Motor nucleus of facial nerve

At a glance

English

Motor nucleus of facial nerve

Latin (TA2)

Nucleus nervi facialis

System

Nerve

Region

Head

Side

Paired, left and right

About the motor nucleus of facial nerve

The motor nucleus of the facial nerve, or facial motor nucleus, is the source of all the facial nerve's motor fibres. Its large lower motor neurons supply the muscles of facial expression together with stapedius, stylohyoid and the posterior belly of digastric. Because these muscles develop from the second pharyngeal arch, the nucleus is classed with the branchial (special visceral efferent) motor nuclei, alongside the trigeminal motor nucleus and the nucleus ambiguus.

It lies in the lower part of the pons, deep in the ventrolateral tegmentum, above the level of the olive. Its axons follow a strange path. Instead of heading straight out, they first run backwards and upwards towards the floor of the fourth ventricle, loop over the abducens nucleus as the internal genu, and then turn forwards and downwards again to leave the side of the lower pons, passing on the inner side of the spinal trigeminal nucleus. This hairpin raises the facial colliculus in the ventricular floor. The motor root then joins the intermediate nerve, which carries the facial nerve's taste, sensory and parasympathetic fibres.

Inside the nucleus the cells are arranged by target. Those for the upper face, chiefly frontalis and the upper part of orbicularis oculi, sit in a dorsal group; those for the lower face sit in a ventral group. The upper motor neurons that drive them begin in the motor cortex and descend in the corticobulbar fibres through the internal capsule. The dorsal group receives fibres from both hemispheres, whereas the ventral group is driven mostly by the opposite hemisphere.

This split explains a basic bedside distinction. A lesion above the nucleus, such as a stroke affecting one hemisphere, weakens only the lower face on the opposite side, because the forehead still receives input from the other hemisphere; the brow can wrinkle and the eye can close. A lesion of the nucleus or of the nerve, by contrast, weakens the whole face on the same side, upper and lower. When no cause is found this lower motor neuron pattern is called Bell's palsy. A lesion of the nucleus itself often catches the abducens nucleus or nerve close by, adding a weak outward gaze.

Course and supply

Supplies

Holds the lower motor neurons for the muscles of facial expression, stapedius, stylohyoid and the posterior belly of digastric; its dorsal part (upper face) is driven by both hemispheres, its ventral part (lower face) by the opposite hemisphere.

Blood supply

Branches of the basilar artery, chiefly its pontine branches and the anterior inferior cerebellar artery.