Olfactory receptor neuron

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Overview
An olfactory receptor neuron, also called an olfactory sensory neuron, is the primary transduction cell in the olfactory system.

In vertebrates
Humans have about 40 million olfactory receptor neurons. In vertebrates, olfactory receptor neurons reside on the olfactory epithelium in the nasal cavity. These 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.

Structure
Many tiny hair-like cilia protrude from the olfactory receptor cell's dendrite into the mucus covering the surface of the olfactory epithelium. The surface of these cilia is covered with olfactory receptors, a type of G protein-coupled receptor. Each olfactory receptor cell expresses only one type of olfactory receptor, but many separate olfactory receptor cells express olfactory receptors which bind the same set of odors. The axons of olfactory receptor cells which bind the same odors glomeruli in the olfactory bulb.

Function
A given olfactory receptor can bind to a variety of odor molecules with varying affinities. The activated olfactory receptor in turn activates the intracellular G-protein GOLF, and adenylate cyclase and production of Cyclic AMP 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, generating an action potential.

Regeneration
Individual olfactory receptor neurons are replaced approximately every 40 days by neural stem cells residing in the olfactory epithelium. The regeneration of olfactory receptor cells, as one of the only few instances of adult neurogenesis in the central nervous system, has raised considerable interest in dissecting the pathways for neural development and differentiation in adult organisms.

In insects
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.