Olfactory Nerve (CN I): Pathway, Testing and Anosmia

Nuclei and fibre types

The olfactory nerve carries a single fibre type: special visceral afferent (SVA) fibres for smell. It has no brainstem nucleus. Its first-order neurons are bipolar olfactory receptor cells lying in the olfactory epithelium, a patch of specialised mucosa over the roof of the nasal cavity, the upper nasal septum and the superior nasal concha.

Each receptor cell sends a dendrite to the mucosal surface, where non-motile cilia carry the odorant receptors, and an unmyelinated axon upward. The axons gather into roughly 20 bundles, the olfactory fila, and these fila are the olfactory nerve proper. Basal stem cells in the epithelium replace receptor cells throughout adult life, a rare example of neuronal turnover.

The fila synapse in the olfactory bulb on mitral and tufted cells, the second-order neurons. The bulb and tract grow out from the forebrain (telencephalon), so strictly they are central nervous tissue, not peripheral nerve.

Emergence and skull foramen

CN I does not emerge from the brainstem. With the optic nerve, it is one of two cranial nerves attached to the forebrain rather than the brainstem. The fila pass up through the small foramina of the cribriform plate of the ethmoid bone, pierce the dura and arachnoid, and enter the underside of the olfactory bulb in the anterior cranial fossa.

Each filum takes a short sleeve of meninges with it. That sleeve is the route by which nasal infection can reach the subarachnoid space, and the site of a CSF leak when the cribriform plate fractures.

Course and branches

The smell pathway reaches the cerebral cortex without a thalamic relay, which sets it apart from every other sensory pathway. The olfactory bulb rests on the cribriform plate beneath the orbital surface of the frontal lobe. The olfactory tract runs back from the bulb in the olfactory sulcus, lateral to the gyrus rectus.

At the anterior perforated substance the tract splits into two olfactory striae:

• Lateral olfactory stria: the main pathway, ending in the primary olfactory cortex, which comprises the piriform cortex, the periamygdaloid cortex and the uncus of the temporal lobe, beside the entorhinal cortex.
• Medial olfactory stria: runs to the septal area, and some fibres cross in the anterior commissure to the opposite side.

Onward links to the orbitofrontal cortex, hypothalamus and limbic system tie smell to appetite, emotion and memory. The nerve has no named peripheral branches.

Clinical testing

Smell is tested one nostril at a time using familiar, non-irritant odours. With the eyes closed and one nostril occluded, the patient is asked to detect and then name a scent such as coffee, peppermint, cloves or soap. Inspect the nose first, because blocked nasal passages reduce smell without any nerve damage.

Avoid ammonia and other pungent agents. They stimulate free trigeminal nerve endings in the nasal mucosa, so a patient with complete anosmia still reacts to them and the test is falsely reassuring. Standardised scratch-and-sniff identification tests give a quantitative score when one is needed.

Lesions and palsies

A CN I lesion causes anosmia (absent smell) or hyposmia (reduced smell), and patients often describe it as loss of taste. Flavour depends largely on smell, while the basic tastes of sweet, salt, sour, bitter and umami, carried by CN VII, IX and X, are preserved.

Foster Kennedy syndrome combines ipsilateral anosmia and optic atrophy, from direct compression, with contralateral papilloedema from raised intracranial pressure; an olfactory groove meningioma is the classic cause. Irritation of the uncus produces uncinate fits: temporal lobe seizures heralded by a brief, usually unpleasant, smell hallucination.