Cranial Nerve 1 – Olfactory nerve

Cranial nerve 1 is the olfactory nerve, and it is all about the special sense of smell. The fibers in CN 1 are SVA, special visceral afferent fibers dedicated only to olfaction. Check out the Intro to cranial nerves post for more information about what it means to have SVA fibers.

Sense of smell

The human sense of smell is pretty weak compared to our sense of vision, or to what is had by our friendly canine companions. But, most of that has to do with available surface area for the neurons that make up cranial nerve 1. While not as prominent as our sense of vision, for example, the sense of smell is hard-wired deeply into our brain in ways other cranial nerves are not. Let’s discuss some of the anatomy related to cranial nerve 1 to see how and why.

Olfactory region of the nasal cavity

Though technically still a cranial nerve, the neurons that begin the sense of smell (first-order neurons at the start of a chain leading back to the brain) are housed with the olfactory region of the nasal cavity. This region occupies the upper ⅓ of the nasal cavity. The rest of the nasal cavity is devoted to more respiratory functions.

Surrounded by supporting epithelial cells, the neurons belonging to the olfactory nerve are literally embedded in the thin mucosal lining of the olfactory region of nasal mucosa. Recall that most cranial nerve neurons are housed within specific locations within the brain stem. CN 1’s set up is therefore rather unique. Upwards of 25 million olfactory receptor cells are on each side of the nose! They do have the capacity to regenerate. 

The sensory receptor cells have a bipolar-shaped nerve cell body, meaning it has a fat little body and two arms coming off the top and bottom ends of it (central and peripheral processes). The process on the bottom, or the one related to the olfactory mucosa, has long cilia that radiate out across the mucosal surface. As odors and chemicals are dissolved on the mucosa the nerves of CN 1 are triggered, they are chemoreceptors after all. 

Transmission electron microscope (TEM) image of an olfactory receptor cell (blue) and cilia in nasal mucosa (purple).

Ethmoid bone and olfactory bulb

They send the information upwards, through unmyelinated axons which travel through tiny bony canals, or foramina, in the cribriform plate of the ethmoid bone. There, they are met by the olfactory bulb, an outpocket of the telencephalon, and home to the second-order neurons that will pick up the information and relay it deeper into the brain. The first-order neurons that began in the nasal cavity synapse in olfactory glomeruli within the bulb, on mitral and tufted cells (second-order neurons). 

Deeper brain connections

From the bulb the information proceeds further into the brain, forming the olfactory tracts. The olfactory tracts terminate at the medial and lateral olfactory striae, cross or decussate to the contralateral or opposite side of the brain, by way of the anterior commissure

Fibers then connect to the primary olfactory cortex of the brain (found near the uncus, a bump on the parahippocampal gyrus), and the amygdaloid nucleus. These regions form connections with the thalamus, the primary sensory relay station in the brain. The thalamus sends the information to the orbitofrontal cortex where the information finally reaches conscious perception and interpretation.

Smell is hardwired to our limbic system, by way of the connections to the parahippocampal gyrus and the amygdala. The intimate relationship of smell information passing directly into the forebrain is unique among cranial nerves. The connection between smell and emotions are strong for this reason. Baking bread, burning leaves, bad vodka, all conjure up not just what those items smell like, but how those items make you feel, too. 

Sense of smell

It is possible to lose your sense of smell, though it is usually temporary unless significant damage to the olfactory mucosa or nerves has occurred. Loss of the sense of smell is called anosmia. Commonly this is the result of fracture of the cribriform plate of the ethmoid bone, as might happen from a blow to the eyes or nose. Feeling stuffy, or having a lot of snot and mucous in your sinuses can also dampen the sense of smell since odors don’t dissolve quite so well through all the snot. In these cases, the loss of smell will typically be unilateral, and on the same side.

Lesions of the olfactory n.

Lesions of CN1 within the cranial cavity, perhaps due to meningitis, meningiomas, or abscesses of the frontal cerebral lobes near the olfactory apparatus, may also damage the olfactory apparatus and maybe even impact CN2 which is nearby. Persons who have lesions in the temporal lobes (housing the limbic system) may experience olfactory hallucinations, or the false sense of smell.

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