All other tastes come from a combination of these four basic tastes. Actually, a fifth basic taste called "Umami" has recently been discovered. Umami is a taste that occurs when foods with glutamate (like MSG) are eaten. Different parts of the tongue can detect all types of tastes. Morever, the simple tongue "taste map" that is found in many textbooks has been criticized for several reasons (also here). The actual organ of taste is called the "taste bud." Each taste bud (and there approximately 10,000 taste buds in humans) is made up of many (between 50-150) receptor cells. Receptor cells live for only 1 to 2 weeks and then are replaced by new receptor cells. Each receptor in a taste bud responds best to one of the basic tastes. A receptor can respond to the other tastes, but it responds strongest to a particular taste.

The Taste Bud Image from Biodidac

There are two cranial nerves that innervate the tongue and are used for taste: the facial nerve (cranial nerve VII) and the glossopharyngeal nerve (cranial nerve IX). The facial nerve innervates the anterior (front) two-thirds of the tongue and the glossopharyngeal nerve innervates that posterior (back) one-third part of the tongue. Another cranial nerve (the vagus nerve, X) carries taste information from the back part of the mouth. The cranial nerves carry taste information into the brain to a part of the brain stem called the nucleus of the solitary tract. From the nucleus of the solitary tract, taste information goes to the thalamus and then to the cerebral cortex. Like information for smell, taste information also goes to the limbic system (hypothalamus and amygdala). Another cranial nerve (the trigeminal nerve, V) also innervates the tongue, but is not used for taste. Rather, the trigeminal nerve carries information related to touch, pressure, temperature and pain.

Cranial Nerves used for taste

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The complete inability to taste is called ageusia, the reduced ability to taste is called hypogeusia, and the enhanced ability to taste is called hypergeusia. Ageusia is a rare disorder. It may be rare because there are three different nerves that carry taste information to the brain. Older people have a reduced sense of taste because their taste buds are not replaced as fast those in younger people. Taste disorders can also be caused by drugs used to treat epilepsy, Parkinson's disease, diabetes and high blood pressure. Damage to the areas of the brain such as the brain stem, thalamus and cerebral cortex may also cause taste problems. A giraffe's tongue can be 29 inches (74 cm) in length. (Turin, M.S. Aardvarks to Zebras, New York: Citadel Press, 1995.) The tongue of a nectar bat (Anoura fistulata) can extend to 150% of its body length. (Muchhala, N., Nature, 444:701-702, 2006.) The tongue of a chameleon can extend to 200% of its body length. (Herrel1, A., Meyers, J.J., Aerts, P. and Nishikawa, K.C., The Journal of Experimental Biology, 204:3621-3627, 2001.)