The Nervous System: Sensory Systems

Somatosensory Receptors

  • Mechanoreceptors: Respond to mechanical stimuli (touch, pressure, vibration).
    • Rapidly adapting: Pacinian corpuscles, Meissner's corpuscles, Hair follicle receptors.
    • Slowly adapting: Free nerve endings, Merkel's disks, Ruffini's endings.
  • Thermoreceptors: Respond to temperature changes.
    • Utilize Transient Receptor Potential (TRP) ion channels; show rapid adaptation.
    • Warm receptors: Free nerve endings, respond to 30-43^\circ \text{C}. Frequency increases with temperature increase.
    • Cold receptors: Respond to 20-35^\circ \text{C}. Frequency increases with temperature decrease. Paradoxical cold at 45^\circ \text{C}.
  • Nociceptors: Respond to noxious stimuli (pain).
    • Free nerve endings, carried by A-delta or C fibers.
    • Classes: mechanical, thermal (>44^\circ \text{C}), polymodal (mechanical, extreme cold/heat, chemicals).
    • Activated by chemicals from damaged tissue: histamine, bradykinin, prostaglandins.

Somatosensory Cortex

  • Organized into columns, each processing one sensory modality.
  • Topographically oriented, representing body regions (sensory homunculus).
  • Area devoted to a body region is proportional to its sensitivity (acuity), not its physical size.

Somatosensory Pathways

  • Dorsal Column-Medial Lemniscal Pathway:
    • Transmits fine touch, pressure, and proprioception.
    • First-order neurons synapse in dorsal column nuclei of medulla.
    • Decussation occurs in the medulla.
    • Second-order neurons ascend via medial lemniscus to thalamus.
  • Spinothalamic Tract (Lateral):
    • Transmits pain and temperature sensations.
    • First-order neurons synapse in the dorsal horn of the spinal cord.
    • Decussation occurs in the spinal cord.
    • Second-order neurons ascend to the thalamus.

Pain Perception

  • Sensation produced by actual or potential tissue damage; elicits autonomic, emotional, and reflexive responses.
  • Fast Pain: Carried by A-delta fibers; sharp, pricking, easily localized, velocity 12-30 \text{ m/sec} (myelinated).
  • Slow Pain: Carried by C fibers; dull, aching, poorly localized, velocity 0.2-1.3 \text{ m/sec} (unmyelinated).
  • Chemical Activators: Potassium, histamine, prostaglandins, bradykinin, serotonin.
  • Pain Pathways:
    • First-order neurons (A-delta or C fibers) terminate in dorsal horn; use neurotransmitters like Substance P, glutamate, CGRP.
    • Specific pathway: Spinothalamic tract.
    • Nonspecific pathway: To reticular formation, hypothalamus, limbic system.
  • Visceral Pain: Originates in internal organs.
  • Referred Pain: Visceral pain perceived on the body surface due to shared pathways.
  • Modulation of Pain:
    • Gate-Control Theory: Large diameter (A-beta) fibers can inhibit pain transmission, e.g., rubbing a sore spot or Transcutaneous Electrical Stimulation (TENS).
    • Endogenous Analgesia: Involves periaqueductal gray matter, raphe magnus, and reticular formation.
    • Endogenous Opioids: Enkephalin blocks Substance P release from first-order neurons and acts as competitive inhibitor on second-order neurons.
  • Hyperalgesia: Enhanced pain sensitivity, often associated with cytokines.
  • Phantom Limb Pain: Activation of central pain pathways for a missing limb, possibly due to convergent stimulation.