Facial movement is a complex operation that involves input from both pyramidal and extrapyramidal areas.³ The extrapyramidal circuits involve the premotor cortex, parietal cortex, temporal cortex, cingulate gyrus, hippocampus, amygdala, hypothalamus, basal ganglia, cerebellum, and midbrain tegmentum. These varied connections help explain the tight association of facial movement with emotions and the limbic circuit, as well as the apparent paradox seen in persons who have lost voluntary control of facial movement yet may laugh normally upon hearing a joke.

The cortical area that subserves volitional facial movement is located in the lower third of the precentral gyrus, with representation of the upper face situated rostral to the lower face. These descending motor fibers unite in the internal capsule, decussate in the pons, and terminate in the facial nucleus, which is located in the lateral tegmentum of the caudal pons.⁴ Both crossed and uncrossed fibers descend to the upper face, whereas only crossed fibers descend to the lower face (Fig. 1). Ascending sensory fibers traveling in the medial lemniscus synapse in the ventral posterior medial nucleus of the thalamus. Medial lemniscal fibers originate in the nucleus solitarius, the gustatory gray of the spinal tract of nerve V, and the main sensory nucleus of nerve V. Much of the primary sensory information is interpreted by the thalamus, while other information is relayed to the postcentral gyrus of the parietal cortex for more complex sensory interpretation. An extensive number of connections within the brainstem are also made, allowing for a richly diverse pattern of reflexes. Lastly, muscle spindle information probably reaches the cerebellum from the mesencephalic nucleus of nerve V in the caudal midbrain.

The facial motor nucleus extends approximately 3 to 4 mm in length; its caudal portion is an extension of the nucleus ambiguus, and its rostral portion reaches the abducens nucleus dorsomedially and the motor nucleus of nerve V laterally.⁵ In its rostral extent is found a separate, discrete bunch of large multipolar cells representing the accessory facial nucleus.⁶ Both nuclei control motor function. Running rostromedially and dorsally, these motor fibers sweep around the abducens nucleus to form the internal genu, which bulges into the fourth ventricle. On its course laterally to exit the brainstem, the nerve passes by the superior salivatory nucleus, where visceral motor fibers destined for the submandibular and sphenopalatine ganglia are picked up by the facial nerve.^3,7

Sensory information is transported to the brainstem via small, medium, and large fibers. Each specializes in a different sensory modality, and each synapses on different sensory nuclei in the brainstem or cervical cord. The terminology of the brainstem can be confusing because many of the tracts and nuclei serve many different cranial nerves (see next paragraph). Yet, these structures are often named after the cranial nerve with the largest input (i.e., descending tract of nerve V).

The descending tract of nerve V and associated gray receive small fiber somatic pain and temperature input from cranial nerves V, VII, IX, and X. The solitary tract and nucleus receives small fiber visceral information from the nasal mucosa and anterior two thirds of the tongue, as well as information from nerves IX and X. The main sensory nucleus of nerve V receives medium fiber somatic input for light touch and vibration from both nerves V and VII. Large fiber muscle spindle information is much less clearly defined, but many believe that the mesencephalic nucleus of nerve V receives this input.^3,8

Within the cerebellopontine angle, the somatic motor component of the facial nerve (motor root) is closely approximated to the nervus intermedius and the auditory nerve. Enclosed in a leptomeningeal sheath that contains an extension of the subarachnoid space, the three nerves enter the internal auditory meatus, traveling laterally.⁹ At the point of entry, the nervus intermedius is between nerve VIII and the motor root. The motor root and nervus intermedius enter the facial canal (aqueduct of Fallopius) within the substance of the temporal bone and then widen to form the geniculate ganglion. It is in the geniculate ganglion where the first parts of the nervus intermedius (see next section) diverge, while other parts continue with the motor nerve (Fig. 2).

The motor nerve then turns posteriorly at a sharp angle, forming the external genu. After the genu, the nerve enters the tympanic (horizontal) portion of the facial canal. This is located below and medial to the horizontal semicircular canal and above the pyramidal eminence that houses the stapedius muscle. The proximal part of this segment is housed behind a fragile, easily fractured tympanic wall; this is a common site of facial nerve damage in traumatic head injuries. Not infrequently, the nerve prolapses into the oval window niche, partly or completely concealing the footplate of the stapes. This anomaly is of clinical importance when the stapes is removed or manipulated during ear surgery.¹⁰

At the posterior aspect of the middle ear, the nerve curves downward, at which point a branch innervates the stapedius muscle. This minute muscle acts to dampen wide acoustic input surges from the delicate bony transmission system. The downward course takes it into the anterior wall of the mastoid process of the temporal bone. It is within this segment that the chorda tympani arises and marks the final separation of fibers that form the nervus intermedius. The chorda tympani transmit afferent taste sensation and efferent parasympathetic fibers to the submandibular ganglion (also known as the submaxillary ganglion; see Fig. 2).

The motor nerve continues in the temporal bone until it exits at the base through the stylomastoid foramen. Three branches immediately diverge to innervate the posterior auricular, posterior belly of the digastric, and stylohyoid muscles. It is also in this location that connections are made with nerves IX and X, the auriculotemporal branch of nerve V, and the cervical plexus.

Running anteriorly approximately 2 cm from the stylomastoid foramen the nerve divides into upper and lower divisions, which traverse the superficial portion of the parotid gland (Fig. 3). Pathology of the parotid may impair facial nerve function. Emerging from the parotid gland, the nerve passes over the fascial plane of the masseter muscle, with variable communications between upper and lower divisions. This rich anastomotic network becomes clinically important in cases of aberrant regeneration and in the surgical treatment of synkinetic movements. Eventually the nerve divides further into its five major branches: temporal, zygomatic, buccal, mandibular, and cervical. The distal ends of these branches anastomose with those of the trigeminal nerve; the clinical significance of these communications is open to speculation.¹¹

All of the muscles of facial expression are supplied by the facial nerve, with the exception of the levator palpebrae superioris, which is supplied by the oculomotor nerve.

The nervus intermedius of Wrisberg is made up of all the components of the facial nerve except the somatic motor component, as discussed above (Fig. 2). Broadly categorized, information is transmitted for visceral motor function and for visceral and somatic sensation.

The superior salivatory nucleus is the origin of the visceral motor preganglionic parasympathetic nerve destined for both the submandibular and sublingual glands via postganglionic fibers from the submandibular ganglion. These fibers run in the nervus intermedius through the geniculate ganglion, where they follow the somatic motor nerve through the facial canal. Along its downward course within the temporal bone, the preganglionic fibers separate to run with the chorda tympani. The course of the chorda tympani is complicated, running upward and anteriorly over the incus, under the malleus, across the tympanic cavity, and through the petrotympanic fissure to join with the lingual nerve. Finally these parasympathetic fibers reach their destination in the submandibular ganglion.

The lacrimal nucleus, often considered part of the superior salivatory nucleus, is the origin of the visceral motor preganglionic parasympathetic nerve destined for the lacrimal gland via postganglionic fibers from the sphenopalatine ganglion.⁹ These fibers run in the nervus intermedius, traverse the geniculate ganglion, and form the greater superficial petrosal nerve. On its course, the greater superficial petrosal nerve joins the deep petrosal nerve carrying sympathetic information from the carotid plexus to form the vidian nerve, which ends in the sphenopalatine ganglion. Postganglionic fibers travel with the zygomaticotemporal branch of cranial nerve V to the lacrimal gland.

Visceral sensory fibers may take two routes on their way back to synapse in the nucleus of solitarius. Regardless of the route, their unipolar cell bodies lie in the geniculate ganglion. Taste from the anterior two thirds of the tongue runs with the lingual nerve, chorda tympani, through the facial canal to the geniculate ganglion, and via the nervus intermedius to the brainstem. Sensation from the palate, nose, and pharynx runs with the maxillary nerve to the sphenopalatine ganglion, vidian nerve, greater superficial petrosal nerve to the geniculate ganglion, and via the nervus intermedius to the brainstem.

Somatic sensory fibers from parts of the tympanic membrane, external auditory canal, and periauricular area have their cell bodies in the geniculate ganglion; via the nervus intermedius, they synapse in the nucleus of the descending tract of nerve V and the main sensory nucleus of nerve V.

In the segment between the geniculate ganglion and the branch to the stapedius muscle, the facial nerve consists of a single bundle bound by a dense, thick perineural sheath. For the remainder of its course, it lies within a loose rubric of connective tissue.¹² It is speculated that this tightly bound portion of the nerve is the pathophysiologic determinant of the degree of facial paresis when there is inflammation of the facial nerve, such as in the mononeuritis of Bell palsy. Any swelling of the nerve fascicles is contained by the perineurium, not by the bony walls surrounding it.¹⁰ Therefore, attempts at facial nerve decompression will be best accomplished in this region.

The vascular supply of the facial nerve is complex and more fully described elsewhere.¹² The posterior vertebrobasilar circulation supplies the proximal and middle portions of the nerve via the anterior inferior cerebellar artery and the internal auditory artery, respectively. Further supply of the middle portion of the nerve comes from the petrosal artery via the middle meningeal artery off the external carotid. Distal segments receive blood from the stylomastoid artery, which is also off the external carotid. The considerable overlap of the arterial supply, especially in the middle portions through the facial canal, make it unlikely that occlusion of any single artery will compromise facial nerve function.¹²

The majority of facial nerve functions can be readily assessed by observation:

Although it is a function of overlapping sensory fibers from the trigeminal, glossopharyngeal, vagus, and greater auricular nerves, abnormal sensation along the posterior aspect of the external auditory canal and tympanic membrane may be an early indication of facial nerve dysfunction.¹⁵

Taste is most reliably tested by electrogustometry, which compares the amounts of electrical current applied to the anterolateral aspect of the tongue necessary to produce taste perception. When associated with infranuclear weakness, an increase in the electrical threshold necessary to evoke the perception indicates that the facial nerve lesion is proximal to the origin of the chorda tympani.¹⁶ The simple application of salt or sugar to the anterior aspect of the tongue, although unreliable, may give useful information regarding the nerve’s integrity.¹⁷

Salivary and lacrimal function can be quantified and evaluated through comparison to normal controls as well as to the contralateral side. Examples of tests of these autonomic functions are the salivary flow test and Schirmer’s test of lacrimal function, respectively. This type of testing may be helpful in the prognosis of facial palsy in select patients, thereby helping to direct more extreme measures of therapy.¹⁸ In general, however, these tests are difficult to quantitate and impractical for use in general practice.