Olfactory nerve (I)

At a glance

English

Olfactory nerve (I)

Latin (TA2)

Nervus olfactorius (I)

System

Nerve

Region

Head

Side

Paired, left and right

About the olfactory nerve (i)

The nerve of smell is cranial nerve I, the olfactory nerve. Unlike most nerves it is not a single cord. It is made of many small bundles of unmyelinated axons on each side, the olfactory fila, which together count as the nerve. It is the shortest cranial nerve and, like the optic nerve, it connects straight to the forebrain rather than to the brainstem.

Its cells are receptor neurons for smell lying in the olfactory epithelium, a small patch of yellowish mucosa in the roof of the nasal cavity, covering the upper part of the septum and the superior concha. Each neuron sends a fine hair-bearing process to the surface, where odour molecules dissolved in the mucus bind to receptor proteins and set off electrical signals. The axon runs the other way, joining others in bundles wrapped by special glial cells. The bundles pass up through the many small holes of the ethmoid's cribriform plate, pierce the dura and arachnoid, and end in the olfactory bulb on the underside of the frontal lobe. There they synapse with mitral and tufted cells, which send their axons back along the olfactory tract to the smell-processing cortex.

The receptor neurons develop from the nasal placode, a thickening of the embryonic surface ectoderm. They are unusual among nerve cells in being replaced throughout life from stem cells in the epithelium, so the nerve can partly regrow after injury.

Each nostril is tested separately with a familiar, non-irritating smell, or with scratch cards that release set odours. Loss of smell (anosmia) has many causes besides nerve damage, including blocked nasal passages and viral infections. Head injury, especially a blow to the back of the head that jolts the frontal lobes, can shear the fila as they cross the cribriform plate, and fractures here may leak cerebrospinal fluid from the nose. Tumours under the frontal lobe and meningitis also affect it. Flavour depends heavily on smell, so people often notice the loss as a change in taste. Sensitivity falls with age and is often reduced early in Parkinson's and Alzheimer's disease. Pain and irritation from the nose are carried by the trigeminal nerve, not this one.

Course and supply

Arises from

Receptor neurons for smell in the olfactory epithelium, in the roof of the nasal cavity, on the upper septum and superior concha.

Ends in

Crosses the ethmoid's cribriform plate and ends by synapsing in the olfactory bulb.

Supplies

Special sensory: carries the sense of smell from the nasal cavity to the olfactory bulb.