Corpus: Olfactory nerve

The olfactory nerve consists of special viscerosensitive, fine nerve fibers (olfactory fibers) that are essential for the perception of odors.

The olfactory nerve is not a cranial nerve in the strictest sense, as it is an extension of the brain, similar to the optic nerve. Unlike cranial nerves III to XII, it does not originate in the brainstem.

The olfactory nerve comprises about 20 to 25 small bundles of medullated nerve fibers. These fibers are the axons of the primary sensory cells located in the olfactory epithelium and constitute the first section of the olfactory pathway. They conduct electrical impulses from the olfactory cells in the nasal mucosa through the lamina cribrosa of the ethmoid bone.

Within the skull, the olfactory nerve fibers predominantly connect with the dendrites of mitral cells in the olfactory bulb. From there, impulses are transmitted via the olfactory tract to the primary olfactory cortex in the telencephalon. The olfactory nerve is the shortest of the cranial nerves.

A distinctive feature of the olfactory nerve fibers is the presence of olfactory ensheathing cells (OECs). These cells are a type of macroglia and resemble Schwann cells but do not form myelin sheaths around the axons. Instead, OECs bundle the axons of the olfactory cells into fascicles, guiding newly sprouting axons as they grow toward the olfactory bulb. This mechanism supports the continuous regeneration of the olfactory nerve fibers, necessary due to the limited lifespan of olfactory cells, which is 6 to 8 weeks.

The olfactory nerve fibers originate from ectodermal tissue and develop from the embryonic nasal placode.

6.1. Diagnostics

• Imaging: CT, MRI
• Functional tests: Olfactometry

6.2. Diseases

Damage to the olfactory nerve fibers can lead to a reduced sense of smell, manifesting as hyposmia or anosmia. Skull base fractures or other head traumas can cause the olfactory nerve fibers to be torn.

The olfactory nerve fibers create a pathway between the nasal cavity and the brain, through which nanoparticles and pathogens, such as those causing naegleriasis, can enter the intracranial cavity.

Tumors originating from the olfactory nerve fibers are rare. These include olfactory neurocytoma, olfactory neuroblastoma, and olfactory neuroepithelioma.