Facial nerve
From Wikipedia, the free encyclopedia
| Nerve: Facial nerve | ||
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| The course and connections of the facial nerve in the temporal bone. | ||
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| Latin | n. facialis | |
| Gray's | subject #202 901 | |
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| MeSH | A08.800.800.120.250 | |
The facial nerve is seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and taste to the anterior two-thirds of the tongue.
Its main function is motor control of most of the facial muscles and muscles of the inner ear. It also supplies parasympathetic fibres to the submandibular gland and sublingual glands via chorda tympani nerve and the submandibular ganglion, and to the lacrimal gland via the pterygopalatine ganglion. In addition, it receives taste sensations from the anterior two-thirds of the tongue. There is also a small amount of cutaneous sensation carried by the nervus intermedius from the skin in and around the auricle (earlobe).
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Anatomy of the facial nerve
The motor part of the facial nerve arises from the facial nerve nucleus in the pons while the sensory part of the facial nerve arises from the nervus intermedius.
The motor part of the facial nerve enters the petrous temporal bone into the internal acoustic canal (intimately close to the inner ear) then runs a tortuous course (including two tight turns) through the fallopian canal, emerges from the stylomastoid foramen and passes through the parotid gland, where it divides into five major branches. Though it passes through the paratid gland, it does not innervate the gland. This action is the responsibility of cranial nerve IX, the glossopharyngeal nerve.
No other nerve in the body travels such a long distance through a bony canal.
Five branches
The five main branches of the facial nerve are, from top to bottom:
- temporal branch of the facial nerve
- zygomatic branch of the facial nerve
- buccal branch of the facial nerve
- marginal mandibular branch of the facial nerve
- cervical branch of the facial nerve
A helpful mnemonic device for remembering the major branches is the phrase, "To Zanzibar By Motor Car".
Aside from the five main branches, the facial nerve also gives rise to the posterior auricular nerve which controls movements of some of the scalp muscles around the ear.
Two divisions occur in the parotid gland - temporofacial and cervicofacial divisions. The temporofacial division is made up of: temporal branch, zygomatic branch and buccal branch. Cervicofacial branch is made up of cervical branch, marginal mandibular branch and buccal branch. The two buccal branches later join.
Facial nerve nucleus
The cranial nerve motor nucleus of the facial nerve, is located in the lower pons. It is branchial in origin, so located more lateral than other motor nuclei.
Axons leaving the facial nucleus loop around the more posterior nucleus of the abducens nerve, then exit the pons anteriorly to form the nerve.
Cortical input to the facial nuclei
The primary motor cortex is where motor output comes from the cortex. The area supplying the face is quite lateral, just above the parietal lobe.
Neurons supplying the lower face in the cortex, project axons to the contralateral (opposite side) facial nerve nucleus. Neurons controlling the upper face project to both facial nuclei.
Testing the facial nerve
Voluntary facial movements, such as wrinkling the brow, showing teeth, frowning, closing the eyes tightly, pursing the lips and puffing out the cheeks, all test the facial nerve. There should be no noticeable asymmetry.
In an upper motor neuron lesion, only the lower part of the face will be affected, due to the crossing over of input to the nuclei.
Taste can be tested on the anterior of the tongue, this can be tested with a swab dipped in a flavoured solution, or with electronic stimulation (similar to putting your tongue on a battery).
Facial nerve pathology
People may suffer from acute facial nerve paralysis, which is usually manifested by facial paralysis. Bell's palsy is one type of idiopathic acute facial nerve paralysis, which is more accurately described as a multiple cranial nerve ganglionitis that involves the facial nerve, and most likely results from viral infection.
