Neurophysiology 2 – Receptors & Somatosensory System Notes

Sensory Transduction

  • Definition: Process by which sensory receptors convert stimuli (e.g., heat, pressure) into electrical signals understood by the brain.
  • Importance: Critical for brain function; allows interpretation and reaction to environmental changes.

Afferent Signal Encoding

  • The properties of a skin stimulus are recorded through three questions:
    1. What type of stimulus? (e.g., hot vs. painful)
    2. Where is it located? (e.g., toe vs. ankle)
    3. What is its intensity and duration? (light vs. heavy)
  • Key aspects contributing to encoding:
    • Afferent type: Defines stimulus modality.
    • Afferent location: Relates to somatotopy (mapping of specific stimuli to specific body parts).
    • Afferent activity: Encodes intensity and duration through rate coding and population coding solutions.

Understanding Receptor Types and Functions

  • Stimuli initiate receptor potential:
    • Change in ionic permeability and thus, membrane potential.
    • If the receptor potential reaches a threshold, action potentials are generated, traveling to the central nervous system (CNS).
Adaptation to Stimuli
  • Definition: Gradual decrease in neuron response to continual stimulation.
  • Types:
    • Rapidly adapting (phasic): Good for signaling change (e.g., pacinian corpuscles denote movement).
    • Slowly adapting (tonic): Good for detecting constant intensity (e.g., joint receptors monitor ongoing pressure).

Receptive Fields

  • Definition: Area where stimulation activates a neuron.
    • Can expand over a larger region when receptors are sensitive, allowing activation by stimuli distant from the receptor location, indicating higher sensitivity levels.
  • Specificity: Each neuron’s receptive field should specify the type and intensity of the stimulus for accurate signaling.

Somatosensory Modalities and Their Receptors

  • Types of Fibres in Somatosensory System:
    • Aδ fibres: small, myelinated (5-30 m/s); responsible for thermal and pain sensing.
    • Aβ fibres: larger, myelinated (30-70 m/s); involved in touch (e.g., Meissner's and Pacinian corpuscles) and proprioception.
    • C fibres: unmyelinated (0.5-2 m/s); relay slow burning pain and itch sensations.

Temperature Encoding

  • Temperature Sensation: Encoded through two types of thermoreceptors - cold receptors (Aδ axons) and warm receptors (C axons).
  • Ambiguity in temperature perception can be resolved by using two types of fibres: both convey specific firing rates which determine the exact temperature encountered.

Pain Sensation and Nociceptors

  • Classifications of Pain:
    • Somatic pain: Fast, localized (Aδ fibres) and slow, poorly localized (C fibres).
    • Visceral pain: Dull, diffuse pain primarily from C fibres associated with internal organs.
  • Referred Pain: Phenomenon where pain sensation is perceived at a different location from the actual source due to shared neural pathways.

Central Pathways of Somatosensory Information

  • Dorsal Column/Medial-Lemniscal pathway: Tactile and proprioceptive information travels ipsilaterally before decussating at the medulla and entering the somatosensory cortex.
  • Spinothalamic pathway: Pain and temperature information decussate immediately to the contralateral side upon entering the spinal cord before projecting to the thalamus within the spinothalamic tract.

Effects of Spinal Damage on Sensory Perception

  • Damage (e.g., spinal hemi-lesion) can lead to specific losses in sensation:
    • Left-side damage may leave upper limb sensation unaffected while resulting in loss of touch/proprioception from the left leg and pain/temperature sensation from the right leg.
    • This differential impact occurs due to the distinct pathways taken by touch (dorsal column) and pain (spinothalamic tract).

Summary of Sensory Receptors:

  • Merkel Disks: Slowly adapting, superficial; detect fine touch.
  • Ruffini Endings: Slowly adapting, deep; respond to sustained pressure.
  • Meissner's Corpuscles: Rapidly adapting, superficial; sensitive to light touch.
  • Pacinian Corpuscles: Rapidly adapting, deep; react to vibration and pressure changes.