The receptors for the sense of taste are the taste buds (Fig. 9.32), microscopic nests of cells mainly associated with the papillae of the tongue, although small numbers are also found in the soft palate and in the vicinity of the epiglottis.

Taste buds are about as tall as the epithelium in which they lie and communicate with the oral cavity by taste pores through which solutions enter to stimulate the receptor cells.

The taste buds consist of sustentacular or supporting cells in addition to the receptor or gustatory cells. The gustatory cells have elongated nuclei and at their free tips bear microvilli (taste hairs) that project into the taste pore. Glands deep to the lingual papillae discharge a serous secretion on the surface of the epithelium. It is believed that the secretion cleanses the taste pores and enhances perception by the gustatory cells.

To be discerned, food substances have to be in solution. One of the reasons food is insalivated is to dissolve parts for sampling by the taste buds. The principal taste sensations are sweetness, sourness, and saltiness. In the dog sweetness and saltiness appear to be perceived in the rostral two thirds of the tongue, where taste buds are present on the fungiform papillae. Sour substances are perceived over the entire tongue. The caudal third of the tongue, which incorporates the vallate and foliate papillae, therefore seems to respond only to what tastes sour.

FIG. 9.32 Histologic section of a taste bud. 1, Sustentacular cell; 2, gustatory cells; 3, taste pore; 4, epithelium.

The afferent pathways transmitting these sensations to the brain are similarly divided. In the rostral two thirds of the tongue, the sensory fibers travel first in the lingual nerve and then pass into the chorda tympani, which we encountered in the description of the ear. Neuronal cell bodies associated with these fibers are located in the geniculate ganglion of the seventh cranial nerve, and the centrally directed axons of these sensory neurons then enter the medulla oblongata. The afferent fibers innervating taste buds in the caudal third of the tongue travel in the glossopharyngeal nerve (and to a small extent in the vagus) to the medulla oblongata.