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The Somatic Sensory System Chapter 12 Friday, November 7, 2003 Somatic Sensation Enables us to know what our body parts are doing. Three kinds of receptors: Touch -- mechanoreceptors Pain -- nociceptors Temperature -- thermoreceptors Mechanoreceptors

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the somatic sensory system

The Somatic Sensory System

Chapter 12

Friday, November 7, 2003

somatic sensation
Somatic Sensation
  • Enables us to know what our body parts are doing.
  • Three kinds of receptors:
    • Touch -- mechanoreceptors
    • Pain -- nociceptors
    • Temperature -- thermoreceptors
mechanoreceptors
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.
two point discrimination
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.
sensory pathways
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)
importance of axon diameter
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.
cortical somatopy
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.
cortical plasticity
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.
posterior parietal cortex
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.
nociceptors
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
hyperalgesia
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.
cortical pain pathways
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.
regulation of pain
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.
opioids
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.
thermoreceptors
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.