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Olfactory nerve (CN I)

The olfactory nerve (Latin: nervus olfactorius) is the first cranial nerve (CN I). It is purely sensory in function, as it transmits the sense of smell (olfaction). Although it is considered as a cranial nerve, the olfactory nerve is actually a collection of the olfactory receptor cell axons and is derived from the telencephalon, a part of the brain.


Olfactory pathway

The axons that compose the olfactory nerves originate in the olfactory epithelium of the nasal mucosa on the upper part of the nasal cavity from special sensory cells - neurosensory cells of the nasal mucosa or olfactory receptor cells. These cells are the first-order neurons in the olfactory pathway. There are around 3 million receptor cells in the olfactory epithelium.

The axons of the olfactory receptor cells continue as small nerve fascicles - olfactory fila, which in gross anatomy is referred to as the olfactory nerves (CN I). The olfactory fila pass through the cribriform plate of the ethmoid bone and reach the olfactory bulb of the brain, where the second-order neurons are located.

The axons of the second-order cells form the olfactory tract that serves as a connection between the olfactory bulb and the cerebral hemispheres. It continues as the olfactory trigone that lies in the olfactory groove of the frontal lobe. In the trigone, the fibers separate into olfactory striae, which synapse with the third-order neurons in the anterior perforated substance area.

The olfactory striae divide into lateral and medial olfactory striae. The axons of the third-order neurons reach the olfactory cortex and also synapse with the components of the limbic system.

The lateral olfactory striae project to the uncus and structures associated with the olfactory cortex, while the medial olfactory striae project to the hypothalamus and brainstem nuclei which are involved in such processes as saliva production and gastric contractions triggered by various odors and smells.