The olfactory nerves consist of up to 20 bundles of unmyelinated axons. The neurons are bipolar, and their bodies lie in the olfactory epithelium. The short epithelial fibres end in about 20 non-motile cilia which increase the surface area for olfaction. There are about 1,000 genes which code for different olfactory receptors located in the cilia. Each cell produces only one type of receptor. Unlike other nerves, the olfactory receptor cells are constantly turned over, with an average life expectancy of 3 months.
The nerves pass superiorly through the cribriform plate, where they are covered by dura and pia mater. The dura is continuous with the periosteum of the nasal cavity bones. The pia mater forms a connective tissue sheath which coats the nerve bundles. After emerging into the anterior cranial fossa, the nerves end in the olfactory bulb where they terminate in glomeruli.
The olfactory bulb is continuous with the olfactory tract, which passes posteriorly and carries fibres from the bulb to the olfactory cortex.
a) I - Olfactory Nerve
