Somatosensation
Fall 2002

Soma = body, so somatosensation are the sensations coming from the skin. These include:

Touch carried by mechanoreceptors
Temperature carried by thermoreceptors
Pain carried by nociceptors Mechanoreception

There are 2 types of skin:

Hairy skin covers most of the body. Although hair can detect touch, this skin is not very sensitive.
Glabrous skin is located on the palms, feet, lips, and parts of the genitals. This skin is very sensitive to touch. Our goal is to map the pathway from the skin to the brain. The first step is to examine a map of the skin. The sensitivity of different parts of the skin can be determined by a two point discrimination task. That is, what is the distance between two points when they are first perceived as two points, not one. Results show that the fingertips are the most sensitive region of skin (Figure 12.7).
The fingertip is more sensitive than the back because there are more mechanoreceptors in the skin on the finger than in the skin of the back. A single mechanoreceptor in the back may have many branches to cover a region of skin. Any two points touching this region will be perceived as a single point.

The results of two point discrimination experiments suggest that more cortex is devoted to touch sensation coming from the fingertips than from the back. A cross section of the somatosensory cortex reveals this to be the case (see Figure 12.20).
 
 

The pathway from mechanoreceptors to the brain requires three neurons (Figure 12.15). 1) Mechanoreceptors are the longest neurons in body. The soma is located in the dorsal root ganglion and the axon runs from the skin to the dorsal column nucleus in the medulla. These axons, called A-beta fibers, are large and myelinated which makes them very fast (75 m/s; myelin). These axons enter the spinal cord through one of 31 spinal nerves coming from a specific region of the body called a dermatome (Figure 12.12), and travel up the spinal cord in the dorsal columns (Figure 12.14a).

2) The dorsal column nucleus is located in the medulla. These neurons send axons to the contralateral thalamus.

3) The thalamus relays touch messages to the somatosensory cortex which is located in the postcentral gyrus of the parietal lobe (Figure 12.19).
 

 Mechanoreception from the face follows a similar path:
Cranial nerve #5 (trigeminal nerve) from the face (nerves 5, 7, 9 from the neck and ear) projects to the the trigeminal nucleus in the medulla and then to the thalamus and somatosensory cortex (Figure 12.18).


The whole body is represented on the somatosensory cortex (called a homunculus; Figure 12.20). Electrical stimulation of this cortex will evoke sensations of touch on the opposite side of your body. All somatosensory information from the body projects to the opposite side of the cortex.
 

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