CHEMORECEPTORS
Chemoreceptors respond to chemical stimuli. Two very important chemoreceptive activities are the senses of taste (gustation) and smell (olfaction).

Taste
The taste bud, located on the tongue of humans, is the functional unit that permits us to discriminate between the tastes of sweet, sour, salty, and bitter. Each taste bud contains more than 100 taste receptor cells.

As the chemical substances that are dissolved in the saliva break down into molecules, they activate a signal transduction process. The transduction process for molecules perceived as sweet involves a G protein. Adenylyl cyclase activity is stimulated, raising cyclic AMP levels. This activates a protein kinase that phosphorylates and closes K⁺ channels, setting up a depolarizing receptor potential that synapses with the taste receptor cell.

Smell
Humans can distinguish at least seven core groups of odors: camphor, musk, floral, peppermint, ethereal, pungent, and putrid. Roughly 1000 genes code for 1000 different types of olfactory receptors. In humans, olfaction occurs in the olfactory epithelium, which contains about 100 million olfactory receptor cells. Receptor molecules bind with compounds dissolved in the mucus. The olfactory nerve—the first cranial nerve—transmits the information to the olfactory cortex in the limbic system, which is a part of the brain.