Olfactory receptor neuron

The olfactory receptor neuron is the cell you smell with. It is a bipolar neuron, meaning it has one process coming off each end of the cell body, and that cell body lives in the olfactory epithelium, a patch of specialized tissue high in the back of each nasal cavity.

Its two ends do opposite jobs. Upward, toward the air, come the dendrites, small branches that reach the surface of the epithelium and there intercept odor particles using G protein-coupled receptors, membrane proteins that recognize a molecule and fire a signal inward. Downward comes the axon, an unmyelinated cable that passes through the cribriform plate of the ethmoid, a bone perforated like a sieve in the roof of the nose, and reaches the olfactory bulb, the first relay for smell in the brain. It is a neuron with one end in the air and the other in the olfactory bulb, inside the skull.

There are enormous numbers of them: six to ten million in each nasal cavity. And they are replaced: at the base of the epithelium sit stem cells that take over for them one by one.