Facial nerve (CN VII)
The facial nerve (seventh cranial nerve, CN VII, latin: nervus facialis) is a mixed cranial nerve consisting of motor, sensory, and visceromotor fibers. The main functions of the facial nerve include controlling the muscles of facial expression, and providing taste sensations from the anterior part of the tongue.
Functionally, the facial nerve consists of two parts: somatosensory and visceromotor part. The somatosensory part of the facial nerve provides the innervation for all of the facial expression muscles and the muscles of the neck, while the visceromotor part contributes to the parasympathetic innervation of the lacrimal gland, salivary glands, and mucous glands of the nasal cavity, also providing sensory fibers that transmit taste sensations from the tongue.
The facial nerve arises from the following three nuclei located in the brainstem:
- facial motor nucleus,
- superior salivatory nucleus,
- solitary tract nucleus.
The facial motor nucleus (motor) provides the facial nerve with general somatic efferent fibers, the superior salivatory nucleus (parasympathetic) gives rise to general visceral efferent fibers of the facial nerve, and the solitary tract nucleus or nucleus of the solitary tract nuclear complex (sensory), which is responsible for taste sensation transmitted via special visceral afferent fibers of the facial nerve.
The facial nerve emerges from the brainstem on its ventral surface in the cerebellopontine angle. The facial nerve then travels through the subarachnoid space, specifically, the pontine cistern, to reach the internal acoustic opening, through which the facial nerve enters the internal acoustic meatus and continues its path via the facial canal.
Motor branches of facial nerve
While travelling through the facial canal the facial nerve gives off a small branch, the stapedius nerve, which enters the tympanic cavity to innervate the stapedius muscle. The rest of the motor fibers of the facial nerve exit the facial canal and the skull via the stylomastoid foramen. Immediately upon emerging on the outer surface of the skull the facial nerve gives three motor branches:
- posterior auricular nerve,
- digastric branch,
- stylohyoid branch.
The posterior auricular nerve is a small motor branch of the facial nerve that innervates the auricularis posterior muscle, and the occipital belly of the occipitofrontalis muscle.
The digastric branch of the facial nerve innervates the posterior belly of the digastric muscle.
The stylohyoid branch arises from the facial nerve and innervates the stylohyoid muscle.
The rest of the motor part (carrying the majority of the motor fibers) of the facial nerve divides inside the mass of the parotid gland forming the parotid nerve plexus. The branches from the parotid plexus spread in a fan-like manner to innervate the muscles of facial expression. These branches are:
- temporal branches,
- zygomatic branches,
- buccal branches,
- marginal mandibular branch,
- cervical branch.
The temporal branches are motor branches of the facial nerve, which emerge from the parotid plexus to innervate the auricular muscles and several facial muscles above the mouth. After arising from the parotid plexus located in the parotid gland the temporal branches run ventrally and upwards, and innervate muscles of the auricle - auricularis anterior, auricularis superior - and those muscles of facial expression which are located above the opening of the orbit, such as the frontal belly of occipitofrontalis (or frontalis), orbicularis oculi, and corrugator supercilii muscles.
The zygomatic branches are motor branches of the facial nerve that innervate facial muscles below the eye. After leaving the parotid plexus, the zygomatic branches go obliquely, ventrally, and upward, innervating muscles of the lower half of the orbit - the orbicularis oculi, the zygomaticus major, and the zygomaticus minor muscles.
The buccal branches are motor branches of the facial nerve that innervate muscles of facial expression located around the nose and the mouth. The buccal branches leave the parotid plexus in a fan-shaped manner eventually reaching the area of the mouth and innervating the following muscles of facial expression: zygomaticus, levator labii superioris, levator anguli oris, risorius, buccinator, orbicularis oris, nasalis, and levator labii superioris alaeque nasi.
The marginal mandibular branch is one of the motor branches of the facial nerve emerging from the parotid plexus, and it innervates facial muscles in the chin area. After leaving the parotid plexus, the marginal mandibular branch runs by the base of the mandible, and innervates muscles of facial expression around the chin - depressor labii inferioris, depressor anguli oris, and mentalis.
Intermediate portion of facial nerve
The intermediate portion of the facial nerve (also called the intermediate nerve) carrying parasympathetic and sensory fibers is a part of the facial nerve within the facial canal. The sensory fibers of the intermediate nerve arise from pseudounipolar neurons located in the geniculate ganglion. The dendrites of these neurons transmit taste information from the anterior two thirds of the tongue. The axons of these pseudounipolar neurons travel via the facial nerve to reach the solitary tract nucleus in the brainstem.
The intermediate portion of the facial nerve divides into two branches right after forming the geniculate ganglion. These branches are:
- greater petrosal nerve (parasympathetic),
- chorda tympani (parasympathetic and sensory).
The greater petrosal nerve carries parasympathetic preganglionic fibers and leaves the facial canal through the hiatus for greater petrosal nerve. The greater petrosal nerve innervates the lacrimal gland, and the mucous membrane of the nasal cavity and the palate.
The fibers of the greater petrosal nerve originate from the lower part of the pons. The nerve contains not only preganglionic parasympathetic fibers, but sympathetic fibers as well. The parasympathetic fibers exit the brainstem as a part of a separate division (intermediate nerve) of the facial nerve. When reaching the geniculate body, the greater petrosal nerve breaks away and travels anterolaterally to exit the superior surface of the temporal bone through the hiatus for the greater petrosal nerve. After exiting the hiatus, the nerve continues anteromedially and slightly inferiorly and passes under trigeminal cave toward the foramen lacerum where it joins the deep petrosal nerve from the carotid sympathetic plexus, forming the Vidian nerve (nerve of pterygoid canal).
The chorda tympani is a branch of the facial nerve carrying sensory and parasympathetic preganglionic fibers. The chorda tympani travels almost all of the length of the facial canal and leaves the canal through the canaliculus for chorda tympani to join the lingual nerve. The afferent special (gustatory) fibers of the chorda tympani transmit the taste sensations from the lingual papillae of the anterior two-thirds of the tongue via the lingual nerve, while the efferent parasympathetic preganglionic fibers synapse in the submandibular ganglion to provide secretomotor innervation to the submandibular and sublingual glands.