What is the facial nerve?
The facial nerve is a facial expression of the 7th mixed cranial nerve(CN) which is motor as well as sensory nerve, but predominantly it is motor in nature.
This nerve supplies the muscles of facial expression so-called facial nerve. It is the most frequently paralyzed nerve among the cranial nerves in the human body.
- Special visceral efferent fibers (SVE)
- General visceral efferent fibers(GVE)
- Special visceral afferent fibers(SVA)
- General somatic afferent fibers(GSA)
- General visceral afferent fibers(GVA)
Facial Nerve Nuclei
The facial nerve has three types of nuclei present including the following;
- Main motor nucleus
- Parasympathetic nuclei
- Sensory nucleus
Main Motor Nucleus
It originates from the lower part of the pons of the brainstem of the brain. The part of the nucleus supplying the upper part muscles of the face and receives corticonuclear fibers from both cerebral hemispheres in the brain.
It is available posterolateral to the principal motor nucleus. They are the superior salivatory & lacrimal nuclei. They providing the secretomotor fibers to the lacrimal, submandibular, and sublingual glands in the human body.
The lacrimal nucleus
The afferents fibers arise from the hypothalamus which provides emotional responses and the sensory nuclei of the trigeminal nerve for reflex lacrimation secondary to the annoyance (irritation) of the cornea or conjunctiva.
The sensory nucleus
The upper part of the nucleus of the tractus solitarius & which is closed to the motor nucleus of the brainstem.
The sensations of taste travel throughout the peripheral axons of nerve cells located in the geniculate ganglion on the 7th cranial nerve in the brain.
The efferent fibers pass over the median plane & ascend up to the ventral posterior medial nucleus of the opposite thalamus & to a number of hypothalamic nuclei.
From the thalamus, the axons of the thalamic cells travel throughout the internal capsule & corona radiata to end in the tasting area of the cortex in the postcentral gyrus of the lower part.
The course of the Facial Nerve
The facial nerve consists of both motor and sensory roots of the brainstem. The fibers of the motor root first passes through posteriorly around the medial side of the abducent nucleus.
Then travel around the nucleus below the colliculus facialis on the floor of the fourth ventricle in the human brain and, finally, travel anteriorly to come out from the brainstem.
The nervus intermedius which is sensory root is established by the central processes of the unipolar cells of the geniculate ganglion.
And also contains the preganglionic efferent parasympathetic fibers from the parasympathetic nuclei in the brainstem.
The two roots of the facial nerve come out from the anterior surface of the brain between the medulla and the pons.
And it passes laterally in the posterior cranial fossa with the vestibulocochlear nerve and enters through the internal acoustic meatus in the petrous part of the temporal bone of the head.
At the bottom of the meatus, the nerve goes into the facial canal and runs laterally through the inner ear.
On reaching the medial wall of the tympanic cavity, the nerve enlarges to form the sensory geniculate ganglion and turns sharply backward above the promontory.
At the posterior wall of the tympanic cavity, the facial nerve turns downward on the medial side of the aditus of the mastoid antrum, descends beyond the pyramid, and comes out from the stylomastoid foramen.
Distribution of the Facial Nerve
The motor nucleus supplies the muscles of facial expression in the face, the auricular muscles, the stapedius in the ear, the posterior belly of the digastric, and the stylohyoid muscles in the neck region.
The superior salivatory nucleus supplies the submandibular gland and sublingual salivary glands, the nasal glands, and the palatine glands.
The lacrimal nucleus supplies the lacrimal glands in the eye region.
The sensory nucleus receives taste fibers from the front (anterior) two-thirds of the tongue, the floor of the mouth, and the upper palate.
- Lesions of the facial nerve (supranuclear or infranuclear)– Bell’s palsy.
- Crocodile tears syndrome (due to facial nerve lesion in the brainstem).
- Ramsay Hunt syndrome.
Ramsay Hunt syndrome
It is due to fact that the involvement of geniculate ganglion in herpes zoster infection in human beings. The signs and symptoms are the following;
- Herpetic vesicles on the auricle in the ear.
- Hyperacusis (a disorder of hearing).
- Loss of lacrimation (flow of tears).
- Loss of taste sensations in the anterior 2/3rd of the tongue.
- Complete ipsilateral facial palsy (Bell’s palsy).