.
Gate theory (Melzack-Wall): large A-β input inhibits pain transmission at substantia gelatinosa — rationale for TENS and rubbing an injury.
Descending pain modulation: PAG (midbrain) → RVM (raphe serotonergic + locus coeruleus noradrenergic) → dorsal horn; opioid + SNRI targets.

🔍 Quick ReferencePathway / receptor · Cortical / thalamic processing · Lesion patterns

Pathway / receptor

Internal arcuate fibers at caudal medulla → DCML decussation (fine touch, vibration, proprioception)
Anterior white commissure crossing 1–2 levels above entry → spinothalamic tract (pain, temp, crude touch)
Pacinian corpuscle → high-frequency vibration + deep pressure (rapidly adapting, A-β, 256 Hz tuning fork)
Meissner corpuscle → light touch + low-frequency vibration in glabrous skin (rapidly adapting, fingertip discrimination)
Merkel disc → sustained pressure + 2-point discrimination (slowly adapting, Braille reading)
Ruffini ending → skin stretch + joint position (slowly adapting, proprioceptive contribution)
Free nerve endings (Aδ + C) → pain, temp, itch, crude touch (non-adapting nociceptors; "first" vs "second" pain)
Muscle spindle Ia (annulospiral) → dynamic + static stretch; group II (flower spray) → static length
Golgi tendon organ (Ib) → muscle tension (autogenic inhibition)
Mesencephalic nucleus of CN V → facial proprioception (only PNS-like cell bodies inside CNS)

Cortical / thalamic

VPL nucleus of thalamus → body somatosensory relay (DCML + spinothalamic)
VPM nucleus of thalamus → face somatosensation + taste
LGN / MGN → vision / audition relays (special sensory thalamus)
Postcentral gyrus, Brodmann 3, 1, 2 → primary somatosensory cortex (S1) with contralateral homunculus
Parietal operculum S2 → bilateral higher-order somatosensory processing
Medial paracentral lobule somatosensory strip → contralateral leg/foot (ACA territory)
Hand knob region of postcentral gyrus → contralateral hand sensation
Periaqueductal gray (PAG) → RVM → dorsal horn → descending pain modulation (opioid-rich)
Substantia gelatinosa (Rexed II) → gate-control site for dorsal horn pain modulation

Lesion patterns / syndromes

Ipsilateral motor + DCML loss with contralateral pain/temp loss → Brown-Séquard (hemicord)
Cape-distribution bilateral UE pain/temp loss with preserved touch → syringomyelia
Sensory ataxia + positive Romberg + lost vibration/proprioception → pure posterior column disease (tabes dorsalis, posterior spinal artery infarct); combined dorsal column + lateral corticospinal involvement → dorsolateral column disease (B12, copper, HIV vacuolar myelopathy)
Delayed contralateral burning hemibody pain after stroke → Dejerine-Roussy (thalamic VPL/VPM infarct)
Agraphesthesia, astereognosis, extinction with intact primary touch → cortical (parietal) sensory loss
Crossed face/body sensory loss (ipsi face + contra body pain/temp) → lateral medullary (Wallenberg) syndrome
Stocking-glove distal symmetric sensory loss → length-dependent peripheral neuropathy (DM, alcohol, chemo, B12)
Painful burning feet with normal NCS + reduced IENFD on skin biopsy → small fiber neuropathy (DM, amyloid, Sjögren, sarcoid)
Pseudoathetosis + areflexia + non-length-dependent all-modality loss → sensory neuronopathy (anti-Hu, Sjögren, cisplatin)
Asymmetric multifocal sensorimotor deficits → mononeuritis multiplex (vasculitis, DM, leprosy)
Acute painful shoulder → patchy upper-trunk weakness → Parsonage-Turner brachial neuritis (plexopathy)
Saddle anesthesia + bladder/bowel dysfunction → cauda equina / conus medullaris syndrome
Asymmetric apraxia + alien limb + cortical sensory loss → corticobasal degeneration (CBD)
Burning allodynia + autonomic + trophic skin changes after minor injury → complex regional pain syndrome (CRPS)
Dermatomal sensory loss + reflex loss + myotomal weakness → radiculopathy

Sensory Receptor Types

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

Receptor	Type	Location	Adaptation	Modality
Meissner corpuscle	Encapsulated mechanoreceptor	Dermal papillae, glabrous skin	Rapidly adapting	Light touch, low-freq vibration
Merkel disc	Unencapsulated mechanoreceptor	Basal epidermis, fingertips	Slowly adapting	Pressure, edges, two-point discrimination
Pacinian corpuscle	Encapsulated mechanoreceptor	Deep dermis, periosteum, joint capsules	Rapidly adapting	Deep pressure, high-freq vibration
Ruffini ending	Encapsulated mechanoreceptor	Deep dermis, joint capsules	Slowly adapting	Skin stretch, sustained pressure
Cold receptor	Free nerve ending	Skin (superficial)	Slowly adapting	Cold temperature (10–35°C)
Warm receptor	Free nerve ending	Skin (deeper)	Slowly adapting	Warm temperature (30–45°C)
A-delta nociceptor	Free nerve ending	Skin, viscera	Non-adapting	Sharp/first pain, temperature
C-fiber nociceptor	Free nerve ending	Skin, viscera (polymodal)	Non-adapting	Dull/second pain, burning
Muscle spindle	Intrafusal fiber complex	Skeletal muscle belly	Both (Ia rapid, II slow)	Muscle length, stretch velocity
Golgi tendon organ	Encapsulated receptor	Musculotendinous junction	Slowly adapting	Muscle tension/force
Joint receptors	Mixed (Ruffini/Pacinian-like)	Joint capsule, ligaments	Mixed	Joint position and movement

Board Pearl — Receptors

Two-point discrimination = Merkel discs (slowly adapting, small receptive fields)
Vibration (128 Hz tuning fork) = Pacinian corpuscles → dorsal column pathway; lost early in diabetic neuropathy and subacute combined degeneration
First vs. second pain: A-delta = sharp/fast; C = dull/slow — explains the "double pain" phenomenon with a single noxious stimulus
Ia afferents mediate the monosynaptic stretch reflex — the only monosynaptic reflex in the body

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