Somatosensation

Receptors & Modalities - Touchy Feely Bits

Somatosensation: touch, pressure, vibration, pain, temperature. Specialized receptors detect these.

• Receptor Types & Properties:
  • Mechanoreceptors: Detect mechanical stimuli.
    • Encapsulated: Meissner's, Pacinian, Ruffini's.
    • Unencapsulated: Merkel's, Free nerve endings (FNE).
  • Adaptation: 📌 "MaP out Rapid changes (Meissner, Pacinian = Rapidly Adapting)."
    • Rapidly Adapting (RA/Phasic): Signal changes (e.g., Meissner's, Pacinian).
    • Slowly Adapting (SA/Tonic): Signal intensity/duration (e.g., Merkel's, Ruffini's).

⭐ A-delta (Aδ) fibers (myelinated) transmit fast, sharp, localized pain; C fibers (unmyelinated) transmit slow, dull, diffuse pain.

Ascending Pathways - Info Superhighways

• Dorsal Column-Medial Lemniscus (DCML) Pathway:
  • Senses: Fine touch, vibration, proprioception.
  • Fibers: Aβ (fast). Decussates: Medulla.

• Anterolateral System (ALS) / Spinothalamic Tract:
  • Senses: Pain, temperature, crude touch.
  • Fibers: Aδ & C (slower). Decussates: Spinal cord.

⭐ Syringomyelia, a central cord syndrome, classically damages crossing fibers in the anterior white commissure, causing bilateral loss of pain & temperature at affected levels (cape-like distribution).

Cortical Processing - Brain's Touch Map

• Primary Somatosensory Cortex (S1):
  • Brodmann areas 3, 1, 2 (postcentral gyrus).
  • Receives thalamic input (VPL/VPM nuclei).
• Sensory Homunculus (Somatotopy):
  • Distorted cortical map; area reflects receptor density.
  • Large representation: Lips, hands, face.
  • Small representation: Trunk, legs.
• Columnar Organization:
  • Vertical columns in S1; each responds to specific modality & location.
• Secondary Somatosensory Cortex (S2):
  • Location: Superior bank, lateral sulcus.
  • Input from S1; role in tactile learning & memory.
• Posterior Parietal Cortex (Areas 5, 7):
  • Integrates somatosensory with other senses (e.g., visual).
  • Spatial orientation, body image, attention.

  ⭐ Lesions in posterior parietal cortex (Areas 5, 7) can cause contralateral neglect syndrome or astereognosis (tactile agnosia).

• Cortical Plasticity: Maps can reorganize after injury or with experience (e.g., phantom limb).

Pain & Temperature - Ouch & Hot Spots

• Nociceptors (Pain): Free nerve endings.
  • Aδ fibers: Myelinated; fast, sharp, localized pain (Glutamate).
  • C fibers: Unmyelinated; slow, dull, burning, diffuse pain (Substance P, CGRP).
• Thermoreceptors: Free nerve endings.
  • Cold (Aδ, C): Peak 20-35°C. Paradoxical cold >45°C.
  • Warm (C): Peak 30-45°C. Pain >45°C.
• Pathway: Anterolateral System (Spinothalamic Tract).
• Gate Control: Aβ (touch) fibers activate inhibitory interneurons (SG), reducing Aδ/C pain transmission.

• Pain Modulation:
  • Endogenous opioids: Enkephalins, endorphins.
  • Descending inhibition: PAG → Raphe (5-HT), Locus Coeruleus (NE) → Dorsal Horn.
• 📌 Mnemonic: Aδ = Alert (fast), C = Chronic (slow).

⭐ Allodynia: Pain from non-painful stimuli (e.g., light touch). Common in neuropathic pain.

High‑Yield Points - ⚡ Biggest Takeaways

• DCML pathway carries fine touch, vibration, proprioception; decussates in medulla.
• Anterolateral system transmits pain, temperature, crude touch; decussates in spinal cord.
• Key mechanoreceptors: Meissner's (light touch), Pacinian (vibration), Merkel's (pressure), Ruffini's (stretch).
• Brown-Séquard syndrome: ipsilateral DCML loss, contralateral ALS loss below lesion.
• Thalamic relay: VPL nucleus for body, VPM nucleus for face.
• Dermatomes (e.g., T4 nipple, T10 umbilicus, L4 knee) aid lesion localization.
• Gate control theory modulates pain at spinal cord; basis for TENS therapy.