Anatomy of motor component

The motor branch of the facial nerve innervates the muscles derived from mesoderm of the second branchial arch.

The main motor nucleus of the seventh cranial nerve resides in the reticular formation at the level of the mid pons. The nucleus receives input from the cortex both directly, and indirectly through the reticular formation. The rostral part of the nucleus has both crossed and uncrossed cortical input whereas the more caudal part has just a crossed input.

From the nucleus, the motor root of the facial nerve travels rostrally to arc around the abducens nucleus before passing laterally to exit the brainstem at the cerebellopontine angle with the sensory root.

The sensory and motor roots exit the posterior cranial fossa through the internal auditory meatus with cranial nerve VIII. Passing laterally through the meatus, the facial nerve proper traverses the roof of the internal ear where the geniculate ganglion is given off. The motor fibres project posteriorly beyond this within the facial canal to emerge from the stylomastoid foramen. En route, a nerve is given off to the stapedius muscle to dampen excessive movement of the stapes.

After the stylomastoid foramen, the extratemporal facial nerve passes anteriorly into the superficial part of the parotid salivary gland. Here it divides into branches that supply the muscles of facial expression; the muscles innervated by individual branches are described in the submenu:

• temporal
• zygomatic
• buccal
• marginal mandibular
• cervical

Initially the temporal and zygomatic may form a discrete trunk in the parotid before diverging and the same may be true for the lower branches; there is great variability in the number and distribution of branches.

The bilateral upper motor neurone supply to the more superior facial muscles, and not their lower counterparts, results in sparing of the former after unilateral upper motor neurone lesions.