Nasociliary nerve is a branch of the ophthalmic division of trigeminal nerve. It is intermediate in size as compared to the other two branches of the ophthalmic division; frontal nerve (larger) and lacrimal nerve (smaller).
The Nasociliary nerve enters the orbit through the lower part of the superior orbital fissure, between the two heads of the lateral rectus muscle.
It then crosses above the optic nerve (cranial nerve II), and runs forward along the upper margin of the medial rectus muscle.
Finally the nerve ends by dividing into the anterior ethmoidal nerve and the infratrochlear nerve.
Nasociliary nerve gives off a number of branches, as described below;
Communicating branch to the ciliary ganglion:
This communicating branch is actually composed of sensory fibers that arise in the eyeball. They pass to the ciliary ganglion via the short ciliary nerves. Then they pass through the ciliary ganglion without interruption and join the nasociliary nerve via the communicating branch.
Long ciliary nerves:
These are two or three small branches that arise from the nasociliary nerve while it crosses the optic nerve in the orbital cavity.
The long ciliary nerves pass forward alongside the short ciliary nerves and pierce the sclera of the eyeball. Then they continue forward between the sclera and the choroid to reach the iris.
These nerves contain sympathetic fibers for the dilator pupillae muscle. Thus they have a role in the pupillary reflex (light reflex).
Posterior ethmoidal nerve:
This branch of the nasociliary nerve supplies the sphenoidal and ethmoidal air sinuses.
It is a terminal branch of the nasociliary nerve that passes forward below the pulley (trochlea) of the superior oblique muscle and innervates the skin of the medial part of the upper eyelid. It also innervates the adjacent part of the nose.
Anterior ethmoidal nerve:
It is also a terminal branch of the nasociliary nerve. It passes through the anterior ethmoidal foramen to enter the anterior cranial fossa on the upper surface of the cribriform plate of ethmoid bone.
The nerve then enters the nasal cavity through a slit like opening near the crista galli. It supplies the mucous membrane here.
After supplying the mucous membrane, the anterior ethmoidal nerve appears on the face at the lower border of the nasal bone as the external nasal branch. Here it supplies the skin of the nose as far down as the tip.