The Sense of Taste
Taste is the ability to respond to dissolved molecules (as contrasted with the sense of smell which detects airborne molecules).
Humans detect taste with taste receptor cells. From one to dozens of these (depending on the taste) are clustered in taste buds. 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.
Each taste receptor cell
- has receptor molecules on its apical surface;
- is connected to a sensory neuron in which it can generate action potentials to be carried back to the brain.
The traditional view is that there are only four primary taste sensations:
However, there is evidence for a fifth: response to salts of glutamic acid - like monosodium glutamate (MSG). This taste has been given the name umami. Monosodium glutamate is a flavor enhancer used in many processed foods and in many Asian dishes.
Salty and sour
Salty and sour tastes seem to depend on receptor molecules that are themselves - or are closely associated with - ligand-gated channels located at the apical surface of the receptor cells. These channels are gated by external ligands:
- sodium ions (Na+) for salty and
- protons (H+) for sour
.
Salty
With salty substances (e.g., table salt, NaCl), the sodium ions (Na+) enter directly into the cell reducing its polarization until threshold is reached and an action potential generated.
Sour
In the case of sour substances (acids), the protons (H+) liberated by the acid seems to block potassium channels thus interrupting the normal outflow of K+ that creates the resting potential of the cell. The resting potential of the cell is reduced and if this reaches threshold, an action potential is generated.
Sweet and bitter
The detection of sweet and bitter substances seems to depend on transmembrane proteins in the taste receptor cells that are themselves - or are closely associated with - ligand gated channels at the apical surface. These are gated by internal ligands.
Sweet
Sweet substances (like table sugar - sucrose) appear to bind to transmembrane receptors at the cell surface. This triggers a cascade of intracellular reactions leading to the
- activation of adenylyl cyclase
- formation of cyclic AMP (cAMP)
- the closing of K+ channels that leads to depolarization of the cell.
The mechanism is similar to that used by our odor receptors.
Bitter
The binding of substances with a bitter taste (e.g., urea, quinine sulfate) also triggers a cascade of intracellular reactions leading to the generation of action potentials. One of the molecules in this cascade is gustducin, which is similar in its structure and action to the transducin that plays such an essential role in rod vision.
22 June 1999
