Taste buds

Perception of most taste qualities is mediated by about 4000 taste buds. These onion-shaped structures are about 50p,m wide and X0p.m deep. Nearly half of them arc

Tasre 11

Circum vallate papillae

Circum vallate papillae

Fungiform papillae

Foliate papillae

Fungiform papillae

Fijjur* 2.1 The papillae on the tongue contain 75% ofall taste buds embedded adjacent to either side of the trenches around the nine circum vallate papillae (Purves et at., 2001). Fungiform papillae, which are located in large numbers at the tip and the foremost two-thirds of the tongue, carry about three taste buds each. The foliate papillae, which resemble parallel ridges on either side of the root of the tongue, contain a total of about 600 taste buds. A smaller number resides on the epiglottis. Taste buds were also found on the soft palate by some investigators (Kikuchi-et at,, 1988: Imfeld and Schroeder. 1992 ). but not by others (Cleaton-Jones. 1975).

All taste buds carry receptors at least for all the basic taste qualities, though the quantitative mixture varies among the different locations of the oral cavity. Taste buds that have a high propensity for detecting bitter tastes are especially well represented in the foliate papillae, whereas those detecting sweet taste are more common at the front of the tongue. The sides of the back half of the tongue are especially salt-sensitive, the sides of the middle portion of the tongue detect sour Savors especially well. Whether individual ceils detect more than one taste quality is still a matter of debate (Gilbcrtson et at., 2001: Wilson. 2001; Caicedo and Roper. 2001). Each taste bud is populated by 30 100 dark, intermediate, and light bipolar ceils with microvilli that extend through a taste pore into the trench around the papilla.

The bipolar cells respond when sapid molecules act on ion channels and specific receptors on their apical surface. The receptors are associated with G-proteins. which initiate second-messenger cascades, eventually, this leads to the depolarization of the cell and release of a transmitter (serotonin) into the synaptic cleft at the base of the bipolar cell. Serotonin binds to a serotonin receptor on the adjacent primary sensory-neuron and triggers its depolarization.

The bipolar cells have a limited life span (about a month) and must be replaced by replicating basal cells at the base of the taste buds. Carbonic anhydrasc VI (gustin). a zinc-containing glycoprotein that is specifically expressed in salivary glands, is critical for maintenance of taste hud growth (possibly by acting on bud stem cells) and function (Henkin et at., 1999). Adequate zinc intake appears to promote taste acuity.

Circumvallate papilla

Papillary trench

Figurp 2.2 Taste buds adjacent to trie trenches of a circumvallate papilla

Innervation

The bipolar cells of the taste buds connect to the synapses of afferent gustatory axons. Individual primary sensory neurons carry information on more than one taste quality (Gilbertson et al.. 2001). The corda tympam of cranial nerve VII (facial nerve) carries the gustatory axons from the fungiform and anterior foliate papillae, the lingual branch of cranial nerve IX (glossopharyngeal nerve) carries the axons from the posterior foliate papillae and from the circumvallatc papillae. The superficial petrosal branch of cranial nerve VII is responsible for innervation of the taste buds on the soft palate, and the superior branch of cranial nerve X (vagal nerve) Sinks to the taste buds on the epiglottis.

Some taste categories arc predominantly linked to particular nerves. Sweet and salty tastes are largely associated with the corda tympani of cranial nerve VII. while sour and bitter tastes arc predominantly (but not exclusively) associated with cranial nerve IX. The chorda tympani of cranial nerve VII also is important for the maintenance of actin filaments in taste pore cells.

The gustatory axons travel within their cranial nerves to the nucleus of the solitary tract in the medulla where they link to neurons leading to the medial half of the ventral posterior medial nucleus in thalamus. Another set of neurons then connects to the gustatory cortex of the insula and frontal operculum. The stimulatory effect of sapid compounds can be detected in these cortical areas bv PET scans (Gamier w al„ I l)W), Interneuron connections also exist within the solitary tract to sympathetic and parasympathetic neurons of the visceral organs. Thus, gustatory senses have direct input on the motility of stomach and diaphragm and on the secretion of digestive juices and hormones.

Cranial ri.V

Cranial ri.V

Irritants and pungent compounds Fungiform and anterior foliate papillae Taste buds of soft palate Posterior foliate and circumvallate papillae Taste buds of epiglottis

Lingual branch n.lX Superior branch n.X

Corda tympam n.VII Superficial petrosal n.VII

Irritants and pungent compounds Fungiform and anterior foliate papillae Taste buds of soft palate Posterior foliate and circumvallate papillae Taste buds of epiglottis

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