VII,VIII CRANIAL NERVES

 

Facial nerve (VII cranial nerve)

Special visceral efferent (SVE), general visceral efferent (GVE), general visceral afferent (GVA), special visceral afferent (SVA) and general somatic afferent (GSA)

 

The root of the facial nerve is made up of a motor root and an intermediate root. The motor root is formed by the SVE component. The intermediate root is formed by GSA, GVE, GVA and SVA components coming from the parasympathetic nucleus of the facial nerve to the pterygopalatine ganglion, and from the parasympathetic nucleus of the intermediate nerve to the mandibular and sublingual salivary glands.

The motor and intermediate nerve fibers of the facial nerve leave the brain stem ventrolaterally at the rostral portion of the trapezoid body. They are surrounded by a common sheath of dura mater together with the vestibulocochlear nerve.  After a short distance, they enter the facial canal (located in the petrosal part of the temporal bone) through the internal acoustic meatus. Inside the facial canal, the nerve turns caudally and laterally, forming the external genu of the facial nerve. At this point the geniculate ganglion is located. This ganglion contains the bodies of the afferent neurons (GVA and SVA).

 

The major petrosal nerve (GVE and GVA) leaves the facial nerve at the level of the geniculate ganglion. It is joined by sympathetic postganglionic fibers from the cranial cervical ganglion via the internal carotid plexus. These sympathetic fibers form the deep petrosal nerve. Both nerves (the major and the deep) constitute the nerve of the pterygoid canal, which leaves the skull through a small foramen on the rim of the rostral alar foramen. This nerve reaches the pterogopalatine ganglion where the parasympathetic neurons synapse on postganglionic neurons that innervate the lacrimal gland by means of the lacrimal nerve (a branch of the ophthalmic nerve) and the zygomaticotemporal nerve (a branch of the maxillary nerve). Other postganglionic fibers are incorporated to the pterygopalatine nerve (also a branch of the maxillary nerve) to innervate the palate and nasal glands.

 

The facial nerve continues along the facial canal and gives off the stapedial nerve (SVE) to innervate the stapedius muscle. This muscle modifies the position of the stapes in the vestibular window.

 

The next branch to leave the facial nerve within the facial canal is the chorda tympani (GVE, GVA and SVA). This nerve passes through the canaliculus of the chorda tympani to reach the middle ear cavity. It runs medial to the tympanic membrane and exits the skull through the petrotympanic fissure to join the lingual nerve (a branch of the mandibular nerve). The efferent fibers (GVE) are parasympathetic preganglionic neurons that reach the mandibular and sublingual ganglia in order to innervate the mandibular and sublingual salivary glands respectively. The afferent fibers transmit sensations from the rostral two thirds of the tongue including taste (SVA).

 

The facial nerve is joined by the auricular branch of the vagus nerve. This leaves the vagus nerve at the level of the jugular foramen and constitute the lateral internal auricular nerve (a branch of the facial nerve) that innervates the external ear canal.

 

Once the facial nerve has become external through the stylomastoid foramen, it gets thicker because the epineurium increases. As the facial nerve runs rostrally, it gives rise to the following nerves: the caudal auricular nerve (SVE for the platysma and caudal auricular muscles), the caudal and middle internal auricular nerve (GSA fibers for the auricular concha), the lateral internal auricular nerve (formed by GSA vagal fibers to the external ear canal), the digastric branch (SVE and GSA for the caudal belly of the digastricus muscle) and the stylohyoid branch (SVE and GSA for the stylohyoid and jugulohyoid muscles).

 

When the facial nerve passes the caudal border of the mandible on the surface of the masseter muscle, it sends out the following SVE branches: cervical, buccal and auriculopalpebral. The cervical branches innervate the parotidoauricularis and the sphincter colli muscles before joining the ventral branch of the second cervical spinal nerve. The buccal nerve divides into dorsal and ventral branches. These innervate the buccinator, the orbicularis oris and the nasolabial muscles. The dorsal and ventral buccal branches receive GSA fibers through communicating branches of the auriculotemporal nerve and mylohyoid nerves respectively (these nerves are branches of the mandibular nerve). The auriculopalpebral nerve divides into palpebral and rostral auricular nerves. The palpebral nerve forms a plexus that innervates the rostral auricular, nasolabial and palpebral muscles. The rostral auricular branch innervates the scutuloauricularis muscles.

 

There is no evidence for proprioceptive afferents in the facial nerve that innervates the superficial or mimic muscles of the face, most likely the trigeminal nerve contains afferents from intramuscular and cutaneous receptors involved in mimic movements.

 

Vestibulocochlear nerve (VII cranial nerve)

Special somatic afferent (SSA)

 

The vestibulocochlear nerve consists of fibers that transmit stimuli from receptors located in the inner ear. So this nerve does not course outside that cranium.

 

The vestibulocochlear nerve enters the cranial cavity through the internal acoustic meatus (located in the medial surface of the petrosal part of the temporal bone). It enters the brain stem at the level of the trapezoid body. The fibers of the vestibular root (sense of balance) end in the vestibular nuclei and the fibers of the cochlear root (sense of hearing) reach the cochlear nuclei. Same cochlear fibers are efferent to the organ of Corti.