• The chemical senses rely on specialized receptors that are renewed every few weeks.
• Olfactory receptor neurons are the only central nervous system neurons that are directly exposed to and interact with chemical substances in the environment.
• Odorant binding proteins serve to enhance sensitivity to odorants at very low concentrations.
• Odorant receptors are members of a large receptor superfamily that are coupled to G-protein second-messenger systems.
• Olfactory receptor neurons project to the olfactory bulb containing two well-defined inhibitory feedback loops.
• In addition to the primary olfactory pathway, two other pathways send olfactory information to the cortex to (1) contribute to the perception of flavor and (2) transmit information about pheromones, which influence reproductive behaviors.
• The function of the gustatory pathway is to elicit pleasant or aversive reactions to food.
• Recognition of five basic tastes is achieved by the preferential distribution of sensory receptors coupled to different second-messenger systems.
• Afferent taste fibers project through three cranial nerves to nuclei closely associated with general visceral sensation.
• The large number of distinguishable tastes is represented by population responses of various central taste neurons rather than by activation of individual taste-specific neurons.
The chemical senses, taste and smell, involve a direct interaction of chemical substances in the environment with receptor sites on sensory cells. Thus, compounds must be taken into the body through either aspiration or ingestion in order to elicit a smell or taste. Perhaps it is because ofthis direct interaction with potentially harmful substances in the environment that taste and olfactory receptor cells turn over every 4 to 8 weeks. This is not unusual in the case of taste receptor cells because they are modified epithelial cells, not true neurons. Almost all types of epithelial cells normally turn over; however, olfactory cells are true neurons with cell bodies in the nasal mucosa and axons that project directly to brain structures. These olfactory neurons continually reproduce throughout life, more so than any other type of central nervous system (CNS) neurons. New replacement olfactory neurons are generated by stem cells (basal cells) within the olfactory mucosa, and their axons must grow to proper connection sites within the brain. There is hope that studies of the renewal capability of olfactory neurons will provide important leads to understanding how other neurons in the brain could be replaced after cell loss from disease, stroke, or aging. Even with receptor renewal, however, there is a marked decline in the ability to taste and smell during normal aging. Loss of appetite associated with a decline in the chemical senses can be a serious problem leading to an inability to maintain body weight and a decline in general health.
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