Thermoreception refers specifically to our ability to detect small increases or decreases in environmental temperature by the direct effect that these changes have on our skin. Humans are particularly sensitive to changes in skin temperature with psychophysical studies suggesting that we can detect shifts in skin temperature as small as 0.01 oC.
You will recall that there are two specific sensations induced by low intensity (i.e. non-painful) thermal stimuli and these are what we refer to as cold and warm. Cold and warm sensations are enabled by the presence of two distinct classes of temperature-sensitive primary sensory neurones known as thermoreceptors.
A. Thermoreceptors(i) Cold Receptors.This class of primary sensory neurone is spontaneously active at normal environmental skin temperatures and shows a fairly linear increase in action potential frequency as the skin temperature decreases (see opposite). These neurones typically have small diameter myelinated axons and consequently conduct action potentials in the range of 12 - 30 m.sec-1. The functional properties of these neurones clearly indicates that they are able to reliably encode a decline in skin temperature (i.e. skin cooling). Unlike low threshold mechanoreceptors the peripheral endings of these neurones have no specialised receptor endings. Instead the axons simply terminate blindly amongst the cells of the skin. These types of peripheral endings are known as free nerve endings to reflect their simplicity. |
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(ii) Warm ReceptorsThe second class of thermoreceptors present in human skin are also spontaneously active but their action potential frequency increases as skin temperature increases (see opposite). Warm receptors have small diameter unmyelinated axons so conduction action potentials fairly slowly (0.5 - 2.5 m.sec-1). Like cold receptors these neurones have free nerve endings in the skin. Clearly these neurones are ideally suited to encode information about increases in skin temperature in the normal (i.e non-painful) range. |
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B. Thermoreception Projection PathwayAs we have seen above, there are two classes of primary sensory neurones that encode information about low threshold changes in skin temperature. In this section we will see how this information reaches the cerebral cortex. (i) Primary Sensory NeuronesThe small diameter axons of thermoreceptors project through peripheral nerves and 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 NeuronesThe second-order neurones of the thermoreceptive 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 (opposite) side of the spinal cord. Because of its anatomical position midway between the lateral and ventral white matter this region is known anatomically as the ventrolateral funiculus. Physiologically these neurones form part of a projection pathway known as the spinothalamic tract because the axon terminals of these second-order neurones project out of the spinal cord through the brainstem and terminate with the ventrobasal complex of the thalamus where they synapse with the third-order neurones. (iii) Third-order NeuronesThe cell bodies of the third-order neurones of the thermoreceptive 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 skin cooling or warming occurs. |
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Note that just like mechanoreception, the thermoreceptive pathway crosses the midline. Consequently thermal stimuli reach consciousness in the parietal lobe on the opposite side of the body from where the stimulus occurs. The major difference is that in the thermoreceptive pathway the pathway crosses the midline at the level of the spinal cord.