The Somatic Sensory System

1. The Somatic Sensory System Chapter 12 Friday, November 7, 2003

2. Somatic Sensation • Enables us to know what our body parts are doing. • Three kinds of receptors: • Touch -- mechanoreceptors • Pain -- nociceptors • Temperature -- thermoreceptors

3. Mechanoreceptors • Pacinian corpuscle – quick responding • Meissner’s corpuscle – quick responding • Merkel’s disks – slow adapting • Ruffini’s endings – slow adapting • Hairs – stretches, bends, flattens nearby nerve endings.

4. Two-Point Discrimination • Whether a stimulus feels like one sensation or two distinct sensations depends on the size of the receptive fields of the sensory receptors. • Different areas of the body have sensory receptors with different sized receptive fields. • Smaller receptive fields result in greater sensitivity. • Fingers are more sensitive than backs.

5. Sensory Pathways • Sensory receptors synapse on dorsal root ganglia in the spinal cord. • Pathways go up the spinal cord to: • Brain Stem • Medulla – decussation occurs here • Thalamus (VP nucleus) • Primary somatosensory cortex (S1)

6. Importance of Axon Diameter • Different types of sensory information is carried by axons of different diameters. • Sensory nerves from muscles have largest axons and send fastest messages. • Mechanoreceptors of the skin are second fastest and have medium-large axon diameters. • Pain & temperature -- smaller myelinated axons. • Some pain, temperature, itch axons are unmyelinated and very small diameter.

7. Cortical Somatopy • Areas of the body map onto the sensory cortex so that the relations among body parts are maintained in the brain. • Separate kinds of sensory receptors (e.g., slow adapting vs fast adapting) have distinct alternating locations in the sensory cortex. • The amount of cortex devoted to an area of the body varies with sensory input.

8. Cortical Plasticity • With changes in sensory experience, areas of the sensory cortex can change their mappings. • When a limb is lost, the area of the brain dedicated to that limb’s sensations is taken over by other parts of the body. • Phantom limb syndrome may result from incursions into brain regions previously devoted to a missing limb.

9. Posterior Parietal Cortex • Sensory information is interpreted in the posterior parietal cortex to form an overall understanding of what the body is doing. • Astereoagnosia – inability to interpret sensory input using touch, inability to recognize objects by feeling them. • Neglect syndrome – a part of the body or a part of the world is ignored, denied, suppressed.

10. Nociceptors • Detect harmful stimuli that cause a risk of damage to the body. • Pain is the feeling associated with the sensory process. • Nociceptors trigger pain. • Pain occurs in the cortex, not the nociceptors. • Specialized for different types of harm: polymodal, thermal, chemical

11. Hyperalgesia • Already damaged areas show an increased sensitivity to stimulation of sensory receptors. • Substances released when the skin is damaged appear to modulate the excitability of nociceptors. • Prostaglandin – aspirin reduces it. • Cross-talk between touch and pain pathways also contributes to hyperalgesia.

12. Cortical Pain Pathways • Regulation of pain is complex because it can be affected at multiple locations and pathways. • Subjective experience of pain is affected by concurrent stimulation and also by behavioral context. • Different aproaches to pain management are being developed.

13. Regulation of Pain • Simultaneous activity of low-threshold mechanoreceptors reduces pain. • Rubbing the area around an injury. • Gate theory of pain – inhibition at dorsal horn • Descending regulation – emotion, stress or stoic determination can override or suppress pain. • Periaqueductal gray matter (PAG) involved.

14. Opioids • Opioid receptors respond to endorphins (morphine-like substances) that reduce pain. • Naloxone blocks opioid receptors and also blocks analgesic effects. • Supports the importance of PAG to pain • Opioids block transmission of pain signals from spinal cord and brain stem.

15. Thermoreceptors • Warm receptors detect temperatures within the higher safe range. • Cold receptors detect temperatures at the lower safe range. • Nociceptors detect damaging temperatures. • Most responsive to changes in temperature.