SOMATOSENSORY SYSTEM: Basic Principles

Before going on examine the organisation of the somatosensory system there are a few basic principles that we need to cover (or in some instances revise).

A. Transduction

Transduction is the mechanism by which stimulation of a peripheral tissue induces action potentials in the primary sensory neurone (receptor).

Although the molecular mechanism of transduction for some stimuli has yet to be established, in most cases the stimulus appears to evoke a depolarising graded potential (known as a generator potential) in the primary sensory neurone.

If the generator potential is large enough to reach threshold then action potentials are produced in the primary sensory neurone.

The generator potential appears to involve the opening of stretch-gated ion channels for mechanical stimuli and ligand-gated ion channels when chemical stimuli are involved. More recently some temperature-gated ion channels have been identified that appear to be responsible for the generator potentials associated with changes in temperature.

B. Frequency Encoding

Like the rest of the nervous system, the somatosensory system uses the principle of frequency coding to communicate the size of peripheral stimuli.

So a small stimulus produces a low frequency (i.e. a small number of action potentials per second) response in the primary sensory neurone.

A higher intensity stimulus produces a higher frequency response.

These different frequency responses are relayed through the remaining two neurones in the pathway and are interpreted by the cerebral cortex as stimuli of differing intensities.

C. Receptive Field

If you record the membrane potential of a primary sensory neurone and then stimulate the peripheral tissue that it innervates you will eventually locate the region of that tissue that produces action potentials in that neurone.

This region is referred to as the neurone's receptive field.

Using your cursor scan the peripheral tissue opposite and see of you can identify the receptive fields of three different neurones.

The receptive field of a neurone is quite closely related to the extent of the axon terminals of the primary sensory neurones. Interestingly however the size of receptive fields varies quite significantly in different parts of the body.

D. Innervation Density

The extent to which a particular peripheral tissue is able to detect stimuli is directly related to the number of neurones that innervate it. The number of neurones that innervate a particular unit area is known as innervation density.

The higher the innervation density the more neurones terminate in each cm² of that tissue.

The innervation density of tissues varies quite significantly. For example there may be over 140 pressure receptors in each cm² of skin at the tips of our fingers and less than 1 receptor per cm² in the skin of the back.

We will investigate innervation density in the somatosensory system practical.

E. Adaptation

The term adaptation refers to the way in which a primary sensory neurone responds to a sustained stimulus.

In response to a sustained stimulus (e.g. skin deformation) some neurones show little change in their action potential frequency until the stimulus is removed. These types of neurones exhibit very little adaptation so are said to be are said to be slowly adapting.

Other neurones stop responding after a few seconds of a sustained response and are said to be rapidly adapting.