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 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 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 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 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 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 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 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 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 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 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 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 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 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.