Type III Taste Cells
Taste buds are the sensory end organs of the gustatory system. They contain three basic types of cells—Type I, Type II, and Type III cells. These networks of cells allow for the detection of five distinct taste qualities: sweet, bitter, umami, salty, and sour. Type III cells are sensitive to sour and some salty tastes.
In comparison to other cells in the taste bud, Type III cells are less abundant and smaller in size. These cells are spindle shaped and extend from the base of the taste bud to the apical pore, where an opening in the tongue epithelium allows taste cells to access chemicals present in the oral cavity. Interestingly, Type III cells are the only taste cells to make traditional synapses with afferent nerve fibers. At these sites, the taste cell membrane appears thicker in electron microscope images, indicating an aggregation of proteins important in synaptic transmission. Vesicles (small membranous sacs containing signaling molecules known as neurotransmitters) are also present. Here, vesicles fuse to the outer cell membrane, allowing for the release of neurotransmitters that chemically signal the receiving nerve fiber.
Taste bud cells are continually turning over during the life of an organism. As taste cells are damaged and die, a population of renewing basal cells repopulates the taste bud with new taste receptor cells. Type III cells are the slowest to undergo this process—they may even take months to die off and renew.
Type III cells are sensitive to both sour and salty stimuli. While not neurons, Type III cells behave similarly to neurons. They too fire action potentials—the electrical signal that travels from one end of a neuron to the other—and release neurotransmitters when stimulated with sour and salty substances.
Acid (sour) stimuli cause action potentials and neurotransmitter release in Type III cells via two cooperative mechanisms: (1) acid sensing ion channels located in Type III cells at the apical pore, and (2) a lowering of pH inside the cell (Chang et al., 2010; Ye et al., 2015). Simply put, sour substances contain acids, which have a low pH and freely dissociating hydrogen ions (H+, protons). Type III cells express an ion channel (a protein in the cell membrane that forms a pore and lets certain ions through when open) that allows for the passage of free protons from the oral cavity into the cell. Electrically active cells are negatively charged inside the cell—the entrance of positively charged protons into Type III cells, then, disturbs the electrical balance between inside and out, causing a depolarizing electrical signal (action potential) that travels to the base of the cell. Some acids further promote Type III cell depolarization via intracellular acidification. Many relatively weak acids, like citric and acetic acid, can pass through cell membranes where their associated protons can dissociate and block potassium-passing ion channels. Since these potassium ion channels steady the electrical balance of the cell membrane, blocking them with protons also disturbs the electrical balance of the cell, ultimately encouraging depolarizing action potentials. For this reason, psychophysical experiments report that weak acids often taste more sour than stronger acids (like hydrochloric acid, which does not cross the cell membrane) of the same pH.
When electrical action potentials reach the base of the cell, they cause vesicle fusion to the membrane and neurotransmitter release. Type III cells release serotonin, GABA (gamma-aminobutyric acid), and norepinephrine when stimulated. These neurotransmitters may signal to the afferent nerve as well as neighboring Type II cells, but the details of neurotransmitter signaling are not well understood. Regardless, Type III cells communicate to nerve fibers, causing neurons in taste nerves to fire. These nerves carry the signal to the brainstem and, ultimately, the insular cortex of the brain, which processes taste information.
Courtney E. Wilson
See also: Salty Sensation; Sour Sensation; Taste Aversion; Taste Bud; Taste System; Type I Taste Cells; Type II Taste Cells
Chang, Rui B., Hang Waters, & Emily R. Liman. (2010). A proton current drives action potentials in genetically identified sour taste cells. Proceedings of the National Academy of Sciences of the United States of America, 107(51), 22320—22325. http://dx.doi.org/10.1073/pnas.1013664107
Chaudhari, Nirupa, & Stephen D. Roper. (2010). The cell biology of taste. Journal of Cell Biology, 190(3), 285—296.
Wenlei Ye, Rui B. Chang, Jeremy D. Bushman, Yu-Hsiang Tu, Eric M. Mulhall, Courtney E. Wilson, … Emily R. Liman. (2016). The K+ channel KIR2.1 functions in tandem with proton influx to mediate sour taste transduction. Proceedings of the National Academy of Sciences of the United States of America, 113(2), E229—238.