Embryology - Phylogeny and embryology of the facial nerve and related structures, part 2

Ear, Nose & Throat Journal, Oct, 2003 by Robert Thayer Sataloff, Jesse C. Selber

Editor's note: This article on the embryology of the facial nerve and related structures is the second of a two-part series. Part I, which covered the phylogeny of the facial nerve and related structures, appeared in the September issue. (1)

Abstract

A precise description of the developmental anatomy of the facial nerve and associated ear structures, augmented by an appreciation of phylogenic history, has proven extremely helpful intraoperatively. Predictions of facial nerve position can be made with reasonable accuracy when they are based on a proper analysis of developmental anomalies of other ear structures. In cases in which the facial nerve canal has not developed, it may be impossible to obtain accurate localization of the facial nerve radiographically prior to surgery. In such cases, judgment based on an understanding of embryology may be the surgeon's only guide to the position of the facial nerve. Although these principles have proven valid and reliable so far, it is not prudent for the surgeon to consider them gospel, of course. Extreme caution must always be exercised when operating on an ear with congenitally abnormal anatomy. Nevertheless, it is not necessary for the surgeon to approach a congenitally malformed ear with the fear that "You never know where the facial nerve is going to be." Understanding embryology allows otologic surgery to be planned with reasonable accuracy and carried out safely and expeditiously.

Introduction

In studying the embryology of the facial nerve, it is helpful to keep in mind the final structure toward which development progresses. In the adult, the motor nucleus of the facial nerve (VIIIth cranial nerve) is located deep in the reticular formation of the caudal portion of the pons. The axons leave the motor nucleus and extend dorsally and medially, cranially and superficially. They bend around the abducens nucleus to form the first genu of the facial nerve. The fibers then course deep through the pons and exit from the central nervous system between the olive and the inferior cerebellar peduncle. At this point, the axons join to form the motor root. The sensory root (nervus intermedius) consists of central processes of neurons located in the geniculate ganglion and axons of parasympathetic neurons from the superior salivatory nucleus. The nervus intermedius enters the central nervous system at the pontocerebellar groove lateral to the motor root and synapses with neurons in the upper part of the solitary tract of the medulla oblongata. The facial nerve and nervus intermedius course with the vestibuloacoustic nerve from the brainstem and enter the internal auditory canal (figure 1). For approximately 20% of its course, the nervus intermedius is fused with the VIIIth cranial nerve. (2)

[FIGURE 1 OMITTED]

At the point where the facial nerve enters the middle ear, it bends a second time at the geniculate ganglion (the second genu) and courses horizontally through the middle ear. It then curves (the pyramidal bend) to course vertically through the mastoid bone and exits at the stylomastoid foramen (figure 2). The nerve is ordinarily surrounded by a bony sheath called the fallopian canal. Several branches are given off during the intrapetrosal course. The facial nerve spreads extratemporally to innervate the facial musculature (figure 3).

[FIGURES 2&3 OMITTED]

Medially to laterally, the facial nerve branches include 11 structures of note (figure 4):

[FIGURE 4 OMITTED]

* communications in the internal auditory canal with the VIIIth cranial nerve

* the greater superficial petrosal nerve, which supplies taste fibers to the palatal mucosa, preganglionic parasympathetic axons to the pterygopalatine ganglion, and postganglionic axons to the lacrimal glands, nasal glands, and palatine mucosal glands; it also communicates with the lesser petrosal nerve (2)

* the nerve to the stapedius muscle

* the chorda tympani nerve, which supplies taste fibers to the anterior two-thirds of the tongue, preganglionic parasympathetic fibers to the submandibular gland, and postganglionic fibers to the submandibular and sublingual glands

* the posterior auricular branch--which innervates the auricularis posterior, the cranially oriented muscles of the auricle, and the occipital muscles--communicates with the greater auricular nerve, the auricular branch of the vagus nerve, and the lesser occipital nerve (3)

* the branch to the posterior belly of the digastric muscle

* the branch to the stylohyoid muscle

* the temporal branch that supplies the lateral intrinsic muscle of the auricle, the anterior and superior auricular muscles, the frontalis, the orbicularis oculi, and the corrugator

* the buccal branch, which innervates the procerus, the zygomaticus major and minor, the levator labii superioris, the levator anguli oris, nasal muscles, the buccinator, and the orbicularis oris

* the marginal mandibular branch to the risorius muscle and the muscles of the lower lip and chin

* the cervical branch to the platysma; there are interconnections between the facial nerve and primarily the sensory nerves, including the trigeminal, glossopharyngeal, vagus, and cervical nerves (3)