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Odor and food molecules activate membrane receptors
Reports from our noses and mouths alert us to pleasure, danger, food and drink in the environment. The complicated processes of smelling and tasting begin when molecules detach from substances and float into noses or are put into mouths. In both cases, the molecules must dissolve in watery mucous in order to bind to and stimulate special cells. These cells transmit messages to brain centers where we perceive odors or tastes, and where we remember people, places, or events associated with these olfactory (smell) and gustatory (taste) sensations.
The neural systems for these two chemical senses can distinguish thousands of different odors and flavors. Identification begins at membrane receptors on sensory cells, where odorant or taste molecules fit into molecular slots or pockets with the right "lock and key" fit. This latching together of binding molecule or ligand and membrane receptor leads to the production of an electrical signal, which speeds along a pathway formed by nerve cells (neurons) and their extensions called axons. In this way, information reaches brain areas that perceive and interpret the stimulus.
A membrane receptor will respond to several structurally related molecules
The activation of receptors by discrete chemical structures is not absolute, because a given membrane receptor will accept a number of structurally similar ligands. Nevertheless, we can discriminate many thousands of smells and tastes, even though some chemicals stimulate the same receptor. How are we able to distinguish these? Our ability results from the fact that most substances we encounter are complex mixtures, which activate different combinations of odor and taste receptors simultaneously. Thus, each substance we smell or taste has a unique chemical signature. In the laboratory, researchers frequently test people or animals with pure individual chemicals in order to find the best stimulus for a receptor, but in the real world we seldom encounter these molecules alone.
Although we do have overlap in the response of taste and smell receptors to ligands, scientists have identified quite a number of receptor types. Humans probably have hundreds of kinds of odor membrane receptors, and on the order of 50 to 100 different kinds of taste receptors. It is true that we typically describe only five categories of tastes (see below): this means that each of the categories probably has more than one type of receptor. Further research will show how this puzzle fits together.
The neural systems for taste and smell share several characteristics
Although the neural systems (sensory cells, nerve pathways, and primary brain centers) for taste and smell are distinct from one another, the sensations of flavors and aromas often work together, especially during eating. Much of what we normally describe as flavor comes from food molecules wafting up our noses. Furthermore, these two senses both have connections to brain centers that control emotions, regulate food and water intake, and form certain types of memories.
Another similarity between these systems is the constant turnover of olfactory and gustatory receptor cells. After ten or so days, taste sensory cells die and are replaced by progeny of stem cells in the taste bud. More surprising is the story of olfactory sensory cells. These are not epithelial cells like taste cells, but rather neurons, which are not commonly regenerated in adults (although recent evidence shows that new neurons are produced, even in the brain). Researchers are investigating how taste perception and odor recognition are maintained when cells die and new connections to the nervous system must be generated.
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