Nucleus of abducens nerve

At a glance

English

Nucleus of abducens nerve

Latin (TA2)

Nucleus nervi abducentis

System

Nerve

Region

Head

Side

Paired, left and right

About the nucleus of abducens nerve

The abducens nucleus is where the sixth cranial nerve begins. It is a small rounded group of cells in the lower pons, lying close to the midline beneath the floor of the fourth ventricle, medial to the sulcus limitans like the other somatic motor nuclei of the eye. Fibres of the facial nerve curve over it as the internal genu, and together the two produce the facial colliculus, a swelling at the lower end of the medial eminence of the ventricular floor.

The nucleus holds two different populations. Motor neurons send their axons forwards through the pons to form the abducens nerve, which supplies the lateral rectus on the same side and turns that eye outwards. Interneurons send their axons across the midline, where they climb in the medial longitudinal fasciculus to the oculomotor nucleus of the opposite side and excite the motor neurons for its medial rectus, turning the other eye inwards. One nucleus thus moves both eyes towards its own side at once.

This arrangement makes the nucleus the final common centre for horizontal gaze. Stimulating it makes the two eyes turn together, the same way and through the same angle, and recordings show its two cell types fire in much the same pattern during fast and slow eye movements. Signals from the paramedian pontine reticular formation, the vestibular nuclei and higher gaze centres all converge on it.

Damage gives different pictures depending on the level. Injury to the nerve alone weakens outward movement of one eye. Injury to the nucleus stops both eyes looking towards that side, a horizontal gaze palsy, because motor neurons and interneurons are lost together; the facial nerve fibres looping round it are often caught as well, adding facial weakness on the same side. Injury to the medial longitudinal fasciculus produces internuclear ophthalmoplegia: on side gaze the eye that should turn in lags, although it can still turn in when the eyes converge on a near object, showing the muscle and its own nerve are intact. Multiple sclerosis and small strokes are typical causes.

Course and supply

Supplies

Holds motor neurons for the lateral rectus of the same side and interneurons that, through the opposite medial longitudinal fasciculus, drive the opposite medial rectus, producing conjugate horizontal gaze.

Blood supply

Paramedian branches of the basilar artery.