No. 33 Ⅶ--Ⅻ pairs of Cranial Nerves

Ⅶ Facial Nerve Ⅰ) Fibrous components: It is a mixed nerve, it contains four components: (1) The special visceral motor fibers They take origin from the facial nucleus and supply the muscles of expression. (2) The general visceral motor fibers They arise from the superior salivatory nucleus and are relayed in certain parasympathetic ganglia. The postganglionic fibers are distributed to the lacrimal gland, the submandibular and sublingual salivary glands and glands in the mucous membrane of the nose and palate.

(3) The special visceral sensory fibers They arise from the cells of the geniculate ganglion. The central processes of these cells terminate in the nucleus of solitary tract. The periphery processes of these cells are distributed to the taste buds on the anterior 2/3 of the tongue. (4) The general somatic sensory fibers They connect the somatic sensation of skin in ear and the proprioceptive sensation of the muscles of expression.

Ⅱ) Course: The facial nerve leaves the brain at the bulbopontine sulcus just medial to the vestibulocohlear nerve. It passes laterally and forwards with that nerve to the internal acoustic pore. At the bottom of the meatus, the facial nerve enters the facial canal, where it turns sharply downward and backward behind the middle ear. The geniculate ganglion is located at the entrance of facial canal as an enlargement of the nerve. The nerve emerges onto the side of the face through the stylomastoid foramen, then runs forward into the parotid gland.

Ⅲ) Branches and distribution 1. The branches within the facial canal 1) The greater petrosal nerve It includes secretary fibers of parasympathetic nerve. It arises from the geniculate ganglion and runs forward and passes through the base of skull to the pterygopalatine fossa and enters the pterygopalatine ganglion; the postganglionic fibers are distributed to the lacrimal gland and the glands of the nose and palate.

2) The chorda tympani It arises from the facial nerve about 6 mm above the stylomastoid foramen. It travels upwards into the tympanic cavity going between the manubrium of malleus and tympanic membrane, and pierces the petrotympanic fissure to leave the tympanic cavity to the infratemporal fossa. Then, it runs downwards and forwards to join the lingual nerve. The chorda tympani contains two kinds of fibers:

① Its taste fibers (special visceral sensory fibers) are the peripheral processes of the cells of the geniculate ganglion and are distributed to the mucous membrane covering the anterior 2/3 of the tongue, excluding the vallate papillae. ② Its efferent preganglionic parasympathetic fibers enter the submandibular ganglion, which sends postganglionic fibers to the submandibular and sublingual glands.

3) Stapedial nerve It supplies the stapedius in the tympanic cavity. 2. The branches outside the facial canal After the facial nerve exits from the stylomastoid foramen, it gives off three small branches supplying the occipital belly of the occipitofrontalis, the posterior auricularis, posterior belly of the digastric and the stylohyoid. The principle stem of the facial nerve then runs forward into the parotid gland and forms a network (parotid plexus). At the anterior border of the parotid gland it divides into 5 branches, and fans out to supply the muscles of expression:

② The zygomatic branches ① The temporal branches It supplies the frontal belly of the occipitofrontalis, and orbicularis oculi. ② The zygomatic branches It supplies the orbicularis oculi and the zygomaticus. ③ The buccal branches It supplies the buccinator, orbicularis oris and other muscles around the mouth. ④ The marginal mandibular branch It supplies the mucles of the lower lip. ⑤ The cervical branch It supplies the platysma.

Ⅳ) The parasympathetic ganglia related to the facial nerve 1. The pterygopalatine ganglion It lies in the pterygopalatine fossa under the maxillary nerve. The preganglionic parasympathetic fibers of the greater petrosal nerve enter this ganglion. The postganglionic fibers pass along the zygomatic branch to the ophthalmic nerve to control the secretion of the lacrinal gland. The postganglionic fibers also manipulate the secretion of the glands in the mucous membrane of the nasal cavity and palate.

2. The submandibular ganglion It lies below the lingual nerve, deep to the submandibular gland. The preganglionic parasympathetic fibers of the chorda tympani enter this ganglion. The postganglionic parasympathetic fibers are distributed to the submandibular and sublingual glands and control their secretions.

Ⅴ) The symptoms after injury of the facial nerve 1. The injury outside the facial nerve canal The main symptom is paralysis of muscles of expression. The face looks asymmetrical. The affected side of the face shows that the wrinkles on the forehead are smoothed out, the eye can not shut voluntarily, the nasolabial fold becomes smooth; the affected angle of mouth droop.

2. The injury inside the facial nerve canal There is paralysis of the muscles of expression as wella as the following symptoms: The taste in the anterior 2/3 of the tongue is lost. The secretion of the lacrimal, submandibular, and sublingual glands are decreased or lost.

Ⅷ. The Vestibulocochlear Nerve (CN Ⅷ) It is special somatic sensory nerve, consisting of cochlear nerve and vestibular nerve. Ⅰ) The vestibular nerve It connects the sensation of equilibrium. The vestibular nerve arises from the bipolar nerve cells of the vestibular ganglion situated in the bottom of the internal acoustic meatus. Its peripheral fibers terminate in the crista ampullaris and the macula of utriculi and sacculi; the central fibers pass into the brain at the pontocerebellar angle with the cochlear nerve.

Ⅱ) The cochlear nerve It connects the acoustic sensation. The sensory fibers of the cochlear nerve take origin from the cells of the cochlear ganglion situated in the modiolus. The ganglion consists of bipolar nerve cells, whose peripheral fibers distribute the spiral organs, while the central processes pass into the brain through the internal acoustic meatus, terminating the ventral and dorsal nuclei of cochlear nerve.

Clinical significance: The vestibulocochlear nerve is frequently injured together with the facial nerve in the fracture of the base of skull which involves the internal acoustic meatus, or in the case of an acoustic tumor. Injuries of the internal ear may also cause the deafness and loss of the equilibrium.

Ⅸ. The Glossopharyngeal Nerve (CN Ⅸ) Ⅰ) Fibrous components It is a mixed nerve and contains five components. ① The special visceral motor fibers These fibers arise from the nucleus ambiguus to control the stylopharyngeus. ② The parasympathetic nerve fibers (general visceral motor fibers) They arise from the inferior salivatory nucleus and are relay in otic ganglion. The postganglionic fibers are distributed to the parotid gland and manipulate the secretion of it.

③ The general visceral sensory fibers Its visceral sensory fibers are the peripheral processes of the inferior ganglion, to distribute the mucous membrane of the pharynx, posterior 1/3 of the tongue, tympanic cavity, auditory tube, and the chemoreceptors in the carotid glomus and pressure receptors of the carotid sinus. The central processes terminate in the nucleus of solitary tract.

④ The special visceral sensory fibers The cell bodies of their neurons are in the inferior ganglion. The peripheral processes are distributed to the taste buds posterior 1/3 of the tongue, the central processes terminate in the nucleus of solitary tract.

⑤ The general somatic sensory fibers These fibers are the peripheral processes of the superior ganglion and supply the skin of the posterior surface of the auricle. The central processes of the superior ganglion terminate in the spinal nucleus of trigeminal nerve.

Ⅱ) Route of glosopharyngeal nerve It leaves (enters) the skull through the jugular foramen. It passes forwards between the internal jugular vein and internal carotid artery to reach the styloid process, to the pharynx and breaks up into its terminal branches.

Ⅲ) Branches of glosopharyngeal nerve ① The lingual branches They are the terminal branches of glosopharyngeal nerve, supplying the mucous membrane and taste buds of the posterior 1/3 of the tongue. They connect general and special (taste) sensations. ② The pharyngeal branches They supply the mucous membrane of the pharynx with sensory branches, the stylopharyngeus with motor branch.

③ The tympanic nerve It arises from the inferior ganglion of the glosopharyngeal nerve and ascends to the tympanic cavity to form the tympanic plexus. Some small branches are distributed in the mucous membrane of tympanic cavity, mastoid cells, and auditory tube. The lesser petrosal nerve arises from this plexus and contains the preganglionic parasympathetic fibers from the inferior salivatory nucleus), passes outside the tympanic cavity and joins the otic ganglion. The postganglionic fibers pass through a communicating branch to the auriculotemporal nerve which carry the secretomotor fibers to the parotid gland.

The otic ganglion is situated immediately below the foramen ovale, its pregnaglionic fibers come from the lesser petrosal nerve. The postganglionic fibers pass through a communicating branch to the auriculotemporal nerve which supplly the parotid gland. ④ The carotid sinus branch It is distributed to the wall of carotid sinus and the carotid glomus.

Ⅹ. The Vagus Nerve (CNⅩ) It is a mixed nerve. Its course is the longest and its distribution is the most extensive in the cranial nerves. Ⅰ) Components It contains four components: ① The general visceral motor (parasympathetic) fibers They arise from the dorsal nucleus of vagus nerve, and end in the paraorganic or interorganic parasympathetic ganglions in the thoracic and abdominal cavities. The postganglionic fibers supply the smooth muscles, cardiac muscles and glands of the viscera.

② The special visceral motor fibers. They arise from the nucleus ambiguus and supply the striped muscles of the larynx and the pharynx. ③ The general visceral sensory fibers The cell bodies are located in the inferior ganglion of vagus nerve. The central processes terminate in the nucleus of the solitary tract. The peripheral processes of this ganglionic cells are distributed to the viscera of the neck, thorax and abdomen.

④ The general somatic sensory fibers. The cell bodies of neurons are in the superior ganglion of vagus nerve. Their central processes enter the brain to terminate in the spinal nucleus of trigeminal nerve. The peripheral processes of this ganglionic cells supplies the skin of the auricle and external acoustic meatus and cerebral dura mater.

Ⅱ) Course The vagus nerve is attached to the groove posterior olive, and leaves the skull through the jugular foramen. It then passes vertically down the neck within the carotid sheath behind the internal jugular vein and the internal carotid artery (or the common carotid artery). Both vagus nerves descend through the inlet into the thorax, situating behind the roots of lungs.

The left vagus nerve divides into several small branches to form the anterior esophageal plexus in front of the esophagus and forms the anterior vagal trunk at the lower end of the esophagus. The right vagus nerve forms the posterior vagal trunk on the back of the esophagus. The anterior and posterior vagal trunks enter the abdominal cavity through the esophageal hiatus of the diaphragm and divide into their terminal branches near the cardia of the stomach. The anterior vagal trunk divides into the anterior gastric branches and the hepatic branches; the posterior vagal trunk divides into the posterior gastric branches and the celiac branches.

1. The branches in the neck 1) The superior laryngeal nerve It arises from the inferior ganglion of the vagus nerve, descends by the side of the pharynx and medially to the internal carotid artery. It divides into the internal and external branches at the greater horn of hyoid bone. The internal branch pierces the thyrohyoid membrane and supplies sensory fibers to the mucous membrane of the larynx down as far as the level of the vocal folds. The external branch supplies the cricothyroid only.

2) The cervical cardiac branches They join the cardiac branches of the sympathetic trunk, and form the superficial and deep cardiac plexuses. 3) The pharyngeal branch They form the pharyngeal plexus to the muscles of the pharynx and the soft palate and the mucous membrane of the pharynx. 4) The ear branch It contains the general somatic sensory fibers, supplying the skin of the auricle and external acoustic meatus. 5) The meningeal branch It is distributed in the cerebral dura mater.

2. The branches in the thorax 1) The recurrent laryngeal nerve The right one winds from before backwards round the subclavian artery, and ascends obliquely to the side of the trachea behind the common carotid artery. The left one winds below the aortic arch immediately behind the arterial ligament to the concavity of the arch, and then ascends to the side of the trachea. The nerve on each side ascends in the groove between the trachea and esophagus, and enters the larynx behind the cricothyroid joint, where they are named the inferior laryngeal nerve.

They give off branches to all the muscles of larynx, excepting the cricothyroid, and supply sensory fibers to the mucous membrane of the larynx below the level of the vocal folds. Unilateral injury of the recurrent laryngeal nerve produces the paralysis of vocal fold on the same side, resulting in a weak hoarse voice. Bilateral involvement of the recurrent laryngeal nerves produces variable signs and symptoms. Hoarseness even aphonia is present. Paralysis of the muscles in the aryepiglottic fold may result in the dysphagia (difficult to swallow) and regurgitation.

2) The broncheal branches and the esophageal branches They form the pulmonary plexus and esophageal plexus, supply their smooth muscles and glands.

3. The branches in the abdomen 1) The anterior gastric branches and posterior gastric branches They are distributed to the stomach, the anterosuperior surface of the stomach is mainly supplied by the anterior branches of the left vagal trunk and the posteroinferior surface mainly by the posterior gastric branches of the right one. The branches to the body and fundus of the stomach regulate its gastric acid secretion. The branch to the pyloric part of the stomach regulates the hormone gastrin and in relation to emptying the stomach. The branch reaching to the pyloric canal and pylorus is named the “crow’s foot”.

2) The hepatic branches They join the hepatic plexus and then supply the liver and gallbladder to regulate the bile secretion. 3) The celiac branches They join the celiac plexus and accompany with the celiac artery, superior mesenteric and renal arteries to supply the liver, spleen, pancreas, small intestine, cecum and appendix, ascending colon, right half of transverse colon, kidneys and suprarenal glands.

Ⅳ) Clinical points The secretion of the gastric acid is regulated by the vagus nerve. The section of vagus nerve can reduce the production of acid in the person with peptic ulcer. Often a highly selective vagotomy is done during which only the twigs to the wall of stomach before the “crow’s foot” are sectioned. This operation has twofold effects of reducing the acid secretion and keeping the function of the pylorus and other abdominal organs which are supplied by the vagus nerve.

Ⅺ. The Accessory Nerve (CNⅪ) It is a motor nerve, formed by the union of the cranial and spinal roots. The smaller cranial root arises from the lower part of the nucleus ambiguus and emerges from the side of the medulla below the rootlets of the vagus nerve by 4—5 rootlets. The cranial root passes through the jugular foramen and joins the vagus nerve immediately to form the component of pharyngeal branch and recurrent laryngeal nerve of the vagus nerve to the muscles of the soft palate and intrinsic muscles of the larynx.

The larger spinal root arises from the accessory nucleus in the anterior horn of the upper 5—6 cervical segments. It emerges from the spinal cord by rootlets between the anterior and posterior rootlets of the spinal nerves. The spinal root ascends and enters the skull through the foramen magnum. It then passes upward and laterally to the jugular foramen, where it unites the cranial root to form the accessory nerve and passes through this foramen for a short distance.

It leaves the cranial root, passes backward and downward to the sternocleidomastoid, which it pierces and supplies, and then crosses the posterior triangle of the neck to enter and supply the deep surface of the trapezius. In fracture or other damage involving the jugular foramen, the glossopharyngeal, vagus and accessory nerves may be damaged together. The symptoms and signs of this condition are called the jugular foramen syndrome.

Unilateral lesion of accessory nerve usually does not produce an abnormality in the position of the head. However, weakness in turning the head to the opposite side against resistance can be detected due to the sternocleidomastoid muscle paralysis. Unilateral paralysis of the trapezius may produce dropping of the shoulder.

Ⅻ. The Hypoglossal Nerve (CNⅫ) It is the motor cranial nerve and arises from the hypoglossal nucleus. Its 10—15 rootlets emerge from the medulla oblongata between the olive and the pyramid. The rootlets unite to leave the skull by way of the hypoglossal canal. The nerve descends between the internal carotid artery and the internal jugular vein. At the level of the angle of mandible it passes forward over the internal and external carotid arteries and gives off its descending and thyrohyoid branches. It then goes forwards over the hyoglossus to enter the tongue.

The hypoglossal nerve supplies all the extrinsic and intrinsic muscles of the tongue (with the exception of the palatoglossus). Injury to the hypoglossal nerve results in paralysis and atrophy of the affected side of the tongue. The tongue deviates to the paralyzed side during protrusion because of the action of the unaffected genioglossus muscle on the other side.