Olfactory receptor neurons

An olfactory receptor neuron, also called an olfactory sensory neuron, is the primary transduction cell for olfaction in the olfactory system. Humans have about 40 million olfactory receptor neurons. In verebrates, olfactory receptor neurons reside on the olfactory epithelium in the nasal cavity. The cells are bipolar neurons with a dendrite facing the interior space of the nasal cavity and an axon that travels along the olfactory nerve to the olfactory bulb.

Many tiny hair-like cilia protrude from the olfactory receptor cell's dendrite into the mucus covering the surface of the olfactory epithelium. These cillia contain olfactory receptors, a type of G protein-coupled receptor. Each olfactory receptor cell contains only one type of olfactory receptor, but many separate olfactory receptor cells contain the same type of olfactory receptor. The axons of olfactory receptor cells of the same type converge to form glomeruli in the olfactory bulb.

According to prevaling theory, olfactory receptors are activated by binding to structures on the odor molecule. The activation of the olfactory receptor activates the intracellular G-protein GOLF, which activates the enzyme adenylate cyclase. Cyclic AMP, produced by adenylate cyclase, binds to and opens ion channels in the cell membrane, resulting in an influx of sodium and calcium ions into the cell. This influx of positive ions causes the neuron to depolarize, starting an electrical signal called an action potential.

Individual olfactory receptor neurons die out and are replaced approximately every 40 days, so that there is a constant turnover of neurons in the olfactory epithelium. These cells are replaced by stem cells residing in the olfactory epithelium. Most neurons in other parts of the nervous system cannot be replaced, so research focusing on how stem cells differentiate into olfactory sensory neurons could lead to the use of these stem cells to replace damaged or degenerated neurons in the brain, spinal cord, or other parts of the nervous system.

In insects, olfactory receptor neurons typically reside on the antenna. Much like in vertebrates, axons from the sensory neurons converge into glomeruli in the antennal lobe.

See also: Chemoreceptor, Sensory receptor