SOMATOSENSORY SYSTEM: Introduction

Any stimulus applied to the skin that is sufficiently intense to cause damage usually results in the sensation of pain. As was alluded to earlier in this lesson, there are two types of pain sensation; burning and sharp. Thermal and chemical stimuli produce a sensation that has a burning quality. High intensity mechanical stimuli initially produce pain that has a sharp quality, but this is followed a few seconds later by burning pain. Because of the different temporal aspects of pain produced by a mechanical injury we usually describe these as fast-sharp pain and slow-burning pain.

These two different pain sensations are the result of two distinct classes of pain signalling primary sensory neurones known as nociceptors.

A. Nociceptors

(i) High Threshold Mechanoreceptors

As its name suggests, this class of nociceptor is selectively activated by high intensity mechanical stimuli such as a pin-prick, laceration or pinch.

In fact any mechanical stimulus that has the potential to damage the skin will elicit action potentials in this class of neurone.

High threshold mechanoreceptors are NOT activated by either high temperatures or pain-producing chemicals.

Morphological analysis of this class of nociceptor has revealed that they don’t have specialised receptors (i.e. they have free nerve endings) and that they have small diameter myelinated axons. Because of this myelin sheath they conduct action potentials in the range of 12 - 30 m.sec^-1.

High threshold mechanoreceptors are responsible for the fast-sharp pain associated with a mechanical injury.

(ii) Polymodal Nociceptors

This class of nociceptor is activated by high intensity thermal, chemical and mechanical stimuli (hence the name ‘polymodal’). Like high threshold mechanoreceptors they are stimulated by high intensity mechanical stimuli (pin-prick, laceration or pinch). In addition they are activated by temperatures greater than 45 ^oC and a range of pain-producing chemicals (including acid, histamine, bradykinin and a whole host of irritants found in stinging plants and animals).

Polymodal nociceptors have small diameter unmyelinated axons (so they have conduction velocities in the range 0.5 - 2.5 m.sec^-1) and free nerve endings in the periphery.

Polymodal nociceptors are responsible for the burning pain associated with a thermal or chemical injury.

These neurones are also activated by high intensity mechanical stimuli but have conduction velocities that are much slower than high threshold mechanoreceptors. As a result, information about a mechanical injury encoded by these neurones reaches the central nervous system a lot slower than that encoded by high threshold mechanoreceptors. Consequently these neurones are responsible for the slow-burning pain associated with a mechanical injury.

B. Nociception Projection Pathway

As we have seen, in the skin there are two classes of primary sensory neurones that encode information about high intensity stimuli applied to the skin. In many respects the projection pathway for nociception is the same as that already described for thermoreception, except of course in for the types of stimuli that activate neurones in this pathway.

(i) Nociceptors

The small diameter axons of high threshold mechanoreceptors and polymodal nociceptors enter into the spinal cord through the dorsal roots. These axons enter into the grey matter of the dorsal (posterior) horn of the spinal cord where they form axodendritic synapses with the second-order neurones.

(ii) Second-order Neurones

The second-order neurones of the nociceptive pathway have their cell bodies with the dorsal (posterior) horn of the grey matter of the spinal cord. Their axons project down and across the midline underneath the central canal and enter into the white matter on the contralateral ventrolateral funiculus. Together with the neurones of the thermoreceptive pathway these neurones form the spinothalamic tract. The axon terminals of these second-order neurones project out of the spinal cord and terminate with the ventrobasal complex of the thalamus where they synapse with the third-order neurones.

(iii) Third-order Neurones

The cell bodies of the third-order neurones of the nociceptive pathway are located in the ventrobasal complex of the thalamus. These third-order neurones have axons that project up into parietal lobe of the cerebral cortex and terminate within the postcentral gyrus (i.e. somatosensory cortex) where the conscious perception of either sharp or burning pain occurs.