Facial nerve

Cranial nerve VII or simply CN VII

Function

• Facial expression: The main function of the facial nerve is motor control of all of the muscles of facial expression.
• It also innervates the posterior belly of the digastric muscle, the stylohyoid muscle, and the stapedius muscle of the middle ear. All of these muscles are striated muscles of branchiomeric origin developing from the 2nd pharyngeal arch.

• Facial sensation
• Taste sensations of anterior two-thirds of the tongue. General sensation from the anterior two-thirds of tongue is done by 5th cranial nerve (V-3).

The facial nerve also supplies a small amount of afferent innervation to the oropharynx below the palatine tonsil. There is also a small amount of cutaneous sensation carried by the nervus intermedius from the skin in and around the auricle (outer ear).

Other[edit]

The facial nerve also supplies parasympathetic fibers to the submandibular gland and sublingual glands via chorda tympani. Parasympathetic innervation serves to increase the flow of saliva from these glands. It also supplies parasympathetic innervation to the nasal mucosa and the lacrimal gland via the pterygopalatine ganglion. The parasympathetic fibers that travel in the facial nerve originate in the superior salivatory nucleus.

The facial nerve also functions as the efferent limb of the corneal reflex.

Functional components[edit]

The facial nerve carries axons of type GSA, general somatic afferent, to skin of the posterior ear.

The facial nerve also carries axons of type GVE, general visceral efferent, which innervate the sublingual, submandibular, and lacrimal glands, also mucosa of nasal cavity.

Axons of type SVE, special visceral efferent, innervate muscles of facial expression, stapedius, the posterior belly of digastric, and the stylohyoid.

The axons of type SVA, special visceral afferent, provide taste to the anterior two-thirds of tongue via chorda tympani.

Clinical significance

• Bell's palsy and acute facial nerve paralysis

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 and also sometimes as a result of Lyme disease. Iatrogenic Bell's palsy may also be as a result of an incorrectly placed dental local-anesthetic (inferior alveolar nerve block). Although giving the appearance of a hemiplegic stroke, effects dissipate with the drug. When the facial nerve is permanently damaged due to a birth defect, trauma, or other disorder, surgery including a cross facial nerve graft or masseteric facial nerve transfer may be performed to help regain facial movement.^{[citation needed]} Facial nerve decompression surgery is also sometimes carried out in certain cases of facial nerve compression.

Examination[edit]

Voluntary facial movements, such as wrinkling the brow, showing teeth, frowning, closing the eyes tightly (inability to do so is called lagophthalmos), 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, called central seven, only the lower part of the face on the contralateral side will be affected, due to the bilateral control to the upper facial muscles (frontalis and orbicularis oculi).

Lower motor neuron lesions can result in a CN VII palsy (Bell's palsy is the idiopathic form of facial nerve palsy), manifested as both upper and lower facial weakness on the same side of the lesion.

Taste can be tested on the anterior two-thirds 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).

Corneal reflex. The afferent arc is mediated by the general sensory afferents of the trigeminal nerve. The efferent arc occurs via the facial nerve. The reflex involves consensual blinking of both eyes in response to stimulation of one eye. This is due to the facial nerves' innervation of the muscles of facial expression, namely orbicularis oculi, responsible for blinking. Thus, the corneal reflex effectively tests the proper functioning of both cranial nerves V and VII.