A Review of Facial Nerve Anatomy Terence M. Myckatyn, M.D.1 and Susan E. Mackinnon, M.D.1 ABSTRACT An intimate knowledge of facial nerve anatomy is critical to avoid its inadvertent injury during rhytidectomy, parotidectomy, maxillofacial fracture reduction, and almost any surgery of the head and neck. Injury to the frontal and marginal mandibular branches of the facial nerve in particular can lead to obvious clinical deficits, and areas where these nerves are particularly susceptible to injury have been designated danger zones by previous authors. Assessment of facial nerve function is not limited to its extratemporal anatomy, however, as many clinical deficits originate within its intratemporal and intracranial components. Similarly, the facial nerve cannot be considered an exclusively motor nerve given its contributions to taste, auricular sensation, sympathetic input to the middle meningeal artery, and parasympathetic innervation to the lacrimal, submandibular, and sublingual glands. The constellation of deficits resulting from facial nerve injury is correlated with its complex anatomy to help establish the level of injury, predict recovery, and guide surgical management. KEYWORDS: Extratemporal, intratemporal, facial nerve, frontal nerve, marginal mandibular nerve The anatomy of the facial nerve is among the components of the facial nerve reminds the surgeon that most complex of the cranial nerves. In his initial descrip- the facial nerve is composed not exclusively of voluntary tion of the cranial nerves, Galen described the facial motor fibers but also of parasympathetics to the lacrimal, nerve as part of a distinct facial-vestibulocochlear nerve submandibular, and sublingual glands; sensory innerva- complex.1,2 Although the anatomy of the other cranial tion to part of the external ear; and contributions to taste nerves was accurately described shortly after Galen’s at the anterior two thirds of the tongue. initial descriptions, it was not until the early 1800s that Charles Bell distinguished the motor and sensory components of the facial nerve.3,4 INTRACRANIAL ANATOMY OF Facial nerve anatomy is categorized in terms of its THE FACIAL NERVE relationship to the cranium or temporal bone (intracra- Voluntary control of the branchial branch of the facial nial, intratemporal, and extratemporal) or its four dis- nerve is initiated intracranially by supranuclear inputs tinct components (branchial motor, visceral motor, arising from the cerebral cortex projecting to the facial general sensory, and special sensory). The plastic surgeon nucleus. These cortical inputs are arranged with forehead benefits from a basic knowledge of the intracranial and representation most rostral and eyelids, midface, and lips intratemporal components of the facial nerve to help sequentially caudal to this.5 The pyramidal system is localize facial nerve pathology and distinguish extratem- composed of corticobulbar tracts that project voluntary, poral from facial nerve lesions at other anatomic loca- ipsilateral cortical inputs via the genu of the internal tions. Similarly, a knowledge of the four distinct capsule to the seventh cranial nerve nuclei of the pontine Facial Paralysis; Editor in Chief, Saleh M. Shenaq, M.D.; Guest Editor, Susan E. Mackinnon, M.D. Seminars in Plastic Surgery, Volume 18, Number 1, 2004. Address for correspondence and reprint requests: Susan E. Mackinnon, M.D., Suite 17424, East Pavilion, 1 Barnes-Jewish Hospital Plaza, St. Louis, MO 63110. 1Division of Plastic Surgery, Washington University School of Medicine, St. Louis, MO. Copyright # 2004 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001 USA. Tel: +1(212) 584-4662. 1535-2188,p;2004,18,01, 005,012,ftx,en;sps00103x. 5 6 SEMINARS IN PLASTIC SURGERY/VOLUME 18, NUMBER 1 2004 tegmentum. Cell bodies of the upper facial motor nerves general sensory function of the facial nerve reside in giving rise to the frontal branch receive bilateral cortical the general sensory trigeminal nucleus of the rostral inputs, and neurons to the remainder of the facial medulla and receive afferent inputs from projections of nucleus receive contralateral cortical innervation. Spon- the geniculate ganglion within the temporal bone. The taneous facial movements are centrally transmitted via gustatory nucleus within the pontine tegmentum also the extrapyramidal system, which involves diffuse axonal receives special sensory inputs from the geniculate gang- connections between multiple regions including the lion. These impulses, however, were initially generated basal ganglia, amygdala, hypothalamus, and motor cor- by taste receptors in the anterior two thirds of the tex. The extrapyramidal system regulates resting facial tongue. Ascending sensory inputs from the trigeminal tone and stabilizes the voluntary motor response; hy- and gustatory nuclei are influenced by the thalamic pothalamic inputs modulate the emotional response. nuclei prior to their reception within the sensory cortex. The facial nuclei contain the cell bodies of facial Patients with supranuclear lesions involving the nerve lower motor neurons. These cell bodies receive motor cortex or internal capsule present clinically with supranuclear inputs via synapse formation with axons loss of volitional control of the lower facial musculature traveling through both the pyramidal and extrapyramidal but persistent facial tone and spontaneous facial move- systems. The confluence of these postsynaptic lower ments. Voluntary control of the forehead musculature is motor neurons round the abducens nucleus and form retained because the upper halves of the facial nuclei, the facial colliculus at the floor of the fourth ventricle which are populated by frontal nerve branch cell bodies, (Fig. 1). The branchial motor branch of the facial receive bilateral cortical innervation and so not all input nerve exits the brainstem at the cerebellopontine angle, is lost after a unilateral supranuclear lesion. Voluntary where it is joined by the less robust nervus intermedius. lip, nose, and cheek movements, however, are lost. It These nerves resemble the nerve roots of the spinal cord should also be noted that facial muscle dysfunction in that they are devoid of epineurium but covered in pia caused by central injury is frequently accompanied by mater and bathed in cerebrospinal fluid. The branchial motor dysfunction of the tongue and hand given the motor nerve–nervus intermedius complex travels about proximity of these cortical control centers within the 15.8 mm from the cerebellopontine angle before it motor cortex and internal capsule. Reflex arcs involving begins its course within the temporal bone.6 the facial nuclei, such as the corneal blink (trigeminal- The parasympathetic component of the facial facial), are preserved following supranuclear lesions. nerve is composed of visceral motor fibers whose origi- nating cell bodies are scattered within the pontine tegmentum and collectively known as the superior sal- INTRATEMPORAL FACIAL NERVE ivatory nucleus. These nuclei are influenced by involun- The intratemporal anatomy of the facial nerve has been tary hypothalamic inputs. Cell bodies mediating the extensively studied to minimize morbidity in skull base Figure 1 The facial nerve motor nuclei are located in the pontine tegmentum. (1) The facial motor nucleus. These motor nuclei contain the cell bodies of all the voluntary motor components of the facial nerve except for the nerve to stapedius. (2) The facial nerves round the abducens nuclei (cranial nerve VI) at the floor of the fourth ventricle to form the facial colliculi. (3) The cell bodies of facial motor neurons that contract the upper half of the face (frontal branch) receive bilateral descending cortical inputs. (4) Cell bodies of facial motor neurons innervating the bottom half of the face receive descending cortical inputs from the contralateral side alone. A REVIEW OF FACIAL NERVE ANATOMY/MYCKATYN, MACKINNON 7 surgery while maximizing exposure. In addition, its intraneural topography has been investigated in cadavers and animal models.7–9 Whereas the topography in cer- tain animal models, such as the cat, is shown to be consistent,7 the topography of the intratemporal facial nerve in the human is highly variable and spatial rela- tionships to other intratemporal structures such as the carotid artery and sigmoid sinus are also variable.10–13 The branching pattern of the intratemporal facial nerve, however, is reasonably consistent. The branchial motor and nervus intermedius com- ponents of the facial nerve are loosely associated as they enter the internal auditory meatus of the temporal bone. Both the facial and acoustic nerves enter the temporal bone simultaneously with the facial nerve located superior to the acoustic nerve. The facial nerve, along with the acoustic and vestibular nerves, travel 8 to 10 mm within the internal auditory canal before only the facial nerve enters the fallopian canal. The fallopian canal consists of labyrinthine, tympanic, and mastoid segments. The la- byrinthine segment is the narrowest segment and extends 3 to 5 mm from the edge of the internal auditory canal. The geniculate ganglion resides within the distal part of the labyrinthine segment of the facial nerve and gives rise to the first branch of the facial nerve—the greater petrosal nerve—which carries visceral motor parasympa- thetic fibers to the lacrimal gland (Fig. 2). The external petrosal nerve is a second, threadlike branch that is Figure 2 The voluntary motor component of the facial nerve occasionally