Facial Nerve (Seventh Cranial Nerve)
The facial nerve (CN VII) is a complex, mixed sensory and motor nerve. The motor root contains special visceral efferent fibers that innervate the muscles of facial expression. The sensory root conveys the sense of taste from the anterior two-thirds of the tongue and sensation from the external auditory meatus and the retroauricular skin. It also provides preganglionic parasympathetic innervation by way of the sphenopalatine and submandibular ganglia to the lacrimal, submaxillary, and sublingual glands.
The motor nucleus of CN VII is a cigar-shaped column 4 mm long, located in the caudal third of the pons. It is ventrolateral to the CN VI nucleus, ventromedial to the spinal nucleus of CN V, and dorsal to the superior olive (Fig 3-22; see also Fig 3-2A). The signal for facial movement starts in the primary motor cortex in the precentral gyrus.
The dorsal motor subnucleus controls the upper half of the face and receives corticobulbar input from both cerebral hemispheres, whereas the lateral subnucleus controls the lower half of the face and receives corticobulbar input from the contralateral cerebral hemisphere. Therefore, pathology involving the CN VII nucleus would affect only the contralateral lower face; peripheral CN VII pathology causes an ipsilateral hemifacial palsy.
Figure 3-22 Cross section of the pons at the level of CN VI (abducens nerve) nucleus. CS = corticospinal tract; MLF = medial longitudinal fasciculus; PPRF = pontine paramedian reticular formation.
(Illustration by Sylvia Barker.)
The facial nerve has several important anatomical relationships with adjacent structures. Fibers from the motor nucleus course dorsomedially to approach the floor of the fourth ventricle and then ascend immediately dorsal to the CN VI nucleus. At the rostral end of the CN VI nucleus, the main facial motor fibers arch over its dorsal surface (forming the internal genu of CN VII) and then pass ventrolaterally between the spinal nucleus of CN V and the CN VII nucleus to exit the brainstem at the pontomedullary junction. The bulge formed by the CN VII genu in the floor of the fourth ventricle is the facial colliculus (see Figs 3-2A, 3-22).
CNs VII and VIII (the acoustic nerve) pass together through the lateral pontine cistern in the cerebellopontine angle and enter the internal auditory meatus in a common meningeal sheath.
The main branch of CN VII exits the stylomastoid foramen just behind the styloid process at the base of the mastoid. It then passes through the superficial and deep lobes of the parotid gland and divides into the superior temporofacial branch (which further divides into the temporal, zygomatic, and buccal subbranches) and the cervicofacial branch. Commonly, the temporal branch supplies the upper half of the orbicularis oculi muscle, and the zygomatic branch supplies the lower half, although the inferior orbicularis is sometimes innervated by the buccal branch. The frontalis, corrugator supercilii, and pyramidalis muscles are usually innervated by the temporal branch.
The temporal (or frontal) branch of the facial nerve crosses the zygomatic arch superficially at the junction of the anterior one-third and posterior two-thirds of the arch. It then enters the more superficial layer of the temporoparietal fascia while staying below the superficial musculoaponeurotic system (SMAS). A good approximation of the course of the nerve across the zygomatic arch follows the point at which a line between the tragus and the lateral eyelid commissure is bisected by a line that begins at the earlobe. The nerve can be injured in the context of perizygomatic or temple surgical approaches, such as Tenzel or Mustardé semicircular flap reconstruction of the eyelid, temporal artery biopsy, and cosmetic forehead and midface surgery.
Tear Reflex Pathway
Reflex lacrimation is controlled by afferents from the sensory nuclei of CN V. The tear reflex arc is shown in Figure 3-23. The efferent preganglionic parasympathetic fibers pass peripherally as part of the nervus intermedius and divide into 2 groups near the external genu of CN VII. The lacrimal group of fibers passes to the pterygopalatine ganglion in the greater superficial petrosal nerve. The salivatory group of fibers projects through the chorda tympani nerve to the submandibular ganglion to innervate the submandibular and sublingual salivary glands.
The greater superficial petrosal nerve extends forward on the anterior surface of the petrous part of temporal bone to join the deep petrosal nerve (sympathetic fibers) and form the nerve of the pterygoid canal (vidian nerve). This nerve enters the pterygopalatine fossa; joins the pterygopalatine ganglion; and gives rise to unmyelinated postganglionic fibers that innervate the globe, lacrimal gland, glands of the palate, and nose. The parasympathetic fibers destined for the orbit enter it via the superior orbital fissure, along with branches of the ophthalmic nerve (CN V1). Here, they are joined by sympathetic fibers from the carotid plexus and form a retro-orbital plexus of nerves, whose rami oculares supply orbital vessels or enter the globe to supply the choroid and anterior segment structures. Some of these fibers enter the globe directly; others enter via connections with the short ciliary nerves. The rami oculares also supply the lacrimal gland.
Emotional lacrimation is mediated by parasympathetic efferent fibers originating in the superior salivatory nucleus and the lacrimal nucleus in the caudal pons, both of which lie posterolateral to the motor nucleus. The lacrimal nucleus receives input from the hypothalamus, mediating emotional tearing; there is also supranuclear input from the cortex and the limbic system.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.