Transduction of taste
There are primarily five tastes like salty, sour, sweet, bitter, and umami. The salt taste is mainly due to the Na+ ions. So the salty food causes opening of the Na+ channels. The substance namely Amiloride inhibits the receptor channels and diminishes the salt sensation of the receptors.
The sour taste is due to the H+ ions in the food. The sour perception is done via the Na+ channels, which are permeable to H+ ions, but compete with them and so, it is done only in species which have less Na+ ions in their saliva. In other animals, it is by acid-sensing ion channels. It causes opening of the Na+ channels in response to the change in pH.
The sweet perception is by the binding of the sweet substance to the G-protein which activates G-protein gustducin, which signals the adenylate cyclase pathway. High sweet containing substances also use IP3-mediated signal transduction.
The umami is caused by L-glutamate and other amino acids present in the food. There are two different types of receptors. One resembles that of the glutamate receptor in the brain. When glutamate binds to the G-protein, it changes it conformation and releases phosphodiesterase enzyme that degrades cAMP to AMP and that causes increase in neurotransmitter release. The other receptor resembles that of the sweet perception.
The bitter perception is complex and it is estimated that at least 25 genes code for bitter-taste receptors in human beings.
Each taste receptor cell expresses more than one taste receptor protein. The taste receptors are not neurons instead they are epithelial cells which secrete neurotransmitter onto a primary afferent neuron. Single taste neurons synapse with more than one taste receptor cells. Olfaction and gustation work together closely and that is the reason why the taste perception is dependent on smell perception of an item.
