내몸 ≫ 감각 ≫ 미각기관

미각경로 : gustation pathway

감각 기관
- 미각 메커니즘 : 수용체, 경로
- 후각 메커니즘 : 수용체, 경로
- OFC 감각연합 : G protein

- 식욕 매커니즘
- 고립로핵



Gustatory Reception.
(a) Taste receptors are located in taste buds, which form pockets in the epithelium of fungiform or circumvallate papillae. (b) Taste buds in a circumvallate papilla. (c) A taste bud, showing receptor (gustatory) cells and supporting cells. The diagrammatic view shows details of the taste pore that are not visible in the micrograph.

Filiform papillae provide friction that helps the tongue to move objects around in the mouth, but do not contain taste buds. Each small fungiform papilla contains about five taste buds; each large circumvallate papilla contains as many as 100 taste buds. The circumvallate papillae form a V near the posterior margin of the tongue

Taste Receptors
Taste buds are recessed into the surrounding epithelium, isolated from the relatively unprocessed contents of the mouth. Each taste bud ( Figure 17–2b,c ) contains about 40 slender, spindle–shaped cells. There are at least four different types of cells within a taste bud. Basal cells appear to be stem cells. These cells divide to produce daughter cells that mature in stages corresponding to the other types of cell. The cells of the last stage are called gustatory cells . Each gustatory cell extends slender microvilli, sometimes called taste hairs , into the surrounding fluids through the taste pore , a narrow opening.
Despite this relatively protected position, it's still a hard life: A typical gustatory cell survives for only about 10 days before it is replaced. Although everyone agrees that gustatory cells are taste receptors, it remains uncertain as to whether the cells at earlier stages of development provide taste information as well. (Cells at all three stages are innervated by sensory neurons.)

Gustatory Pathways
Taste buds are monitored by cranial nerves VII (facial), IX (glossopharyngeal), and X (vagus). The facial nerve monitors all the taste buds located on the anterior two–thirds of the tongue, from the tip to the line of circumvallate papillae. The circumvallate papillae and the posterior one–third of the tongue are innervated by the glossopharyngeal nerve. The vagus nerve innervates taste buds scattered on the surface of the epiglottis. The sensory afferents carried by these cranial nerves synapse in the solitary nucleus of the medulla oblongata, and the axons of the postsynaptic neurons enter the medial lemniscus. There, the neurons join axons that carry somatic sensory information on touch, pressure, and proprioception. After another synapse in the thalamus, the information is projected to the appropriate portions of the primary sensory cortex.


A conscious perception of taste is produced as the information received from the taste buds is correlated with other sensory data. Information about the texture of food, along with taste–related sensations such as "peppery" or "burning hot," is provided by sensory afferents in the trigeminal nerve (V). In addition, the level of stimulation from the olfactory receptors plays an overwhelming role in taste perception. Thus, you are several thousand times more sensitive to "tastes" when your olfactory organs are fully functional. By contrast, when you have a cold and your nose is stuffed up, airborne molecules cannot reach your olfactory receptors, so meals taste dull and unappealing. This reduction in taste perception occurs even though the taste buds may be responding normally.


Gustatory Discrimination
You are probably already familiar with the four primary taste sensations : sweet, salty, sour, and bitter. There is some behavioral evidence for differences in sensitivity to tastes along the axis of the tongue, with greatest sensitivity to salty–sweet anteriorly and sour–bitter posteriorly. However, there are no differences in the structure of the taste buds, and taste buds in all portions of the tongue provide all four primary taste sensations.
At least two additional tastes have been discovered in humans:

Umami. Umami (oo–MAH–me) is a pleasant taste that is characteristic of beef broth, and chicken broth, and parmesan cheese. This taste is produced by receptors sensitive to the presence of amino acids, especially glutamate, small peptides, and nucleotides. The distribution of these receptors is not known in detail, but they are present in taste buds of the circumvallate papillae.
Water. Most people say that water has no flavor. However, research on humans and other vertebrates has demonstrated the presence of water receptors , especially in the pharynx. The sensory output of these receptors is processed in the hypothalamus and affects several systems that deal with water balance and the regulation of blood volume. For example, a mouthful of distilled water held for 20 minutes will inhibit ADH secretion and promote water loss at the kidneys.

The mechanism behind gustatory reception resembles that of olfaction. Dissolved chemicals contacting the taste hairs bind to receptor proteins of the gustatory cell. The different tastes involve different receptor mechanisms. Salt receptors and sour receptors are chemically gated ion channels whose stimulation produces depolarization of the cell. Receptors responding to stimuli that produce sweet, bitter, and umami sensations are G proteins called gustducins –protein complexes that use second messengers to produce their effects. The end result of taste receptor stimulation is the release of neurotransmitters by the receptor cell. The dendrites of the sensory afferents are tightly wrapped by folds of the receptor cell membrane, and neurotransmitter release leads to the generation of action potentials in the afferent fiber. Taste receptors adapt slowly, but central adaptation quickly reduces your sensitivity to a new taste.
The threshold for receptor stimulation varies for each of the primary taste sensations, and the taste receptors respond more readily to unpleasant than to pleasant stimuli. For example, we are almost a thousand times more sensitive to acids, which taste sour, than to either sweet or salty chemicals, and we are a hundred times more sensitive to bitter compounds than to acids. This sensitivity has survival value, because acids can damage the mucous membranes of the mouth and pharynx, and many potent biological toxins produce an extremely bitter taste.
Taste sensitivity differs significantly among individuals. Many conditions related to taste sensitivity are inherited. The best–known example involves sensitivity to the compound phenylthiourea , also known as phenylthiocarbamide , or PTC . Roughly 70 percent of Caucasians can taste this substance; the other 30 percent are unable to detect it.

Aging and Gustatory Sensitivity
Our tasting abilities change with age. We begin life with more than 10,000 taste buds, but the number begins declining dramatically by age 50. The sensory loss becomes especially significant because, as we have already noted, aging individuals also experience a decline in the number of olfactory receptors. As a result, many elderly people find that their food tastes bland and unappetizing, whereas children tend to find the same foods too spicy.

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안와전두피질 : orbitofrontal cortex


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