Sense of taste (gustation): The sense of taste and smell work closely together. If we cannot smell something we cannot taste it either. One reason why we cannot taste (or smell) food well with a common cold is that with the nasal passages inflamed and coated with thick mucus layer the smell receptors are practically nonfunctional. The receptor cells for taste are located in taste buds. Humans have about 10,000 taste buds. The majority of taste buds are located in pockets around the papillae (peg-like projections of the mucous membrane) on the surface and sides of the tongue, but there are some on the surface of the pharynx and the larynx. Each taste bud contains about 40 specialized receptor cells or gustatory cells, many more supporting cells and some basal cells that replace the worn out cells of the taste buds. Unlike the receptors for smell, that are modified sensory neurons, the receptor cells for taste are not neurons, but rather specialized cells with slender microvilli on their outer ends. The microvilli protrude into the surrounding fluids through a narrow opening called the taste pore. Dissolved chemicals contacting the microvilli bind to specific receptor present on the microvilli, thereby depolarizing the cell. The dendrites of the associated sensory neurons coil intimately around the receptor cells and synapse with them so that, when a receptor cell is stimulated and depolarized, it releases neurotransmitter which leads to the generation of an action potential in the associated sensory neuron. Each dendrite receives signals from several receptor cells within the taste bud. Nerve fibers emerging from the taste buds pass to the brain stem. From here the nerve impulse is relayed to the taste centre in the cerebral cortex of the brain that perceives the taste sensation.
Normally our taste sensations are complicated mixture of qualities. In humans, there are four basic taste senses: sweet, sour, salt, and bitter. The receptors for these four basic tastes have their areas of greatest concentration on different parts of the tongue: sweet and salty on the front bitter on the back, and sour on the sides. A few substances stimulate only one of the four types of receptors, but most stimulate two, three, or all four types to varying degrees. The sensation and flavour of the food we experience are thus produced by a combination of these four basic sensations, modified by accompanying sensations of smell, texture and temperature.
