Facial nerve
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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.
No other nerve in the body travels such a long distance through a bony canal.
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
- mandibular branch of the facial nerve
- cervical branch of the facial nerve
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.
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 (on the precentral gyrus) 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 that results from damage to the facial nerve. This usually happens at the narrowest point of the fallopian canal.
External links
- Emedicine's writeup on this (http://www.emedicine.com/ent/topic8.htm)
- Another good VII anatomy writeup (http://www.ijri.org/articles/archives/19990901/neurorad01.htm)
- Some good CT images (http://css.engineering.uiowa.edu/~jhe/research/mansegment/)
- position of facial nerve on MRI (http://www.med.harvard.edu/AANLIB/cases/caseM/mr1_t/015.html)