9. Chemical Senses: Olfaction and Gustation
Part 1 of 4

Max O. Hutchins, Ph.D.
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An appreciation of the flavor of foods requires the diverse interaction of several sensory systems. Taste and smell are the principal systems for distinguishing flavors. However, tactile, thermal, and nociceptive sensory input from the oral mucosa contributes to food quality. Saliva also is an important factor in maintaining acuity of taste receptor cells (Figure 9.1). Its mechanisms of action include; acting as a solvent for polar solutes, transporting solutes to the taste receptors, buffering action for acidic foods and reparative action on the lingual epithelium.

Gustatory System

Recent technical advances in neurophysiology have made it possible to identify the physiological mechanisms of signal transduction for the detection and discrimination of various taste stimuli by the taste receptor cells.

Morphology of Taste Buds and Cell Types

Taste buds are located on papillae and distributed on the surface of the tongue. Taste buds are also found on the oral mucosa of the palate and epiglottis. These pear-shaped structures contain about 80 cells arranged around a central taste pore (Figure 9.2).

Transport of Solutes

Taste solutes are transported to the taste pore and diffuse through the fluid layer to make contact with membrane receptor proteins on the microvilli and apical membrane. Taste sensitivity is dependent upon the concentration of the taste molecules as well as their solubility in saliva. Many bitter tasting hydrophobic solutes interact with an odorant binding protein produced by von Ebner’s glands in the posterior region of the tongue.

Sensory Transduction

Taste sensation can be evoked by many diverse taste solutes. The pattern of membrane potential change include depolarization, depolarization followed by hyperpolarization, or only hyperpolarization. Action potentials in the taste receptor cells lead to an increase Ca²⁺ influx through voltage-gated membrane channels with the release of Ca²⁺ from intracellular stores. In response to this cation, neurotransmitter is released, which produces synaptic potentials in the dendrites of the sensory nerves and action potentials in afferent nerve fibers (Figure 9.3).

Salts

The taste of salts is mediated by Na⁺ ions which do not interact with a membrane receptor but diffuse through a Na⁺ channel located in the microvilli and apical membrane. Anions such as Cl^- contribute to the salty taste, but anions are transported into these cells by a paracellular route. The influx of these ions of salt evokes a depolarization in the apical membrane (Figure 9.3).

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