Two categories of chemical senses (gustation or taste and olfaction or smell) are important for organisms to respond appropriately to their environments. The sensation of taste detects environmental chemicals and may have initially helped organisms distinguish between new sources of food and potential poisons. There are five different tastes that humans recognize: saltiness, sourness, sweetness, bitterness, and umami (the meaty flavor of monosodium glutamate or parmesan cheese). The chemistry of stimuli is described here, as well as their interactions with proteins in the membranes of taste receptor cells. These initiate nervous signals traveling via the gustatory nucleus of the brain stem to the cerebral cortex where conscious recognition of taste occurs.
Humans have 2,000 to 5,000 taste buds found scattered over the surface of the tongue in small projections known as papillae (bumps). Each taste bud contains about 50 to 150 taste receptor cells that are relatively selective for the taste that they sense. Chemical stimuli interact with the membranes of taste receptor cells either by binding to membrane receptors (proteins with selective binding sites) or by directly changing the number of ions flowing across the membrane.
Ionic Taste Stimuli
Saltiness is sensed by taste receptor cells that respond primarily to sodium chloride. The proteins in the cell membranes involved in transforming the presence of salt into nervous signals are epithelial sodium channels that allow the sodium ions to enter the cells, initiate the release of chemical neurotransmitters , and stimulate adjacent gustatory afferent axons (nerve cells that carry taste information to the brain). Sourness (hydrochloric acid, citric acid, or acetic acid) is likewise sensed by taste receptor cells in ways that directly affect ion channels. Protons either enter via the epithelial sodium channels or block epithelial potassium channels to initiate the cellular response. The bitterness of quinine and calcium is also sensed by blocking potassium channels in taste receptor cell membranes.
Membrane Receptor Taste Stimuli
In contrast, other tastes including sweetness (as in sucrose, fructose, and artificial sweetener) are sensed by actually binding to specific membrane receptor proteins in taste receptor cells. The chemicals sensed as sweet bind to selective sites on a membrane receptor in a "lock-and-key" fashion (implying that only chemicals of a particular shape can fit in the binding site and initiate the response). Once the sweet chemical is bound, the membrane receptor initiates a series of chemical reactions inside the cell, leading eventually to a change in the flow of ions across the membrane and the release of neurotransmitter. Likewise, the bitterness of some chemicals is sensed by binding to other membrane receptors and then initiating a response. The taste of some amino acids is initiated by binding to a specific site on chemical-gated ion channels (channels that open when a chemical is bound) where the amino acid (the chemical) acts as the key. The umami taste of monosodium glutamate is sensed by binding to another type of membrane receptor (similar to the synaptic glutamate receptors of the brain) that allows ions to cross cell membranes.
Taste and smell are well-known chemical senses; however, the specific genes and proteins involved in some tastes have not yet been fully identified.
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