In addition to acting as a physical barrier that protects the human body from damage inflicted by bacterial, mechanical, thermal and chemical assaults, the skin is also our largest sense organ. It is of course very well positioned to detect stimuli that impinge on our bodies from the external environment. Consequently cutaneous sensation is sometimes referred to as exteroreception.
Psychophysical studies have established that humans are able to discriminate at least three major classes of sensation from stimulation of their skin. These sensations are classified according to the modality of the stimulus that activates them:
A. Low Intensity Mechanical Stimuli
The low intensity (i.e. non-painful) mechanical stimuli are responsible for three distinct sensations:
- pressure - the degree of skin indentation.
- touch - the rate at which a skin indentation is applied.
- vibration - the frequency of a vibratory stimulus.
These sensations are referred to collectively as mechanoreception.
B. Low Intensity Thermal Stimuli
The two sensations that can be produced by the small changes in skin temperature that occur as part of daily life are known as cold and warm. These are low intensity in the sense that they do not produce damage to the skin and are non-painful. Collectively these sensations are known as thermoreception.
C. High Intensity (Painful) Stimuli
Stimuli with sufficiently high intensity to produce damage to the skin produce the sensation of pain. It is well established that there are two distinct pain sensations and these can be clearly demonstrated by a mechanical injury such as that associated by a paper cut or pin prick.
Very shortly after a mechanical injury we experience a pain that has a 'sharp' quality. A few seconds later we experience a second pain sensation that is usually described as being 'burning'. We can also elicit the burning pain by high temperatures and by the application of pain-producing chemicals (such as acid).
So there are two distinct pain sensations (sharp and burning) that differ in their quality. Pain sensations are known collectively as nociception.
Each of these seven sensations is the result of activation of a specific class of primary sensory neurones and the transmission of information about these stimuli to the cerebral cortex through a dedicated projection pathway. In the remainder of this lesson we will examine the class of primary sensory neurone and projection pathway that is responsible for each of these classes of cutaneous sensation.