Common Chemical Sense (Chemesthesis)

In 1912, G. H. Parker described the sensory system responsible for detecting chemical irritants as “the common chemical sense.” Although Parker noted that the common chemical sense was mediated by irritant-detecting free nerve endings resembling pain receptors, he concluded that it was a separate sense with its own set of receptors that produced distinct sensation qualities different from taste and smell. Free nerve endings are axons that do not terminate in specialized structures and are devoid of myelin (fatty insulation around axons). Today, we know that the nerves that respond to chemical irritants do not constitute a separate, independent sense, as Parker hypothesized. Rather, they are part of the general somatic sensory system: a subset of pain and temperature sensitive fibers that can be found throughout the skin and mucosal membranes of the nose, mouth, respiratory tract, eye, and anal and genital orifices. Recently, the term chemesthesis has been used to describe the sensations elicited by the chemical stimulation of free nerve endings. Chemesthesis is thus analogous to somesthesis, which describes sensations produced by mechanical stimulation of the general somatic sensory system. We stimulate our chemesthetic sense every time we eat chili peppers, sniff ammonia, or cut onions. These same nerve endings respond to [Page 276]other environmental irritants that may affect our health and determine the food we eat and the personal products we buy. This entry describes trigeminal chemoreception, stimulation of nerve endings, solitary chemoreceptor cells, and reversal of aversion.