Facial Nerve is 7th cranial nerve and is mixed nerve which means that it has both motor and sensory fibers. Motor fibers are those which brings signals from the central nervous system that is brain and spinal cord.
Sensory nerves fibers are the fibers which carry signals towards the central nervous system that is brain and spinal cord.
Before discussing the course of facial nerve let’s discuss facial nerve nuclei.
Nuclei of the facial nerve :
Facial nerve has three nuclei these are
- The main motor nuclei
- The parasympathetic nuclei.
- The sensory nuclei.
Let’s discuss each nucleui of facial nerve in detail.
Main Motor Nucleus of Facial Nerve :
Location of the main motor nucleus of the facial nerve:
This nucleus lies deep in the reticular formation of the lower part of the pons. The part of the muscles that supply the muscles of the upper part of the face receives corticonuclear fibers from both cerebral hemispheres. The part of the nucleus that supplies the muscles of the lower part of the face receives only corticonuclear fibers from the opposite cerebral hemisphere.
This is voluntary control pathway of the facial muscles. There is another involuntary pathway that controls mimetic or emotional changes in facial expression. This pathway forms a part of the reticular system.
Parasympathetic Nucleus of Facial Nerve :
Location of the Parasympathetic nucleus of the facial nerve :
This nucleus lies posterolateral to the main motor nucleus the main motor nuclei are superior salivatory and lacrimal nuclei.
Each nucleus receives afferent fibers.
Superior Salivatory nucleus:
The superior salivary nucleus receives afferent fibers from the hypothalamus through the descending autonomic pathways .information of the taste is also received from the nucleus of the solitary tract from the mouth.
The Lacrimal Nucleus:
Lacrimal Nucleus receives afferent fibers from the hypothalamus for emotional responses and from the sensory nuclei of the Trigeminal nerve for reflex lacrimation secondary to irritation of the cornea or conjunctiva.
Sensory Nuclei of the Facial Nerve:
Location of Sensory Nuclei of the Facial Nerve:
The sensory nucleus is the upper part of the nucleus of the tracts solitarius and lies close to the motor nucleus. Sensation of the taste through the peripheral axons of nerve cells situated in the geniculate ganglion on the seventh cranial nerve. The central processes of these cells synapse on nerve cells in the nucleus.
Efferent fibers cross the median plane and ascend to the ventral posterior medial nucleus of the opposite thalamus and to a number of hypothalamic nuclei. From the thalamus, the axons of the thalamic cells pass through the internal capsule and corana radiata to end in the taste area of the cortex in the lower part of the postcentral gyrus.
Coarse of the Facial nerve :
As we have discussed in the beginning that facial nerve is mixed nerve and it has both sensory and motor fibers so here we will discuss the coarse of both fibers.
The fibers of the motor root first travel posteriorly around the medial side of the Abducent nucleus. They then pass around the nucleus beneath the colliculus facialis in the floor of the fourth ventricle and finally pass anteriorly to emerge from the brainstem.
The sensory root also called nervus intermedius is formed of the central processes of the unipolar cells of the geniculate ganglion. It also contains the efferent preganglionic parasympathetic fibers from the parasympathetic nuclei.
The two roots of the facial nerve emerge from the anterior surface of the brain between the pons and medulla oblongata. They pass laterally in the posterior cranial fossa with the vestibulocochlear nerve and enter the internal acoustic meatus in the posterior part of the temporal bone. At the bottom of the meatus, the nerve enters the facial canal and run laterally through the inner ear. On reaching the medial wall of the tympanic cavity the nerve expands 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 behind the pyramid, and emerges from the stylomastoid foramen.
Distribution of the facial nerve:
The motor nucleus supplies the muscles of facial expression, the stapedius, the auricular muscle, the posterior belly of the digastric and stylohyoid muscle.
The superior salivatory nucleus supplies the submandibular and sublingual salivary glands and the nasal and palatine glands.
The Lacrimal nucleus supplies the Lacrimal gland.
The sensory nucleus receives taste fibers from the anterior two third of the tongue, the floor of the mouth and the palate.