Taste is determined by receptors, called taste buds. A greater number of taste buds appears to give a greater sensitivity to sweetness, sourness, saltiness, and bitterness (not the size of the tongue although someone with a larger tongue will have more taste buds). It is also thought that the shape of the taste buds may also play a role in sensitivity to taste. In humans, the taste buds are located on the surface and sides of the tongue, the roof of the mouth, and the entrance to the pharynx. The mucous membrane lining these areas is invested with tiny projections of papillae, each of which in turn is invested with 200 to 300 taste buds. The papillae located at the back of the tongue, and called circumvallate, are arranged to form a V with the angle pointing backward; they transmit the sensation of bitterness. Those at the tip of the tongue transmit sweetness, whereas saltiness and sourness are transmitted from the papillae on the sides of the tongue.
Each flask-shaped taste bud contains an opening at its base through which nerve fibers enter. These fibers transmit impulses directly to the brain. In order for a substance to stimulate these impulses, however, it must be in solution, moistened by the salivary glands.
Contrary to popular belief, sensations of taste have been determined to be more strongly related to sensations of smell. While the nose, equipped with olfactory nerves, is the special organ of smell, these nerves also account for differing tastes of substances taken into the mouth, that is, most sensations that appear to us as tastes are really smells. This is probably one way that temperature of food affects the taste of things- warmed food gives of more odors that can stimulate the olfactory nerves. Eating spicy foods affects the pain fibers on the tongue, not the taste buds. The sense of taste can be lost if their facial nerve is damaged in some way. For example, Bell's palsy may stop the facial nerve working properly and prevent or reduce chewing function (and, therefore, alter taste). It is uncommon for every taste nerve (bitter, salty, sweet and sour) to be affected.
Another disorder known as Sjogren's syndrome causes reduced saliva production, which in turn reduces the sense of taste. This is because the taste buds can only detect flavor when food is properly mixed with saliva.
Glossodynia, a condition characterized by a burning sensation on the tongue, is also linked to loss of taste in some cases.
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