The Five Basic Tastes

Humans perceive five basic tastes: salt, sour, sweet, bitter, and umami. Further distinctions are made based on combinations of these basic sensations. Additional subtleties in flavor can be discerned by the combination of smell and taste and, to a lesser extent, temperature and texture of food.

The five basic tastes are derived from different trans-duction mechanisms located on taste receptor cells and their specific interactions with different types of molecules. In general, acids elicit a sour taste; salt elicits a salty taste; sugars, some proteins, and amino acid artificial sweeteners such as saccharin and aspartame invoke sweetness; ions such as potassium and magnesium and organic compounds such as quinine and coffee taste bitter. Umami, recently recognized as an additional basic taste, is associated with some amino acids such as glutamate, the common culinary form of which is monosodium glutamate (MSG). These compounds interact with taste receptors and cause an increased release of transmitter, which in turn stimulates primary afferent taste fibers.

There are four mechanisms by which chemicals cause increased transmitter release from taste receptors: (1) direct passage of ions through ion channels, (2) blockage of ionic channels, (3) opening of ionic channels, and (4) activation of second-messenger systems through ligand interactions with membrane receptors (Fig. 6).

A. Taste sensitivity of the tongue

B. Taste papillaae

A. Taste sensitivity of the tongue

glossophangeal (IX) facial (VII)

bitter salty sweet sour vagus (X)

glossophangeal (IX) facial (VII)

C. Taste bud surface of tongue vallate bitter foliate salty sweet fungiform vallate foliate fungiform

taste buds surface of tongue taste buds

taste pore taste cell supporting cell basal cell afferent neuron

FIGURE 5 Cellular constituents of taste buds. (A) Taste buds are distributed around the edge of the tongue and down the throat. Areas of salt-sensitive taste buds overlap with sweet and sour areas at the front. Fibers from the anterior two-thirds of the tongue send taste information to the brain in the facial nerve (cranial nerve VII). The back of the tongue sends information through the glossopharyngeal nerve (IX), and the palate, pharynx, and epiglottis use the vagus nerve (X). (B) Taste buds are located within specialized papillae. Fungiform papillae cover the areas of the tongue innervated by the facial nerve and contain one to five taste buds each. Foliate papillae, located along the back edge of the tongue, and vallate papillae, located along the posterior midline, contain thousands of taste buds each. (C) Taste buds contain 50 to 150 receptor cells, each with microvilli on their apical surface and synaptic connections with afferent neurons near their base. Taste buds also contain basal cells that replace the short-lived receptor cells every few days. There are also supporting cells that appear to be similar to the glial cells of the central nervous system.

sour

A. Salty taste: Na+ and H+ move through open channels microvilli microvilli

B. Sour taste: H+ closes K+ channel

C. Umami taste glutamate receptor

D. Sweet taste: receptor -G-protein - cAMP - PKA

F. Bitter - 2 taste: release internal Ca++

F. Bitter - 2 taste: release internal Ca++

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