Two-Point Discrimination

  • Ability to perceive two separate stimuli as distinct.
  • Improved by:
    • Smaller receptive fields.
    • Specific receptor types (Merkel cells, Meissner corpuscles).
    • Higher innervation density in areas (e.g., fingertips).
    • Cortical representation in the somatosensory cortex (somatotopy).
  • Distribution:
    • High sensitivity: fingers, face (5-10 mm threshold).
    • Low sensitivity: back, thigh, upper arm (>40 mm threshold).

Dermatomes

  • Strips of skin covered by specific nerves entering the spinal cord.
  • Importance: Diagnosing spinal cord injuries or peripheral nerve damage based on loss of sensation in specific dermatomes.

Somatosensation Receptor Structure

  • Specialized peripheral endings of pseudounipolar cells, located in dorsal root ganglion.
  • Mechanical deformation > ion channels open > receptor potential > action potential if threshold met.

Adaptation of Receptors

  • Slowly Adapting Receptors: Continuous response to stimulus (Merkel cells, Ruffini corpuscles).
  • Rapidly Adapting Receptors: Signal changes in stimulation (Meissner corpuscles, Pacinian corpuscles).

Types of Somatosensory Receptors

  • Superficial Receptors (Merkel cells, Meissner corpuscles): Small receptive fields.
  • Deep Receptors (Ruffini corpuscles, Pacinian corpuscles): Larger receptive fields, less effective at two-point discrimination.

Somatosensory Pathway

  • Three-cell circuit:
    1. Pseudounipolar cells enter spinal cord, synapse in medulla.
    2. Cross midline in medulla > synapse in thalamus.
    3. Thalamus to primary somatosensory cortex.

Pain Fibers

  • Aδ fibers: Myelinated, fast (5-30 m/s), sharp pain sensation.
  • C fibers: Unmyelinated, slow (0.5-2 m/s), dull pain and itch sensations.

Nociception vs Mechanoreception

  • Nociceptive pathways cross midline immediately in spinal cord.
  • Touch pathways cross in medulla, leading to different clinical manifestations.

Auditory System Overview

  • Divided into outer, middle, and inner ear.
  • Outer Ear: Pinna and external auditory meatus (direct sound).
  • Middle Ear: Houses ossicles and controls sound transmission.
  • Inner Ear: Cochlea transforms mechanical signals to neural signals.

Cochlea Structure

  • Three cochlear components: scala vestibuli, scala tympani (both perilymph), and scala media (endolymph).
  • Basilar membrane: Tonotopically organized (stiff base for high frequencies, loose apex for low frequencies).

Hair Cell Function

  • Hair cell depolarization via deflection of stereocilia > neurotransmitter release changes with movement direction.

Vestibular System

  • Informs on head position and movement with otolith organs (linear motion) and semicircular canals (angular motion).
  • Tonic activity allows detection of bidirectional movement.

Visual System

  • Light as electromagnetic energy, photon properties (intensity, wavelength).
  • Eye layers: outer (sclera, cornea), middle (uveal tract), inner (retina).
  • Retina: phototransduction via rods (light intensity) and cones (color).