SOMATOSENSORY SYSTEM 

A. Low Threshold Mechanoreceptors

(i) Pressure

Humans are very good at detecting very subtle differences in indentations of their skin. This is enabled by a class of primary sensory neurone that responds in a sustained fashion to skin indentation such that their action potential frequency is directly proportion to the magnitude of the stimulus.

In these neurones, small indentations produce a low frequency of action potentials and progressively larger indentations produce a higher frequency response.

Because of their slowly adapting response to a sustained stimulus these neurones are referred to as slowly adapting mechanoreceptors. Clearly these neurones are well suited for encoding the duration and magnitude of skin indentation.

Slowly adapting mechanoreceptors have specialised receptor endings associated with each of the branches of their axons in the skin. There are two major classes of receptor endings associated with slowly adapting mechanoreceptors. These are known as Merkel’s corpuscles and Ruffini’s endings after the microscopists who first identified them.

(ii) Touch

Psychophysical studies in humans have also revealed that we are able to detect very subtle differences in the rate of application of a skin indentation. This is because we have a population of primary sensory neurones that are selectively sensitive to the speed at which a stimulus is applied.

Whilst a relatively slowly applied indentation produces a low frequency of action potentials, this increases in a linear fashion as the rate of application increases. Note that these neurones do not continue to respond throughout the skin indentation and so are referred to as rapidly adapting mechanoreceptors.

Clearly these neurones can provide little information about the duration of a skin deformation (because action potentials cease before the stimulus is removed) but they are very well suited to encode the rate at which a mechanical stimulus is applied.

In glabrous (hairless) skin rapidly adapting mechanoreceptors have specialised receptor endings known as Meissner’s corpuscles. In hairy skin these neurones innervate hair follicles and are responsible for detecting the rate of hair movement.

(iii) Vibration

Most primates are very sensitive to vibration and can discriminate between very small differences in vibration frequency in the range of 50 to 300 Hz. Vibration is in effect a cyclical skin indentation and encoding of this stimulus is enabled by a population of very rapidly adapting mechanoreceptors.

These neurones adapt so quickly that only one action potential is produced in response to each skin indentation. Clearly these neurones can provide very little information about the magnitude, duration or rate of application of a skin indentation. However because each phase of the vibration produces a single action potential, these neurones can distinguish between different vibration frequencies quite readily.

Like the other classes of mechanoreceptors, very rapidly adapting mechanoreceptors have specialised receptor endings known as Pacinian corpuscles. These corpuscles can be up to 2 mm long and 1 mm wide and consist of concentric layers of connective tissue surrounding the axonal ending.

The pathways for all three types of neurones are essentially the same so we will consider them together.

(i) Primary Sensory Neurones

The large diameter myelinated axons of low threshold mechanoreceptors course through the peripheral nervous system and enter into the spinal cord through the dorsal roots. These axons take up a position in the white matter on the dorsal (posterior) aspect of the spinal cord just lateral to the midline. These structures run the length of the spinal cord and are known as the dorsal columns.

The axons of these mechanoreceptors project through the spinal cord in the dorsal columns and synapse with the second-order neurones on the posterior surface of the medulla oblongata in structures known as the dorsal column nuclei.

(ii) Second-order Neurones

The cell bodies of the second-order neurones of the mechanoreceptive pathway are located in the dorsal column nuclei and their axons cross the midline in a structure known as the medial leminiscus. These axons course through the core of the brain and terminate in the thalamus contralateral (on the opposite side) to their cell bodies.

The thalamus is part of the diencephalon and is large complex collection of sensory nuclei located lateral to the third ventricle. The second order neurones of the mechanoreceptive pathway synapse in a group of nuclei that are collectively referred to as the ventrobasal complex.

(iii) Third-order Neurones

The third-order neurones in the mechanoreceptive pathway have their cell bodies in the ventrobasal complex of the thalamus and axons that project up into the parietal lobe of the cerebral cortex. The major site of termination of these neurones is a thin strip of cortex immediately behind the central sulcus that is referred to anatomically as the post central gyrus of the parietal lobe.

Physiologically we refer to this region as somatosensory cortex and this is where the conscious perception of mechanoreception occurs.

One of the interesting things about the mechanoreceptive projection pathways is that there is extensive convergence of neurones at each of the synaptic relays. However this convergence is between neurones that have adjacent receptive fields in the periphery. Furthermore neurones with adjacent receptive fields terminate in adjacent parts of somatosensory cortex.

The end result of this convergence and organised pattern of terminations is that within somatosensory cortex there is a precise map of the body surface (see diagram on the left).

In effect this means that if you recorded from adjacent neurones in somatosensory cortex they would have adjacent receptive fields on the skin. This map is referred to as a homunculus and is shown in the diagram on the left.

One of the most striking things about the homunculus is that the representation of different body regions is not in proportion to their actual size. As can be seen in the diagram opposite, the torso occupies about the same area of cortex as the thumb.

It turns out that the amount of cortex related to a body part is completely determined by the innervation density of the skin. The higher the density of mechanoreceptors the more second- and third-order neurones and consequently the more cerebral cortex is required to interpret the information carried by these neurones.

If the actual size of our body parts was proportional to the surface area of the cerebral cortex involved in the conscious perception of mechanoreception then we would look quite a bit different.