Nucleus of trochlear nerve

At a glance

English

Nucleus of trochlear nerve

Latin (TA2)

Nucleus nervi trochlearis

System

Nerve

Region

Head

Side

Paired, left and right

About the nucleus of trochlear nerve

The trochlear nucleus is the smallest of the eye-movement nuclei and the origin of the fourth cranial nerve. It lies in the lower midbrain at the level of the inferior colliculus, just below the oculomotor complex, in the grey matter in front of the cerebral aqueduct. Like the other somatic motor nuclei it is close to the midline, and it sits in a notch on the dorsal surface of the medial longitudinal fasciculus, almost embedded in that bundle.

Its axons take a path unlike any other cranial nerve. Instead of running forwards, they curve backwards and downwards around the aqueduct and cross the midline in the superior medullary velum, the thin roof of the upper fourth ventricle. They then leave the back of the brainstem just below the inferior colliculus. The trochlear nerve is therefore the only cranial nerve that emerges from the dorsal surface and the only one whose fibres all cross, so each nucleus supplies the superior oblique muscle of the opposite eye.

The midbrain also houses the oculomotor nucleus and the mesencephalic nucleus of the trigeminal nerve, which carries stretch signals from the chewing muscles and helps set biting force. The trochlear nucleus is the motor part of this set lying lowest in the midbrain, and its link through the medial longitudinal fasciculus allows it to work with the vertical gaze centres and the vestibular nuclei. The superior oblique turns the eye inwards (intorsion) and helps it look down, especially when it is already turned towards the nose.

Because the fibres cross before they leave, a lesion of the nucleus weakens the superior oblique of the opposite eye, whereas a lesion of the nerve after it emerges weakens the eye on the same side. Either way the person sees double, with one image above and tilted relative to the other, worst when looking down and in, as on stairs or reading. They often tilt the head away from the affected eye to compensate. The thin nerve's long course behind the brainstem makes it easily hurt in head injury, sometimes on both sides where the fibres cross in the velum.

Course and supply

Supplies

Holds the motor neurons for the superior oblique of the opposite eye, which intorts the eye and helps depress it, especially in adduction.

Blood supply

Branches of the superior cerebellar artery and paramedian branches of the basilar and posterior cerebral arteries.