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Facial nerve

from Latin: facies - face
Synonyms: Intermediate nerve, nerve VII, 7. cranial nerve, intermediofacialis nerve, nervus facialis
German: Nervus facialis

1 Definition

The facial nerve is the seventh of the cranial nerves leaving the brain from cranial to caudal direction. It contains the motor fibers drawing to the structures of the second hyoid arch, as well as parasympathetic, sensory, and sensitive fibers via the intermediate nerve.

In order to highlight the composed character of the nerve more clearly, it is also called intermediofacialis nerve.

2 Embryology

The facial nerve is counted among the hyoid arch nerves, and it develops from the nerve of the 2nd hyoid arch. Therefore, it provides the specific motor innervation (visceromotor innervation) to all the muscles that have developed from the muscle stem of this hyoid arch.

3 Fiber qualities and innervation

The facial nerve is the only cranial nerve that conducts motor, sensory, sensitive, and parasympathetic fibers at once. It is formed by around 10.000 nerve cells, of which around 7.000 send myelinated motor fibers. The remaining 3.000 cells are to be counted to the intermediate nerve, and they possess unmyelinated parasympathetic, sensory and sensitive fibers.

3.1 Motor fibers

The motor parts of the facial nerve from the facial motor nucleus innervate especially the mimic muscles, as well as the posterior parts of the suprahyoid muscles, and the stylohyoideus muscle and the posterior venter of the digastric muscle. Moreover, there are fibers that draw to the stapedius muscle and are responsible for the fine tuning of the auditory sensation.

3.2 Sensory fibers

The sensory fibers of the nerves draw alongside the chorda tympani via the intermedius part into the nucleus of the solitary tract. They provide the innervation of the taste buds in the region of the papillae of the tongue in the anterior 2/3rds of the tongue. The perikarya of the pseudounipolar nerve cells are located in the geniculate ganglion, which corresponds to a spinal ganglion.

3.3 Sensitive fibers

The sensitive fibers of the facial nerve draw to the skin of the external acoustic meatus and the eardrum; from there, aside from pain and tactile stimuli, it predominantly conducts temperature sensations (see caloric reflex test).

3.4 Parasympathetic fibers

The facial nerve receives parasympathetic nerve fibers from the superior salivary nucleus via the intermediate nerve, which draw with the chorda tympani to the oral cavity and provide the innervation of the oral salivary glands. Moreover, parasympathetic fibers of the facial nerve draw to the lacrimal gland and innervate it.

4 Devolution

The nuclei of the facial nerve can be found in the area of the medulla oblongata After its fibers have run around the nucleus abducens nucleus (internal geniculum of the facial nerve), they leave the brain in the area of the cerebellopontine angle. Via the internal auditory porus, the nerve draws to the internal auditory meatus of the petrosal bone,at whose fundus, it enters the facial canal.

In the region of the petrosal bone, the facial nerve also forms the geniculate ganglion, where the perikarya of the afferent fibers can be found, which then draw further to the nucleus of the solitary tract. With the bent course of its nerve canal, the nerve forms a second external "knee", the geniculum of the facial nerve.

4.1 Branches

Still within the petrosal bone, the facial nerve emits three of its branches, the greater petrosal nerve, which contains parasympathetic fibers for the innervation of the lacrimal gland, the stapedius nerve for the supply of the stapedius muscle, and the chorda tympani.

After the exit from the petrosal bone through the mastoid foramen, further branches go off, namely the posterior auricular nerve and the stylohyoid and digastric branches, which draw to the muscles of the neck.

The facial nerve passes further within the parotid gland, where it forms a fine nerve plexus, the parotid plexus. It can be divided into two main branches, the superior temporofacial part, and the inferior cervicofacial part, which then quickly further divide into finer branches.

4.2 Parotid plexus

The parotid plexus innervates predominantly the mimic muscles. The following branches derive from it:

5 Pathology

The facial nerve can be damaged in a variety of diseases; mostly, the consequences are paralysis symptoms (facial nerve paralysis).
Such paralysis can either appear in the peripheral region of the nerve and be caused by infectious diseases (lyme disease, herpes zoster oticus), mechanical damage (petrosal bone fractures), tumors, or by a central damage. Further importanat causes are brain hemorrhage, cerebral infarction, or brain tumors (especially tumors of the cerebellopontine angle).

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