Thermal Sensation capsaicin

Chemesthesis. The term chemesthesis has been introduced to classify thermal and painful sensations experienced in the mouth (26). Chemesthesis refers to a chemical sensibility (mouthfeel) in which certain chemicals direcdy activate nerve fibers at the level of the basal membrane in the mouth. The sensations are analogous to similar effects at the skin surface where there is a close anatomical and functional relationship. Sensations include the "hot" of capsaicin and piperine, which are active components of chili and pepper, the coolness of menthol and the irritation of chemicals such as salt at high concentrations [FIGURE 4]. Some of the descriptive terms used to make qualitative distinctions in food sensations include pungency, freshness, tingling, burning and sharpness. 

Preexposure to menthol and capsaicin can desensitize the response to other stimuli, i.e. the thermal response to a warm solution or the irritation upon a second application of capsaicin (26,27). After capsaicin desensitization, the nerves no longer respond to burning or stinging but still give sensations of numbness or warmth. This effect is most noticeable upon further stimulation with capsaicin and less for the irritants, cinnamic aldehyde and NaCl. This sensitization and the fact that these sensations build in intensity on repeat application differentiate them from the "basic tastes", which adapt to the sensation on repeated application. 

Of these receptor sites, it is the distinct subsets of thermoreceptors in combination with nociceptors that give the sensations of heat and cooling by chemical stimulus in the mouth. In mammals it is proposed that a set of ion channels, called transient receptor potential (TRP) channels, are the primary molecular transducers of thermal stimuli. One such molecular transducer is the vanilliod receptor (VRl) channel, which is an ion-gated channel that is activated by temperatures above 43°C and by chemical irritants, such as, capsaicin and acidic pH [34]. Vanilliod receptor. 

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