Olfactory transduction and pathways

The sensation of smell, also called olfaction, is carried out by the olfactory nerve or cranial nerve I, and it comes from specialized sensory neurons located in the roof of the nasal cavity, within the nose.

The nasal cavity is made up of three regions. The first, is the nasal vestibule which is the area just inside the nostrils.

The second is the respiratory region, which is just above the nasal vestibule and is separated by three shelf-like bony structures; the superior, middle and inferior nasal conchae.

Lining the respiratory region is a layer of epithelial cells that create mucus to moisten the air and trap pathogens.

The third, is the olfactory region which is at the top of the nasal cavity and is involved in smelling.

Lining the olfactory region is a layer of special epithelial cells which form the olfactory epithelium.

The olfactory epithelium consists of olfactory receptor cells which are chemoreceptors that respond to molecules, called odorants.

The olfactory epithelium also contains columnar epithelial cells which support those olfactory receptor cells.

Below the olfactory epithelium is a layer of connective tissue called the lamina propria.

The lamina propria contains olfactory glands or Bowman’s glands which produce nasal mucus that protects the surface of the olfactory epithelium.

Below the lamina propria is the roof of the olfactory region which is formed by the cribriform plate of the ethmoid bone.

Now, if we zoom in a bit, we can see that the olfactory receptor cells are bipolar neurons, meaning that they have two projections outside the olfactory epithelium.

One projection carries their dendrites to the bottom of the epithelium and gives off hair-like structures called the olfactory hairs, or cilia.

These cilia protrude beyond the nasal mucosa so that they can come into contact with odorants trapped by the mucus.

The other projection is an axon that joins up with axons of other receptors to form tiny olfactory nerves - collectively called cranial nerve 1.

These tiny olfactory nerves pass through small openings of the cribriform plate of the ethmoid bone, called the olfactory foramina, to enter inside the olfactory bulb.

The olfactory bulb contains second order neurons in the olfactory pathway, and it sends information through the olfactory tract to the olfactory cortex in the temporal lobe.