Facial Nerve - Cranial Nerves (V, VII - XII)

Facial Nerve

The seventh cranial nerve supplies motor fibers to the muscles of facial expression; in a nerve bundle emerging separately from the brain stem, called the intermediate nerve, it carries taste fibers and viscero-efferent secretory (parasympathetic) fibers. The motor fibers (AB1) originate from the large, multipolar neurons in the nucleus ofthe facial nerve (AB2). They arch aroundthe abducens nucleus (AB3) (internal genu ofthe facial nerve) and emerge on the lateralaspect of the medulla oblongata from the lower border of the pons. The cells of the preganglionic secretory fibers (AB4) form the superior salivatory nucleus (AB5). The taste fibers (AB6) originate from the pseudo-unipolar cells in the geniculate ganglion (BC7) and terminate in the cranial section of the solitary nucleus (AB8). The visceroefferentand taste fibers do not arch around the ab-ducens nucleus but join the ascending limb of the nerve and emerge as intermediatenerve (B9) between the facial nerve and thevestibulocochlear nerve.

Both parts of the nerve pass through the inner auditory canal, the internal acousticmeatus (petrous part of temporal bone, in-ternal acoustic pore, see vol. 1), and enter the facial canal as a nerve trunk. At the bend of the nerve in the petrous bone (externalgenu of the facial nerve) lies the geniculate ganglion (BC7). The canal continues abovethe tympanic cavity and turns caudally toward the stylomastoid foramen (BC10), through which the nerve leaves the skull. The nerve ramifies into terminal branches (parotid plexus) (E11) in the parotidgland.

The greater petrosal nerve (BC12), the stapedius nerve (BC13), and the chorda tympani (BC14) branch off inside the facialcanal. Thegreater petrosal nerve(pregan-glionic secretory fibers for the lacrimal gland, nasal glands, and palatal glands) originates from the geniculate ganglion, ex-tends through the hiatus for the lesserpetrosal nerve into the cranial cavity andover the anterior aspect of the petrous bone through the foramen lacerum and finallythrough the pterygoid canal to the pterygo-palatine ganglion (C15). Thestapedius nervesupplies the stapedius muscle in the middle ear. The chorda tympani (BC14) branches off above the stylomastoid foramen, runs beneath the mucosa through the tympanic cavity and further to the petrotympanic fissure, and finally joins the lingual nerve (C16). It contains taste fibersfor the anterior two-thirds of the tongue (D) and preganglionic fibers for the subman-dibular and sublingual glands as well as various lingual glands.

Before it enters the parotid gland, the facial nerve gives off the posterior auricular nerve (E17) as well as branches to the posterior belly of the digastric muscle (CE18) and to the stylohyoid muscle (C19). The parotid plexus gives off the temporal branches (E20), the zygomatic branches (E21), the buccalbranches (E22), the marginal mandibular branch (E23), and the cervical branch (E24)for the platysma . The branches provide innervation to all the muscles of fa-cial expression.

Ramifications of the cervical branch lying beneath the platysma form the superficialcervical ansa by anastomosing withbranches of the sensory transverse cervical nerve. The small branches departing from the ansa are mixed sen-sorimotor nerves. The terminal ramifica-tions of temporal branches, buccal branches, and marginal mandibular branch form similar plexuses with branches of the trigeminal nerve.

Clinical Note: Injury to the nerve results inatony of all muscles of the affected half of the face. The mouth region drops, and the eye can no longer close (F). There is increased sensitivity to sound, hyperacusis.

C25 Trigeminal ganglion.