Olfactory Nerve (CN I)

Special sensory pathway for smell · olfactory cleft to primary olfactory cortex

Reference Images

Diagram of the olfactory filaments, cribriform plate, bulb, tract, and striae — CN I CourseClick to enlarge

Illustrated overview of cranial nerve I anatomy and physiology — Anatomy OverviewClick to enlarge

Coronal MRI showing the olfactory bulbs, olfactory sulci, gyri recti, orbital gyri, crista galli, and falx — Coronal MRI LandmarksClick to enlarge

At a Glance

FunctionCN I conveys special sensory information for smell from the olfactory mucosa of the nasal cavity.

Location

Olfactory cleft in the roof of the nasal cavity.
Cribriform plate of the ethmoid bone.
Olfactory groove of the anterior cranial fossa near the midline.

What is actually CN I?

Strictly, the olfactory nerve comprises approximately 20 bundles of first-order axons—the olfactory filaments. The bulb, tract, and striae are extensions of the central nervous system rather than peripheral nerve.

Key imaging landmarks

Crista galli and cribriform plate.
Paired olfactory bulbs and tracts.
Gyrus rectus medially and orbital gyri laterally.
Olfactory sulcus between the gyrus rectus and orbital gyri.

Course

1 · Olfactory filaments

Bipolar olfactory sensory neurons lie in specialized neuroepithelium within the olfactory cleft. Their central axons form fine filaments that traverse the cribriform plate, pierce the dura, and synapse in the olfactory bulb.

2 · Olfactory bulb

The paired bulbs lie above the cribriform plate in the olfactory grooves. Mitral cells within the bulb form the principal second-order neurons.

3 · Olfactory tract

The tract courses posteriorly in the olfactory groove, inferior to the olfactory sulcus, between the gyrus rectus and orbital gyri.

4 · Olfactory striae

Near the anterior perforated substance, the tract expands at the olfactory trigone and divides into medial and lateral striae. The lateral stria projects toward the uncus and primary olfactory cortex; medial fibers connect with septal and hypothalamic pathways.

Pathway shorthandOlfactory mucosa → filaments → cribriform plate → bulb → tract → trigone → medial and lateral striae

Blood Supply & Imaging

Arterial supply

Olfactory mucosa and neurons: sphenopalatine plus anterior and posterior ethmoidal arteries.
Bulb, tract, and striae: small olfactory arterial branches, commonly arising from the anterior cerebral artery circulation.

MRI review

Coronal thin-section T2-weighted imaging best depicts the paired bulbs, sulci, and adjacent inferior frontal lobes.
Confirm symmetry, continuity, and expected position of the bulbs and tracts.
Inspect the olfactory grooves, cribriform plate, anterior cranial fossa, sinonasal cavity, and adjacent frontal lobes.

Radiology relevance

Congenital absence or hypoplasia may accompany anosmia.
Trauma can shear filaments at the cribriform plate.
Sinonasal or anterior cranial fossa lesions can interrupt the pathway.