Sense of Taste

Sense of Taste

The special organs of taste, the taste buds, num-ber over 10,000. Most are found on the surface of the tongue, with tiny elevations called papillae (­FIGURE 15- 2). An adult has approximately 3,000 taste buds. Taste buds are found on the tops of mushroom-shaped fungiform papillae, scattered over the surface of the tongue as well as in the epithe-lium of the large and round circumvallate papillae and the side walls of the foliate papillae, which are also called filiform­ papillae . The circumvallate papillae are the least numerous yet largest of the various types of papillae­. Between 7 and 12 circumvallate papillae form an inverted V shape on the posterior tongue.

In addition, about 1,000 taste buds are found on the roof of the mouth, the throat walls, and portions of the larynx. Each taste bud has up to 150 taste cells (gustatory epithelial cells), which are replaced every three days. The spherical, flask-shaped taste buds have openings called taste pores and tiny projections called taste hairs. The taste hairs are the sensitive parts of the taste receptor cells. Stimulation triggers an impulse on a nearby nerve fiber traveling to the brain. There are three or more types of gustatory epithelial cells, which may either form synapses with sensory dendrites and release serotonin or lack synaptic vesicles and release adenosine triphosphate as a neurotransmitter. The taste buds also have basal epithelial cells.

Chemicals must dissolve in saliva, diffuse into taste pores, and contact gustatory hairs before they can be tasted. A food chemical is known as a tastant. When a tastant binds to receptors in a gustatory epithelial cell membrane, a graded depolarizing potential is induced, causing neurotransmitter release. Although taste cells in taste buds are similar in appearance, there are following five types:

■■ Those that sense sweetness (such as sugar)

■■ Those that sense sourness (such as lemons)

■■ Those that sense saltiness (such as table salt)

■■ Those that sense bitterness (such as caffeine)

■■ Those that sense umami or deliciousness (responding to specific amino acids and related chemicals such as monosodium glutamate or MSG)

Other sensations that may be sensed are alkaline and metallic sensations, and it is widely believed that humans can take long-chain fatty acids from lipids. This may explain why foods containing fats are so popular. Additionally, there is water receptors located mostly in the pharynx. Their sensory output is processed in the hypothalamus and affects water balance, regulation of blood volume, and antidiuretic hormone secretion.

Flavors are tasted because of combinations of the primary sensations. The sense of smell also influences the flavors that are tasted. Though it was tradition-ally believed that specific areas of the tongue only responded to one type of “taste,” further research has revealed that all of the tongue is able to taste the sensa-tions of sweetness, sourness, saltiness, bitterness, and umami (deliciousness).

Taste sensations like smell sensations also adapt rapidly. Sensory impulses from the tongue travel to the medulla oblongata via the facial, glossopharyn-geal, and vagus nerves. Then, they move to the thal-amus (located in the diencephalon) and gustatory cortex (located in the parietal lobe of the cerebrum). Mostly, however, afferent fibers that carry taste infor-mation from the tongue are found in two cranial nerve pairs. The chorda tympani, branching off the facial nerve (VII), carry impulses from the anterior two-thirds of the tongue. The posterior third of the tongue and the pharynx located just behind are serviced by the lingual branch of the glossopharyngeal nerve (IX). The vagus nerve (X) is responsible for carrying taste impulses from the epiglottis and lower pharynx. The afferent taste fibers synapse in the solitary nucleus of the medulla. Impulses then move to the thalamus and end up in the gustatory cortex of the insula. Humans can appreciate tastes because of fibers that project to the hypothalamus and structures of the limbic system.

The sense of taste is actually mostly based on smell, to about 80%. Food lacks taste when the nasal passages are congested. Without smell, the sense of taste would be very inefficient, and much of what we enjoy from var-ious flavors could no longer be ­appreciated. The sense of taste is intensified by food temperatures and textures. Thermoreceptors, mechanoreceptors, and nociceptors in the mouth are involved here. An example is a spicy pepper that becomes more pleasurable because, in addi-tion to its taste, it excites pain receptors in the mouth.

Taste sensitivity among individual people differs widely. A variety of inherited conditions influences your taste sensitivity, such as the compound known as phenylthiourea or phenylthiocarbamide. Approxi-mately, 70% of people throughout the world perceive this substance as bitter, whereas others cannot taste it at all. It does not occur in food, but related chemicals do, mostly in certain Asian or Australian fruits.

1. Name the five primary taste sensations.

2. Describe which types of receptors are used in the senses of smell and taste.

3. Explain the foliate and fungiform papillae of the taste buds.