Facial (VII) nerve &Hypoglossal (XII) nerves Dr. Nimir Dr. Safaa

Objectives Describe the nuclei of the facial nerve Follow up the course of facial nerve from its point of central connections, exit and down to its target areas. Discuss the various modalities of its fibers. Review your knowledge of its target areas Follow up the course from the central connections; exit from the brain and down to its target organs of XII nerves Make a list of types of nerve modalities conveyed by this nerve.

Facial Nerve (Cranial NerveVII): The facial nerve is both motor & sensory nerve Facial Nerve Nuclei: It has three nuclei: (1) Main motor nucleus (SVE). (2)Parasympathetic nuclei (GVE). (3) Sensory nucleus(SVA).

Main Motor Nucleus: It lies deep in reticular formation of lower part of the pons . The part that supplies muscles of upper part of face receives corticonuclear fibers from both cerebral hemispheres. The part that supplies muscles of lower part of face receives only corticonuclear fibers from the opposite cerebral hemisphere.

Parasympathetic Nuclei: They lie posterolateral to main motor nucleus. They are: Superior salivatory nucleus Lacrimal nucleus . Superior salivatory nucleus receives afferent fibers from hypothalamus through descending autonomic pathways. Information concerning taste also is received from nucleus of solitary tract from mouth cavity. Lacrimal nucleus receives afferent fibers from hypothalamus for emotional responses and from sensory nuclei of trigeminal nerve for reflex lacrimation secondary to irritation of cornea or conjunctiva.

Sensory Nucleus: It is the upper part of nucleus of tractus solitarius and lies close to motor nucleus. Sensations of taste travel through peripheral axons of nerve cells situated in geniculate ganglion on 7th cranial nerve. Their central processes synapse on nerve cells in the nucleus.

Efferent fibers cross median plane and ascend to ventral posterior medial nucleus of opposite thalamus and to a number of hypothalamic nuclei. From thalamus, axons of thalamic cells pass through internal capsule and corona radiata to end in taste area of cortex in lower part of postcentral gyrus .

Course of the Facial Nerve: It consists of a motor and a sensory root. Fibers of motor root first travel posteriorly around the medial side of abducent nucleus. They then pass around the nucleus beneath facial colliculus in floor of fourth ventricle and, finally, pass anteriorly to emerge from brainstem.

Sensory root (nervus intermedius) is formed of central processes of unipolar cells of geniculate ganglion. It also contains efferent preganglionic parasympathetic fibers from parasympathetic nuclei.

The two roots of facial nerve emerge from anterior surface of brain stem between pons and medulla oblongata. They pass posterior cranial fossa with vestibulocochlear nerve and enter internal acoustic meatus in petrous bone. At bottom of meatus, nerve enters facial canal and runs laterally through inner ear.

On medial wall of tympanic cavity it expands to form sensory geniculate ganglion and turns backward then downward and emerges from stylomastoid foramen.

Hypoglossal Nerve (Cranial Nerve XII): It is a motor nerve that supplies all muscles of the tongue except palatoglossus. Hypoglossal Nucleus (GSE): It is situated close to midline immediately beneath floor of lower part of fourth ventricle. It receives corticonuclear fibers from both cerebral hemispheres. Cells responsible for supplying genioglossus muscle only receive corticonuclear fibers from opposite cerebral hemisphere.

Course of the Hypoglossal Nerve: Fibers emerge emerge as a series of roots on anterior surface of medulla oblongata between pyramid and olive . The nerve leaves skull through hypoglossal canal. It passes downward and forward in the neck to reach lower border of posterior belly of digastric muscle, then lying on lateral surface of hyoglossus muscle it sends branches to muscles of the tongue . Hypoglossal nerve controls movements and shape of tongue.