Touch, Thermoception, and Nociception
The skin can convey many sensations, such as the biting cold of the wind, the comfortable pressure of a hand holding yours, or the irritating itch from a woolen scarf. The different types of information activate specific receptors that convert the stimulation of the skin to electrical nerve impulses, a process called transduction.
Types of receptors
There are three main groups of receptors in our skin:
- mechanoreceptors respond to mechanical stimuli, such as stroking, stretching, or vibration of the skin.
- thermoreceptors respond to cold or hot temperatures.
- chemoreceptors respond to certain types of chemicals either applied externally or released within the skin (such as histamine from an inflammation).
Receptors and Pain: Nociceptors
The experience of pain usually starts with the activation of nociceptors—receptors that fire specifically to potentially tissue-damaging stimuli. Most of the nociceptors are subtypes of either chemoreceptors or mechanoreceptors. When tissue is damaged or inflamed, certain chemical substances are released from the cells, and these substances activate the chemosensitive nociceptors. Mechanoreceptive nociceptors have a high threshold for activation—they respond to mechanical stimulation that is so intense it might damage the tissue. Sensory information collected from the receptors and free nerve endings travels up the spinal cord and is transmitted to regions of the medulla, thalamus, and ultimately to the somatosensory cortex, which is located in the postcentral gyrus of the parietal lobe.
A number of receptors are distributed throughout the skin to respond to various touch-related stimuli (Figure 1):
Low-threshold mechanoreceptors
- Meissner’s corpuscles respond to pressure and lower frequency vibrations
- Pacinian corpuscles detect transient pressure and higher frequency vibrations
- Merkel’s disks respond to light pressure
- Ruffini corpuscles detect stretch (Abraira & Ginty, 2013)
