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
• Meissners corpuscle quick responding
• Merkels disks slow adapting
• Ruffinis 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 limbs 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.