Taste buds can be found on various types of taste papillae located on the tongue, palate, pharynx, and larynx. The anterior and lateral aspects of the tongue have fungiform and foliate papillae respectively, while circumvallate papillae are located on the base of the tongue. These circumvallate papillae can contain several hundred taste buds. In humans, the tongue can have several thousand taste buds. The distribution of taste buds for different taste qualities is slightly variable as taste buds responding to each taste type are widely spread out. The innervation of taste buds is carried out by three cranial nerves. Taste buds on the anterior two-thirds of the tongue are innervated by the chorda tympani branch of the facial nerve (CN VII), while taste buds on the posterior third of the tongue are innervated by the glossopharyngeal nerve (CN IX). A few taste buds in the larynx and upper esophagus are innervated by the vagus nerve (CN X). The cell bodies of taste fibers in CN VII, IX, and X are located in the geniculate, petrosal, and nodose ganglia respectively. The central processes of the afferent fibers enter the medulla, join the solitary tract, and synapse in the nucleus of the solitary tract. In certain animals, such as various rodent species, the second-order taste neurons of the solitary nucleus project to the ipsilateral parabrachial nucleus, which in turn projects to the parvocellular part of the ventroposterior medial (VPMpc) nucleus of the thalamus. In monkeys, the solitary nucleus directly projects to the VPMpc nucleus. The VPMpc nucleus is connected to two different gustatory areas in the cerebral cortex, one in the face area of the S1 cortex and the other in the insula. A distinctive characteristic of the central gustatory pathway is that it is predominantly an uncrossed pathway, unlike the predominantly crossed central somatosensory pathways. While the sense of smell is more developed in some animals, humans also rely on olfaction for emotional experiences and the retrieval of memories. Additionally, odor helps individuals detect spoiled food and hazardous situations. For instance, an unpleasant odor is added to odorless, colorless natural gas to facilitate the detection of leaks. Odor possesses more primary qualities compared to taste.
Taste buds are located on different types of taste papillae found on the tongue, palate, pharynx, and larynx. Types of taste papillae include fungiform and foliate papillae on the anterior and lateral aspects, respectively, of the tongue and circumvallate papillae on the base of the tongue (see
Fig. 8.29C).
The circumvallate papillae may contain several hundred taste buds. The tongue in humans may have several thousand taste buds. The sensitivity of different regions of the tongue for different taste qualities varies slightly because taste buds responding to each type of taste are widely distributed. The taste buds are innervated by three cranial nerves. The chorda tympani branch of the facial nerve (CN VII) innervates taste buds on the anterior two thirds of the tongue, and the glossopharyngeal nerve (CN IX) innervates taste buds on the posterior third of the tongue (see
Fig. 8.29C ). The vagus nerve (CN
X) innervates a few taste buds in the larynx and upper esophagus.
The cell bodies of taste fibers in cranial nerves VII, IX, and X are located in the geniculate, petrosal, and nodose ganglia, respectively. The central processes of the afferent fibers enter the medulla, join the solitary tract, and synapse in the nucleus of the solitary tract (see
Fig. 4.6
D to E). In some animals, including several rodent species, the second-order taste neurons of the solitary nucleus project rostrally to the ipsilateral parabrachial nucleus. The parabrachial nucleus then projects to the small-celled (parvocellular) part of the ventroposterior medial (VPMpc) nucleus of the thalamus. In monkeys, the solitary nucleus projects directly to the VPMpc nucleus. The VPMpc nucleus is connected to two different gustatory areas of the cerebral cortex: one in the face area of the S1 cortex and the other in the insula. An unusual feature of the central gustatory pathway is that it is predominantly an uncrossed pathway (unlike the central somatosensory pathways, which are predominantly crossed).
The sense of smell is much better developed in some animals (macrosmatic animals) than in humans. The ability of dogs to track other animals on the basis of odor is legendary, as is the use of pheromones by insects to attract mates. However, olfaction contributes to humans’ emotional life, and odors can effectively conjure up memories. It also helps
Release of • Fig. 8.29 A, A taste bud is shown with the taste pore at the top and its innervation below. B, basal cell; R, ciliated taste receptor cells; S, supporting cells. B, Taste receptor cell showing second messenger, ligand-gated, and direct depolarization resulting in depolarization of the cell.
• Fig. 8.29, cont’d C, Distribution of the taste buds on the tongue and their innervation. (Redrawn from Squire LR, et al [eds]. Fundamental Neuroscience. San Diego, CA: Academic Press; 2002.) people avoid consuming spoiled food and detect dangerous situations. For example, an unpleasant odorant is added to odorless, colorless natural gas so that people can easily detect a leak.
Odor has more primary qualities than taste does.