Olfactory nerve (CN I)

Nervus olfactorius

Also known as: CN I, first cranial nerve, fila olfactoria

The nerve of smell (purely sensory, SVA). About 20 bundles of unmyelinated axons from olfactory receptor neurons pass through the cribriform plate of the ethmoid to synapse in the olfactory bulb.

Overview

Fibre typeUnmyelinated, among the thinnest axons in the body

Number of fila≈20 per side

ExitCribriform plate of the ethmoid

Receptor turnover≈30–60 days (lifelong neurogenesis)

The olfactory "nerve" is really a collection of about 20 bundles (fila olfactoria). They are made of the very fine, unmyelinated axons of bipolar olfactory receptor neurons, whose cell bodies lie in the olfactory epithelium of the roof of the nasal cavity. The fila are ensheathed by olfactory ensheathing glia. They pass through the foramina of the cribriform plate of the ethmoid and synapse on mitral and tufted cells in the glomeruli of the olfactory bulb. The olfactory receptor neurons are unusual because they are continually replaced from basal stem cells throughout life.

From the bulb, the olfactory tract runs back along the orbital surface of the frontal lobe and divides into the lateral and medial olfactory striae. The lateral stria projects to the primary olfactory cortex: the piriform cortex, periamygdaloid cortex and entorhinal cortex in the medial temporal lobe (uncus). Olfaction is the only sensory modality that reaches the cortex without first relaying in the thalamus. Its close links with the amygdala and hippocampus explain how strongly smell evokes emotion and memory.

Location

Roof of the nasal cavity (olfactory epithelium), then the cribriform plate of the ethmoid, then the olfactory bulb in the anterior cranial fossa.

Parts & subdivisions

Functional components

Sensory only, SVA (smell). Its first-order neurons are the bipolar olfactory receptor neurons themselves. There is no separate sensory ganglion.

Clinical testing

Test each nostril separately, with the patient's eyes closed, using a familiar non-irritant odour such as coffee, peppermint or soap. Avoid ammonia, which stimulates trigeminal (CN V) nociceptors instead.

Lesion signs

Anosmia or hyposmia, usually noticed as "loss of taste" because flavour depends mainly on smell. Parosmia (distorted smell) and phantosmia (smelling odours that are not there) can occur, for example the olfactory auras of uncinate seizures.

Anatomy

Course

Olfactory epithelium (superior nasal concha, upper septum and roof), then fila olfactoria through the cribriform plate, wrapped by meninges, then the olfactory bulb (mitral and tufted cells). From there the olfactory tract runs in the olfactory sulcus, then the olfactory striae, and finally the piriform and entorhinal cortex and the amygdala.

Relations

The olfactory bulb and tract lie on the cribriform plate and gyrus rectus of the frontal lobe, beneath the orbital surface. The tract is close to the anterior cerebral and anterior communicating arteries.

Clinical significance

Fracture of the cribriform plate

Anterior cranial fossa fractures tear the fila as they pass through the cribriform plate. This causes anosmia and CSF rhinorrhoea (clear, glucose- and β2-transferrin-positive nasal discharge), with a risk of meningitis. Shearing of the fila in head injury, even without a fracture, is the commonest cause of post-traumatic anosmia.

Anosmia as an early sign of disease

Loss of smell is common after viral infection (e.g. COVID-19) and in chronic rhinosinusitis. It is an early prodromal feature of Parkinson disease and Alzheimer disease. An olfactory groove meningioma can cause unilateral anosmia (and, with optic nerve compression, Foster Kennedy syndrome). Congenital anosmia with hypogonadotropic hypogonadism is Kallmann syndrome, caused by failed migration of GnRH neurons with the olfactory placode.