Integrative Oral Sciences 1507
Chemical Sensory System Functions (continued)

by Dr. M. Hutchins


TASTE BUDS AND CELLS

Figure 5
Figure 6
Figure 7
Circumvaliate
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Foliate
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Fungiform
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Taste buds are found primarily in the tongue papillae. The tongue contains 4 types of papillae, the most common type, filiform, are thin and wire shaped and do not contain taste buds. On the dorsal, anterior border of the tongue are mushroom shaped papillae, fungiform,these have taste buds located near the middle or in a cleft of the papillae. The foliate papillae are leaf  shaped with taste buds on the side of the papillae, and these are along the border.  The circumvallate papillae contain taste buds along the sides of whorls and are located in the posterior third of the tongue in the shape of a V. Taste buds are also located in the oral mucosa of the palate and epiglottis.

The taste cells are modified epithelial cells that function as sensory receptors. About 50-60 taste cells are located in pear-shaped taste buds and the taste cells through microvilli project into a taste pore. There are non-receptor basal cells which are located on the basement membrane which do not project into the taste pore. These basal cells differentiate through a series of morphological steps into a mature taste cell. The taste cells are replaced about every 10 days.

Figure 8
In the past, it was believed that taste buds for sweet only existed on the tip of the tongue (Fig. 9) and for bitter (Fig. 10) were in the posterior region, but currently it is known that the tongue does not have regional differences for the taste qualities. Furthermore, taste cells can respond to the stimuli of many different taste molecules that are sweet, salt, sour or bitter.
Figure 9
Figure 10

SENSORY TRANSDUCTION OF TASTE

Transduction of chemical stimuli into nerve impulses begins with saliva transporting the dissoled polar molecules that are sweet or bitter i.e. sugar, caffeine to the taste pore (Fig.12). Polar molecules do not enter the taste cells, but bind to receptors on the microvilli in the taste pore. These receptors are coupled to a G-protein called gustaducin followed by activation of a 2nd messenger which controls a ligand-gated channel protein to the reduce the efflux of  K+ through the ion-gated channel protein in the cell membrane. A receptor potential is generated which opens a voltage-dependent Ca++ channel  and the subsequent discharge of a neurotransmitter into the synapse.

An action potential is then generated in the afferent nerve ending and conducted by the stimulated cranial nerve into the CNS.  Electrolytes, such as salts or acids interact directly with a receptor on the taste cell membrane to open an ion-gated channel protein and increase the flux of Na+ and K+. Non-polar or lipophillic stimuli are transported to the taste cell, but bind to a protein-binding protein, possibly produced by small serous glands, von Ebner’s glands. A similar transduction process converts non-polar solutes into nerv impulses. Many drugs are non-polar solutes and some are bitter in taste.

Figure 11

Figure 12

TRANSMISSION AND PERCEPTION OF ACTION POTENTIALS AS A NEURAL CODE

Neural input from one taste bud does not transmit directly through a single axon into the central nervous sytem, instead many taste cells in many taste buds are stimulated by the diverse quality of masticated food.  A single branch of the facial nerve has been demonstrated to branch extensively and diverge to several papillae and to different taste buds within a single papillae.  Stimulation of the hundreds of taste buds on the tongue generates a neural code that is specific for each type of solute  molecules that may have a sweet, bitter, salt or acid taste. Humans can detect, distinguish and identify many substances that have a specific taste quality because the neural code for each type of taste stimuli is decoded by the central nervous system.

Cranial afferent nerve fibers, VII, IX, X, project from the tongue into the nucleus solitarius to synapse with  2nd order sensory neurons. Fibers from the nucleus solitarius ascend to the thalamus and diverge to the parabrachial nucleus. From the thalamus, nerve fibers project to the gustatory cortex and from the parabrachial nucleus, fibers are relayed to the hypothalamus and amygdala.
Figure 13

DISORDERS OF TASTE

A deficiency in the function of taste may be caused by age, drugs, disease, or trauma. For example as humans age, there is a decrease in the number of papillae containg taste buds and a reduction in the sensitivity to many taste solutes; however, older people still enjoy sweet, tasting foods.

The role of saliva in taste perception is very important because saliva assist in dissolving solutes and in transporting these to the taste pore. It is difficult to taste anything if the mouth is dry. Several drugs reduce salivary flow and  will cause dental patients to complain of a dry mouth. Patients that receive radiation treatment for oral cancer have a severe deficiency in taste because the treatment produces salivary gland atrophy and reduces the number of tongue papillae. These patients will complain of oral soreness and the loss of taste.

Causes of Taste Disorders

  • Acute trauma, burn to surface of the tongue
  • Many types of drugs cause dryness of the mouth
  • Vitamin A defiency
  • Xerostomia

Taste deficiency is reported by patients with vitamin A deficiency, diabetes, hypothroidism, renal failure and with an autoimmune condition, such as  Sjogren’ syndrone. Neural trauma to one of the afferent cranial nerves for taste e.g.  the facial nerve, will compromise the sense of taste and the flavor of foods. A common acute loss of taste is the contact of hot liquid  with the oral tissues which damages the papillae and taste buds, fortunately recovery of the damaged taste buds usually occurs within a 10 day period.



Content questions should be directed to: Dr. Max Hutchins
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