The Sense of Taste

Taste is the ability to respond to dissolved molecules and ions called tastants.

Humans detect taste with taste receptor cells. These are clustered in taste buds and scattered in other areas of the body. Each taste bud has a pore that opens out to the surface of the tongue enabling molecules and ions taken into the mouth to reach the receptor cells inside.

There are five primary taste sensations:

Properties of the taste system.

Salty

In mice, perhaps humans, the receptor for table salt (NaCl) is an ion channel that allows sodium ions (Na+) to enter directly into the cell depolarizing it and triggering action potentials in a nearby sensory neuron.

In lab animals, and perhaps in humans, the hormone aldosterone increases the number of these salt receptors. This makes good biological sense:

Sour

Sour receptors detect the protons (H+) liberated by sour substances (acids). This closes transmembrane K+ channels which leads to depolarization of the cell [Link], and the release of the neurotransmitter serotonin into its synapse with a sensory neuron.

Sweet

Sweet substances (like table sugar — sucrose) bind to G-protein-coupled receptors (GPCRs) at the cell surface.

The hormone leptin inhibits sweet cells by opening their K+ channels. This hyperpolarizes the cell making the generation of action potentials more difficult. Could leptin, which is secreted by fat cells, be a signal to cut down on sweets?

Bitter

The binding of substances with a bitter taste, e.g., quinine, phenylthiocarbamide [PTC], also takes place on G-protein-coupled receptors that are coupled to gustducin and the signaling cascade is the same as for sweet (and umami).

Humans have genes encoding 25 different bitter receptors ("T2Rs"), and each taste cell responsive to bitter expresses a number (4–11) of these genes. (This is in sharp contrast to the system in olfaction where a single odor-detecting cell expresses only a single type of odor receptor.)

Despite this — and still unexplained — a single taste cell seems to respond to certain bitter-tasting molecules in preference to others.

The sensation of taste — like all sensations — resides in the brain. Transgenic mice that

So it is the activation of hard-wired neurons that determines the sensation of taste, not the molecules nor the receptors themselves.

Umami

Umami is the response to salts of glutamic acid — like monosodium glutamate (MSG) a flavor enhancer used in many processed foods and in many Asian dishes. Processed meats and cheeses (proteins) also contain glutamate.

The binding of amino acids, including glutamic acid, takes place on G-protein-coupled receptors that are coupled to heterodimers of the protein subunits T1R1 and T1R3. The signaling cascade that follows is the same as it is for sweet and bitter.

Taste Receptors in Other Locations

Taste receptors have been found in several other places in the body. Examples: So the function of "taste" receptors appears to be the detection of chemicals in the environment — a broader function than simply taste.

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16 March 2013