Olfactory Stimulation

Biologists are not certain how stimulated receptors encode specific smells, but a leading hypothesis is that each odor stimulates a distinct set of receptor subtypes. The brain then recognizes the combination of receptors as an olfactory code. For example, perhaps there are ten types of odor receptors. Banana might stimulate receptors 2, 4, and 7; garlic receptors 1, 5, and 9. Some investigators have proposed seven primary odors, but others hypothesize that the number is much higher and may reflect the functioning of hundreds of genes.

Because the olfactory organs are high in the nasal cavity above the usual pathway of inhaled air, sniffing and forcing air over the receptor areas may be necessary to smell a faint odor. Olfactory receptors undergo sensory adaptation rather rapidly, so the intensity of a smell drops about 50% within a second following the stimulation. Within a minute, the receptors may become almost insensitive to a given odor, but even though they have adapted to one scent, their sensitivity to other odors persists.

The olfactory receptor neurons are the only nerve cells in direct contact with the outside environment. Because of their exposed positions, these neurons are subject to damage; they are the only example of damaged neurons that are regularly replaced.

H Where are the olfactory receptors located?

Trace the pathway of an olfactory impulse from a receptor to the cerebrum.

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