Somatosensory System Presentation Notes

Presentation Overview

Introduction to the Somatosensory System

The somatosensory system encompasses the senses of touch, temperature, pain, and body position, with a focus on the sensation of touch in this presentation.

Learning Objectives
  1. Properties of Touch Receptors
       - Explore different types of touch receptors present in the skin.
  2. Touch Sensitivity
       - Understand how heightened sensitivity to touch is achieved.
  3. Neural Pathways of Touch
       - Identify key neural pathways that are significant in the sensation of touch.
  4. Role of Cortical Areas
       - Understand how the somatosensory cortex and posterior parietal cortex are involved in touch sensation.
Suggested Reading
  • The neuroscience textbooks by Bayer and Kandell are recommended for additional reading on this topic.

Types of Skin and Receptors

Skin Types

  • Hairy Skin: Skin that contains hair follicles.
  • Glabrous Skin: Hairless skin, such as the palms and soles.

Major Touch Receptors

  1. Pacinian Corpuscle
       - Located deep in the dermis.
       - Highest receptor density found in the fingers.
       - Largest and most studied receptor.

  2. Ruffini Endings
       - Found in both hairy and glabrous skin.
       - Slightly smaller than Pacinian corpuscles.

  3. Meissner's Corpuscles
       - About one-tenth the size of Pacinian corpuscles.
       - Located in glabrous skin (fingertips).

  4. Merkel's Discs
       - Located in the outer skin layer.
       - Composed of a nerve terminal and a flattened Merkel cell.

Microneurography

Overview

  • Microneurography records from single sensory axons, primarily from the arm.
  • Involves inserting a microelectrode into a peripheral nerve and testing various skin sites.

Mechanoreceptor Identification

  • Capable of identifying mechanoreceptors and their receptive fields.
      - Meissner's Corpuscles & Merkel's Discs: Small receptive fields.
      - Pacinian Corpuscles & Ruffini Endings: Large receptive fields.

Mechanoreceptor Response Characteristics

Adaptation Rates

  • Mechanoreceptors show variations in their response to persistent stimuli, categorized as:
       - Rapidly Adapting: e.g., Meissner's and Pacinian corpuscles, stop firing quickly after stimulus onset.
       - Slowly Adapting: e.g., Merkel's discs and Ruffini endings, produce sustained responses during long stimuli.
Receptive Field Sizes
  • Functionally, mechanoreceptors may be defined by:
      - Rate of Adaptation: Rapid vs. slow.
      - Receptive Field Size: Small vs. large.

Mechanoreceptor Sensitivity to Frequency

Frequency Response

  • Different mechanoreceptors are sensitive to specific stimulation frequencies:
       - Pacinian Corpuscles: Most sensitive at high frequencies (200-300 Hz).
       - Meissner's Corpuscles: Most sensitive at lower frequencies (around 50 Hz).

Two-Point Discrimination

Definition

  • Two-point discrimination measures spatial acuity and identifies the minimum distance at which two stimuli are perceived as distinct.

Measurement Method

  • Utilizes a calibrated compass to measure distinctness.

Variability Across the Body

  • Highest resolution in fingertips; lowest on back and legs.
Factors for Increased Sensitivity in Fingertips
  1. Higher density of mechanoreceptors.
  2. Numerous receptor types with smaller receptive fields (e.g., Merkel’s discs).
  3. Greater brain tissue dedicated to sensory input from fingertips.

Sensory Axons and Their Properties

Axon Classification

  • Sensory axons vary widely in diameter associated with the type of sensory receptor:
       - A alpha: Largest
       - A beta: Second largest, responsible for touch.
       - A delta: Smaller diameter.
       - C Fibers: Smallest, unmyelinated; mediate pain, temperature, and itch.

Nerve Conduction Speeds

  • A beta axons can transmit at speeds of up to 75 m/s; C fibers conduct at roughly 0.5 - 1 m/s.

Spinal Cord and Sensory Information

Overview of Spinal Cord Structure

  • Encased in the bony vertebral column; consists of 30 segments:
      - Cervical (C1-C8)
      - Thoracic (T1-T12)
      - Lumbar (L1-L5)
      - Sacral (S1-S5)

Dermatomes

  • Each spinal segment receives sensory input from specific skin areas called dermatomes.
  • Dermatomes visually appear as bands, with some overlap in boundaries for redundancy.
Surgical Implications
  • To lose completely the sensation in one dermatome, three adjacent dorsal roots must be cut.

Touch Pathway: Dorsal Column Medial Lemniscal Pathway

Axon Pathway

  • A beta axons enter the dorsal horn, synapsing in a branch to generate reflexes; the other ascends to the brain.

Sensory Processing Overview

  1. Enters the spinal cord through the dorsal column.
  2. Axons synapse in the dorsal column nuclei, crossing midline.
  3. Ascend via the medial lemniscus to synapse in the thalamus.
  4. Thalamic neurons project to the primary somatosensory cortex.

Sensory Pathways for face and head

  • Distinct from the dorsal column medial lemniscal pathway.
Synaptic Processing
  • Information is modified at each synapse, often via projections to inhibitory interneurons, enhancing the contrast between activated neurons.

Somatosensory Cortex and Further Processing

Location and Function

  • Most somatosensory processing happens in the parietal lobe.
  • The primary somatosensory cortex occupies Brodmann area 3B, responding to sensory stimuli.

Other Areas Involved

  • Areas 3a (body position), 1 (texture), and 2 (size/shape).
  • Posterior Parietal Cortex: Areas 5 and 7 involved in perception, visual-motor integration, and attention.

Somatotopy and Cortical Representation

  • Sensory information from the thalamus mainly projects to layer four of the cortex.
  • Somatotopic maps correlate with the density of sensory input, producing a homunculus representation of the body on the cortex.
  • Higher density regions (e.g., face, hands) occupy larger cortical space.

Functional Implications of Damage

  • Injury to the posterior parietal cortex results in agnosia (inability to recognize objects) and may result in astereognosis (inability to recognize objects by touch).
  • Neglect syndrome may manifest, where parts of the body or environment are ignored.
Case Example
  • A patient with right posterior parietal cortex damage drew only the right side of objects, demonstrating neglect.

Conclusion

Ensure mastery of the key points presented above before proceeding to subsequent topics. This presentation covered the structure and function of the somatosensory system, focusing on the aspects of touch perception.