Lecture 06 Somatosensory System

The Somatosensory System

Goals

  • Understand the organization and function of sensory systems, with a focus on the somatosensory system.
  • Explore receptor types, pathways, and cortical representations of somatosensory input.
  • Discuss plasticity in somatosensory cortical maps and related phenomena.

General Principles of Sensory Systems

  • Purpose: Translate external stimuli into neural signals for adaptive behavior.
  • Common Features:
    • Receptors: Transduce specific modalities (e.g., mechanoreceptors, photoreceptors).
    • Thalamus: Major relay center for sensory information, with modality-specific nuclei.
    • Primary Sensory Cortex: Receives sensory input in a topographic map, e.g., somatosensory homunculus.
    • Feature Maps: Organized representation of sensory input (e.g., somatotopic in S1, retinotopic in V1).

The Somatosensory System

Submodalities of Somatosensation

  1. Exteroception:
    • Sense of the external world.
    • Includes touch, vibration, pain, and temperature.
  2. Proprioception:
    • Awareness of body position and movement.
    • Derived from muscle spindles and Golgi tendon organs.
  3. Interoception:
    • Sense of internal organs.
    • Often unconscious but can be brought to awareness (e.g., bladder fullness).

Receptor Types

Tactile Mechanoreceptors

  • Meissner Corpuscles: Superficial, rapidly adapting, sensitive to light touch.
  • Merkel Discs: Superficial, slowly adapting, detect texture and pressure.
  • Pacinian Corpuscles: Deep, rapidly adapting, sensitive to vibration.
  • Ruffini Endings: Deep, slowly adapting, respond to skin stretch.

Proprioceptive Receptors

  • Muscle Spindles: Measure muscle stretch.
  • Golgi Tendon Organs: Measure tension in tendons.

Nociceptors (Free Nerve Endings)

  • Detect pain and temperature.
  • Respond to chemical signals released by damaged tissue (e.g., prostaglandins, bradykinin).

Neural Pathways

Dorsal Column–Medial Lemniscal System

  • Function: Processes touch, proprioception, and vibration.
  • Pathway:
    • First-order neurons ascend ipsilaterally in the dorsal columns.
    • Synapse in the medulla, where they decussate.
    • Second-order neurons project to the ventral posterior lateral (VPL) nucleus of the thalamus.
    • Third-order neurons project to the primary somatosensory cortex (S1).

Anterolateral System (Spinothalamic Tract)

  • Function: Processes pain and temperature.
  • Pathway:
    • First-order neurons synapse in the dorsal horn.
    • Second-order neurons decussate immediately and ascend contralaterally.
    • Project to the thalamus and then to S1.

Spinocerebellar Tract

  • Function: Carries unconscious proprioceptive information to the cerebellum.
  • Pathway:
    • Ascends ipsilaterally to the cerebellum.
    • Essential for motor coordination.

Cortical Representation

Primary Somatosensory Cortex (S1)

  • Located in the postcentral gyrus.
  • Contains a somatotopic map (sensory homunculus).
    • Medial regions represent lower limbs.
    • Lateral regions represent the face.
  • Divided into Brodmann areas:
    • 3a: Proprioception.
    • 3b: Tactile input.
    • 1: Texture processing.
    • 2: Size and shape processing.

Secondary Somatosensory Cortex (S2)

  • Receives input from S1 and thalamus.
  • Processes bilateral and integrative sensory information.

Sensory Plasticity

  • Cortical Reorganization:
    • Amputation or changes in sensory input can remap cortical areas.
    • Example: Phantom limb sensation linked to adjacent cortical representation.
  • Plasticity in Skill Acquisition:
    • Violinists exhibit expanded cortical representation for fingers used in playing.

Disorders of Somatosensation

  • Astereognosis: Inability to recognize objects by touch.
  • Tactile Agnosia: Impairment in identifying objects despite normal touch sensation.
  • Brown-Séquard Syndrome:
    • Damage to one side of the spinal cord.
    • Loss of ipsilateral touch/proprioception and contralateral pain/temperature sensation.

Experimental Insights

  • Penfield’s Studies: Electrical stimulation of somatosensory cortex revealed the sensory homunculus.
  • Ramachandran’s Mirror Box: Demonstrated neuroplasticity in phantom limb pain treatment.
  • Illusions and Confabulations:
    • Cutaneous rabbit illusion and funneling illusion reveal sensory integration mechanisms.

Summary

The somatosensory system is a model sensory system for understanding how external stimuli are represented in the brain. It demonstrates both the precision of neural coding in primary sensory areas and the adaptability of the brain in response to changes in input or experience.

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