The olfactory system transmits smell from detection of odorants at the olfactory epithelium in the nasal cavity to the primary olfactory cortex. It is phylogenetically the most ancient sensory tract and terminates on primitive cortical areas.
Primary olfactory neurons are bipolar with the distal processes sensing airborne chemicals coming into contact with the olfactory mucosa and the proximal processes projecting to the olfactory bulb. These proximal processes pass through the cribriform plate of the ethmoid bone on their way to synapsing with the olfactory bulb. The olfactory nerve (CN I) refers to the sensory fibers (formed from the primary olfactory neurons) passing between the olfactory mucosa and the olfactory bulb. The olfactory nerves are the shortest cranial nerves in the body. It should be noted that the olfactory bulb and tract are themselves a projection of the central nervous system and are frequently incorrectly considered part of the olfactory nerve.
The olfactory bulb and tract pass through in the olfactory sulcus on the orbital surface of the frontal lobe. These tracts synapse at the primary olfactory cortex which includes the anterior olfactory nucleus (AON), amygdala, olfactory tubercle, piriform cortex and rostral entorhinal cortex.
The piriform cortex is by far the largest of these and is located on the uncus of the parahippocampal gyrus. It is thought to project via the medial dorsal nucleus of the thalamus to the orbitofrontal cortex where conscious discrimination of smell can occur.
Differences from other senses
It has two major differences from other sensory modalities. First, it is the only sensory modality which is not relayed to the thalamus prior to synapsing at the cortex. Second, the cortical olfactory areas are part of the evolutionarily older allocortex compared with the neocortical regions processing other sensations.