Chapter 13. Functional Anatomy of the Gustatory System

On average, individual humans have 3,000–12,000 taste buds within small structures called papillae, mostly on the tongue:

• Fungiform papillae, each containing 3–5 taste buds, are located along the edges and top of the front two-thirds of the tongue.
• Foliate papillae, small ridgelike folds located on the sides of the tongue near the back, each contain a few hundred taste buds.
• Circumvallate papillae each contain 200–700 taste buds. The mouth contains only 8–12 of these papillae, all situated in a row at the back of the tongue.

Each taste bud contains 40–100 taste receptor cells (TRCs), elongated neurons with cilia at their outer ends and without axons. Each TRC lives for only a week or so and is then replaced (new TRCs develop from basal cells in the taste bud). The cilia of the TRCs project into a taste pore (an opening onto the surface of the tongue) at the top of the taste bud, where the cilia come into contact with tastant molecules dissolved in saliva.

There are two types of TRCs: receptor cells and presynaptic cells. The membranes of receptor cell cilia contain GPCRs (G-protein coupled receptors) that initiate transduction of sweet, umami, and bitter tastants. Receptor cells do not have synapses with cranial nerve fibers, but presynaptic cells do. In presynaptic cells, receptors take the form of ion channels in the cilia membrane, where transduction of salty and sour tastants is initiated. When tastants are transported through the ion channels, neurotransmitters are released into the synapse and taken up by the nerve fiber, which responds by sending action potentials to the brain.

Receptor cells send signals to the brain by what is termed cell-to-cell signaling: When tastants bind to the receptors on a receptor cell’s cilia, the cell releases ATP into the extracellular fluid within the taste bud. These ATP molecules are taken up by presynaptic cells, which release serotonin into their synapse with cranial nerve fibers. Presumably, differences in the release of ATP and the consequent release of serotonin provide information about the type of tastant transduced by the receptor cell—sweet, umami, or bitter.

Neural signals from taste receptor cells are sent to the brain via cranial nerves VII, IX, and X. These signals travel first to the nucleus of the solitary tract and then through the ventral posterior medial nucleus of the thalamus on their way to the cortex. The first cortical areas to receive taste signals are the anterior insular cortex and the frontal operculum, which constitute the primary taste cortex. Signals then go to the orbitofrontal cortex, the amygdala (where emotion is represented), and the hypothalamus (where hunger is represented).

The orbitofrontal cortex (OFC) appears to play an especially important role in taste and flavor perception. When an animal eats, the responses of neurons in the primary taste cortex aren’t affected by hunger, but neurons in the orbitofrontal cortex respond strongly—indicating high reward value—only when the animal is hungry. This suggests that the primary taste cortex represents taste qualities (sweet, bitter, umami, salty, and sour), whereas the OFC represents the reward value of food.

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