Olfrec.com

Olfactory receptors

This domain, olfrec.com, was created to host a database with information about olfactory receptors. The database now exists, but has not reached a state in which it is presentable to the public.

An olfactory receptor is a receptor in the nose. It is sensitive to certain chemicals in the airstreams which passes through the nose. (I write "airstreams" because a chemical present in the inward airstream may be perceived differently from the same chemical in the outward airstream.

Most species of mammal have 1,000 or more different olfactory receptors. They differ in the protein which is present in them and which detects a chemical (known as a "ligand") or class of chemicals. Most receptor proteins detect more than one ligand, and most detectable ligands are detected by more than one receptor.

Humans have only about 800 genes coding for the characteristc proteins of olfactory receptors. However, less than half of these 800-odd genes actually code for functioning olfactory receptor proteins. This is probably a consequence of reduced selection pressure in humans; for a human, a slightly defective sense of smell is less likely to be fatal than it is for most mammalian species. The non-functioning genes are sometimes known as "pseudogenes".

All known olfactory receptor genes form a single "gene family"; that is, they all descend from a single ancestor. This is the largest known gene family.

The sense of smell

The "sense of smell" is traditionally said to be one of "the five senses", the other four being taste, sight, hearing, and touch. This is an oversimplification. We also have a sense of balance, and can perceive heat, cold, pain, etc..

These pages will only be concerned those senses that detect chemical substances. These fall into three categories:

  1. "The sense of smell". This is the detection of ligands in the nose by the olfactory receptors. These ligands are necessarily volatile. It is the most complicated and the least understood of the three categories, because
  2. "The sense of taste". This is the detection of ligands in the mouth by the sense receptors on the tongue. These ligands are water-soluble. There are five known such detectors:
  3. Sometimes known as piquancy, pungency, chemesthesis or spiciness. This is the "accidental" detection of chemicals by neuroreceptors, where that is not the purpose for which neuroreceptor evolved. Some examples of piquancy are

The terminology is confusing. When we say "this tastes salty", we are indeed referring to "the sense of taste", involving sensors on the tongue. When we say "this tastes of aniseed" we are referring to the "sense of smell", involving olfactory receptors in the nose. And when we say "this tastes hot" we are describing an effect on TRPV1 neurotransmitters, present in all mucous membranes (and elsewhere, but the capsaicin is less likely to reach them elsewhere).

These pages will be mostly concerned with the sense of smell.

For further information

on olfactory receptors and their ligands, see