The facial nerve is directly and indirectly involved in numerous pathological conditions affecting the temporal bone, ranging from infection to neoplasia. In each instance, a solid understanding of its complex anatomy is crucial to the physician’s ability to both diagnose and treat disorders of the facial nerve.
The facial nerve (Figure 72–1) begins its development near the end of the first month of gestation. At this time, the acousticofacial primordium gives rise to both the facial and acoustic nerves. It develops adjacent to the primordial inner ear, known as the otic placode. The geniculate ganglion, which arises from the second pharyngeal (formerly known as branchial) arch, develops early in the second month of gestation. Adjacent to the developing geniculate ganglion, the acousticofacial primordium differentiates into a caudal and a rostral trunk. The caudal trunk progresses into the mesenchyme of the second pharyngeal arch, becoming the main trunk of the facial nerve and innervating the muscles of facial animation. The rostral branch becomes associated with the first arch, eventually developing into the chorda tympani nerve. Thus, although cranial nerve VII is generally associated with the second pharyngeal arch, the chorda tympani then travels with V3 in the lingual nerve, going on to provide taste to the anterior two-thirds of the tongue along the domain of the first arch.
A schematic illustration demonstrating the embryology of the facial nerve. (A) The location of the primitive facial nerve in the developing embryo is shown in relation to other important nerves in the head and neck. (B) The location of the second pharyngeal arch giving rise to the main trunk of the facial nerve is shown in relation to the other pharyngeal arches. (C) Other derivatives of the second pharyngeal arch are shown and help explain the complex innervation pattern of the facial nerve. (Reproduced with permission from Langman J & Sadler TW, Langman’s Medical Embryology, 5th ed. Williams and Wilkins: Baltimore, MD; 1985.)
Both the geniculate ganglion and the nervus intermedius, which arise from the second pharyngeal arch, form independent of the motor division of the seventh nerve. During the sixth week of gestation, the motor division of the facial nerve establishes its position in the middle ear between the membranous labyrinth (an otic placode structure) and the developing stapes (a second arch structure). The nerve then passes into the mesenchyme of the second arch. During this time, the chorda tympani branch may be extratemporal while it becomes associated with the trigeminal nerve, which will carry the chorda tympani on its way to the tongue via the lingual nerve. The greater superficial petrosal nerve, which carries preganglionic parasympathetic fibers toward the pterygopalatine ganglion, also develops during this time period.