Trochlear nerve

Number: IV

Name: Trochlear

Sensory, motor, or both: motor

Origin/Target: Dorsal aspect of Midbrain

Nuclei: Trochlear nucleus

Function: Innervates the superior oblique muscle, which depresses, rotates laterally, and intorts the eyeball. Located in the superior orbital fissure.

Description:

The trochlear nerve, a purely motor nerve, innervates the superior oblique muscle of the eyeball, facilitating downward and lateral eye movements.

Trochlear nerve nucleus:

Located in the midbrain, the trochlear nucleus is positioned inferiorly to the oculomotor nucleus, at the level of the inferior colliculus. Its fibers exit posteriorly, to emerge on the posterior surface of the midbrain. The nucleus receives corticonuclear fibers from both cerebral hemispheres, tectobulbar fibers connecting it to the visual cortex via the superior colliculus, and inputs from the medial longitudinal fasciculus, establishing connections with the nuclei of cranial nerves III, VI, and VIII. Click here to read more about the nucleus.

Course of the Trochlear Nerve:

After emerging dorsally, the slender trochlear nerve decussates with its contralateral counterpart soon after emerging from the midbrain, then wraps around the midbrain and continues anteriorly. It first travels through the lateral wall of the cavernous sinus, prior to entering the orbit via the superior orbital fissure.

Innervation and Actions:

The trochlear nerve innervates the superior oblique muscle, which functions to depress, abduct, and internally rotate the eye. For instance, the superior oblique helps you look downward and outward, crucial for reading or descending stairs.

Clinical Dysfunction:

Lesion of the trochlear nerve leads to superior oblique palsy, resulting in vertical diplopia (double vision) and difficulty moving the eye downward, particularly when looking inward. Patients may adopt a compensatory head tilt away from the affected side to mitigate double vision and maintain proper balance.