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In this article we will discuss about the taste sensation in humans.
Receptors are present in taste buds (Fig. 10.32), situated in the tongue, soft plate, epiglottis and certain regions of the pharynx.
Microvilli project from one end of the pole. Central portion of dorsum of tongue is devoid of papillae (Fig. 10.33).
Afferent Nerve Supply:
i. Anterior two-thirds of tongue—chorda tympani branch of facial nerve
ii. Posterior one-third of tongue—glossopharyngeal
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iii. Epiglottis, pharynx, etc.—vagus
Pathway for Taste (Fig. 10.34):
Afferent fibers (7th, 9th, and 10th) join to form the tractus solitarius. The nerve fiber end over nucleus of tractus solitarius. Nerve fibers arising from nucleus of tractus solitarius cross to opposite side and ascend up along with the medial lemniscus to terminate in the posteroventromedial nucleus of thalamus. Nowadays, it is said that the pathway does not cross, but reaches the thalamus of the ipsilateral side.
From thalamus, the 3rd order neuron fibers that have taken origin end in cerebral cortex (area 3, 1, 2) in the general sensory region. The fibers terminate in the lower region of the sensory area of cortex which is the area of representation for the face.
The 4 primary/basic modalities of taste sensations are:
1. Bitter
2. Sour
3. Sweet
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4. Salt
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A taste bud may respond to any one or more than one as well.
Taste buds at the:
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a. Side of tongue respond best for sour.
b. Tip of tongue respond best for sweet.
c. Middle and tip tongue respond best for salt.
d. Back of tongue respond best for bitter.
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One theory says that taste buds are covered by poly electrolyte field. Therefore, this particular field has electrical charges arranged in a specific fashion. When food is taken, there is alteration in the changes of the membrane. These receptors covered by a protein membrane covering certain amount of receptor site. This region brings about alteration in the changes in the membrane and lead to production of action potentials.
Simultaneous Contrast and Successive Contrast:
Simultaneous contrast:
Over the ½ of tongue apply salt solution. Over the other ½ apply a sweet solution. The sweet solution becomes much sweeter.
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Successive contrast:
Apply sweet and salt solutions one after another. The sweet solution appears much sweeter.
Taste Blindness:
Taste blindness occurs in about 30% of the cases but not for all substances. When low concentration of phenylthiocarbamide (PTC) is applied in normal individual, it gives sour taste. In 30%, it does not give rise to any taste. These individuals are said to be taste blind. This is due to Mendelian recessive gene.
Ageusia means loss of taste sensations and dysgeusia refers to difficulty to perceive the taste sensation.
