The Sense of Smell

Smell depends on sensory receptors that respond to airborne chemicals. In humans, these chemoreceptors are located in the olfactory epithelium — a patch of tissue about the size of a postage stamp located high in the nasal cavity. The olfactory epithelium is made up of three kinds of cells:

The sequence of events

How can one kind of cell enable us to discriminate among so many different odors?

Humans can discriminate between hundreds, perhaps thousands, of different odorant molecules, each with its own structure. How can one kind of cell provide for this?

Now we have a mechanism for discriminating among a thousand or so odorants. However,

This provides the basis for combinatorial diversity. It would work like this:

Assume that The brain then would interpret the two different patterns of impulses as separate odors.
This mechanism appears capable of discriminating between as many as a trillion different mixtures of odorants.

Sperm Also Express Odorant Receptors

Some odor receptors are also expressed in human sperm. These enable sperm to swim towards certain chemicals (a positive chemotaxis). Whether similar chemicals are released in the vicinity of the egg and thus increase the chances of fertilization remains to be proven. (Human sperm also display a positive chemotaxis to a gradient of progesterone which does propel them toward the egg - Link.)

Many mammals have a separate system for detecting pheromones — airborne molecules that elicit mating behaviors. Link to a discussion.

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23 October 2022