Chap 3

Study Guide: Somatosensory System (Chapter 3)

1. Overview of the Somatosensory System

   - Main Topics: Touch, thermal sensations, proprioception, kinesthesis, and pain.

   - Key Concepts:

     - Multimodality: Integration of various senses.

     - Transduction: Conversion of physical stimuli into neural signals.

     - Receptive Fields: Areas that affect neuron activity in response to stimuli.

     - Spatial & Temporal Resolution: Precision in detecting the location (spatial) and timing (temporal) of stimuli.

2. Touch

   - Stimuli: Includes pressure, vibration, temperature, and combined sensations (e.g., smoothness).

   - Mechanoreceptors:

     - Encapsulated Receptors: Meissner, Pacinian, and Ruffini corpuscles.

     - Accessory-structure-associated Receptors: Merkel discs and root hair plexuses.

     - Free Nerve Endings: Respond to pain (nociceptors), heat, and cold (thermoreceptors).

3. Mechanoreceptors and Skin Structure

   - Layers of Skin:

     - Epidermis: Outer layer.

     - Dermis: Contains mechanoreceptors.

   - Types of Skin: Hairy and hairless skin, with different tactile properties.

4. Sensory Neurons (DRG Neurons)

   - Types of DRG Neurons:

     - Aα, Aβ, Aδ fibers: Carry signals from mechanoreceptors.

     - C fibers: Slow, carry pain and warmth signals.

   - Adaptation: Neurons adapt to sustained stimuli, classified as Fast Adapting (FA) and Slow Adapting (SA).

5. Receptive Fields and Resolution

   - Receptive Fields (RFs): Areas affecting neuron firing.

   - Centre-surround RFs: Have excitatory centers and inhibitory surrounds (or vice versa).

   - Spatial Resolution: Determined by the density of receptors in the skin.

   - Temporal Resolution: Determined by how quickly neurons respond to stimuli.

6. Neural Pathways

   - Dorsal Column-Medial Lemniscal Pathway: Fast conduction of kinesthetic and tactile information.

   - Anterolateral Pathway: Slow conduction, carries temperature and pain information.

7. Somatosensory Cortex (S1)

   - Somatotopic Organization: Body parts are represented in the cortex according to their receptor density, not size.

   - Subareas of S1: Process different types of stimuli (tactile, proprioceptive).

   - Neural Plasticity: The cortex can reorganize in response to experience, as seen in musicians with focal dystonia.

8. Perceptual Aspects of Touch

   - Tactile Acuity: The ability to locate touch precisely, measured with two-point thresholds or grating acuity.

   - Vibration Sensation: Important for detecting motion across the skin, involving Meissner and Pacinian corpuscles.

   - Texture Perception: Explained by the Duplex Theory (spatial and temporal cues).

9. Pain Perception

   - Types of Pain:

     - Nociceptive Pain: Caused by intense stimuli (heat, pressure, chemicals).

     - Inflammatory Pain: Results from tissue damage.

     - Neuropathic Pain: Caused by damage to the nervous system (e.g., carpal tunnel syndrome).

   - The Pain Matrix: Involves cortical and subcortical areas like the anterior cingulate cortex and somatosensory areas.

10. Cognitive Effects on Pain

   - Expectations and Attention: Pain can be reduced through distraction or changing expectations.

   - Gate Control Theory (GCT): Suggests pain can be "gated off" at the spinal level by other inputs or top-down processes.

   - Opioids: Both endogenous (endorphins) and exogenous (morphine) can block pain.

11. Proprioception and Kinesthesia

   - Proprioceptors: Detect body position and movement.

     - Muscle Spindle Receptors: Detect muscle stretch.

     - Golgi Tendon Organs: Detect muscle tension.

   - Corollary Discharge Signals: Copies of motor signals that help with error checking and external force compensation.

12. Haptic Perception

   - Active Touch: Involves the use of hands to explore objects.

   - Exploratory Procedures (EPs): Include lateral motion, pressure, enclosure, and contour following.