Somatosensory System

The somatosensory (soma = body) or "touch" sense codes information about the many different kinds of stimuli that people can detect from the skin, muscles, and joints. Because it codes so many different kinds of stimuli, the somatosensory sense is really a combination of several different sensory qualities or kinds of sensory experiences, including, among others, light touch, vibration, pressure, hair movement, joint position, warmth, cooling, and at least two different types of pain.

Each of these sensory qualities has one (or more) type of sensory receptor . Each of these receptors is selectively tuned to respond to one (or sometimes a few) of these kinds of stimuli. That is, each kind of receptor responds selectively to its own adequate stimuli.

Some receptors are very selective, responding only to vibration, only to change in pressure, only to warming, only to (potential) injury, etc., as adequate stimuli. Others are less selective, responding to, for example, light touch as well as warming and damaging stimuli, or some other combination of somatosensory stimuli. Each kind of receptor is part of its own kind of sensory (afferent) neuron, which in turn connect to its own kinds of neurons in the somatosensory (~touch) system in the brain. Experience of different sensory qualities appears to depend on activity in the different kinds of somatosensory neurons in the brain. Figure 1 illustrates this idea.

It shows four different kinds of stimuli activating four different sensory systems, each activated by its own adequate stimulus (there are several more kinds, which are not shown). These different kinds of neurons reach different, though closely related, places in the cerebral touch system. Sensory fibers connect to many other parts of the brain and spinal cord, but these areas appear to be less important for touch perception.

For a detailed description of the somatosensory system going to the cerebral cortex, click HERE . For a paper describing experiments on normal volunteers using fMRI imaging to show the organization of the somatosensory areas of the cerebral cortex, click HERE .

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