No. 32

1. No. 32 • Ⅰ--Ⅵ Pairs of Cranial Nerves

2. Section 2 The Cranial Nerves • 12 pairs of cranial nerves arise from the brain. They leave or enter the cranial cavity through a series of foramina or fissures in the base of the skull. • Fibrous components of the cranial nerves: • The cranial nerves consist of seven fibrous components. ①The general somatic motor fibers • These fibers supply the striated muscles from the myotome, e.g. the extrinsic muscles of the eye and the muscles of tongue.

3. ②The general visceral motor fibers • They belong to the parasympathetic nervous system. Their cell bodies are located in certain nuclei of the brain stem and are small in size. All these fibers end in a certain parasympathetic ganglion near the visceral effectors and control the smooth muscle, cardiac muscle and glands.

4. ③The special visceral motor fibers • These fibers supply the striated muscles from the branchiomere, e.g. the muscles of expression, mastication, pharynx and larynx. ④The general visceral sensory fibers • They conduct the general sensory impulses which originate from the viscera in the head, neck, thorax and abdomen.

5. ⑤The special visceral sensory fibers • They distribute taste buds and olfactory organs. ⑥The general somatic sensory fibers • They transmit the impulses which originate from the skin, muscle, muscular tendon, and most part of oral and nasal mucous membrane.

6. ⑦The special somatic sensory fibers • These fibers distribute the vestibulocochlear and visual organs. • Most of the cranial nerves are mixed nerves composed of both motor and sensory neurons, although some cranial nerves carry only sensory impulses. • With the exception of the vagus nerve, the cranial nerves supply only structures in the head and neck. The cranial nerves are numbered (using Roman numerals) from superior to inferior in the order in which they leave the brain (Table 1).

7. Table 1 The Origins and passages of the Cranial Nerves

10. According to the components and functions the twelve pairs of cranial nerves are divided into three types: 1. The afferent or sensory nerves • They are olfactory, the optic and vestibulocochlear nerves. 2. The efferent or motor nerves • They are the oculomotor, the trochlear, the abducent, the accessory and the hypoglossal nerves. 3. The mixed nerves • They are the trigeminal, the facial, the glossopharyngeal and vagus nerves.

11. Ⅰ. Olfactory Nerve • The olfactory nerves are entirely sensory, carrying impulses associated with the sense of smell. • They arise from receptor cells in the nasal mucosa. Processes of these receptor cells pass through the perforations of the cribriform plate of the ethmoid bone and enter the olfactory bulbs of the telencephalon portion of the brain. In the olfactory bulbs, the nerve fibers synapse with neurons that pass posteriorly in the olfactory tracts. The fibers of the olfactory tracts enter the brain, and many of them travel to the cerebral cortex of the medial sides of the temporal lobes. • In severe injuries of the anterior cranial fossa, the olfactory nerves may be torn, thus producing a loss of the smell (anosmia) and the cerebrospinal fluid leakage into the nasal cavity.

13. Ⅱ. Optic Nerve • It is a special somatic sensory nerve and originates from the ganglionic cell of the retina. • The fibers of the optic nerve converge on the optic disc and they pierce the sclera. As traversing the sclera the nerve fibers receive their medullary sheaths, and run in bundles which are collected to form the optic nerve. • The optic nerve is enclosed in three layers of coverings. These coverings and the spaces between them are continuous with those of the brain. So that, the meningitis may involve the eyes. An increase in SF pressure slows venous return from retina, causing the edema of the optic disc (papilledema), such as in the case of the brain tumor, intracranial abscess, hemorrhage, hypertension, etc.

15. Ⅲ. Oculomotor Nerve • Components of fibers: • It is a motor nerve and contains somatic and general visceral motor fibers. • The somatic motor fibers arise from the oculomotor nucleus. The general visceral motor fibers arise from the cells of the Edinger-Westphal nucleus. • Course: • The oculomotor nerve emerges from the sulcus on the medial side of the cerebral peduncle, and passes within the lateral wall of the cavernous sinus, where it lies above the trochlear nerve. It then divides into a superior and an inferior branches, which enter the orbit through the superior orbital fissure.

17. Branches and distribution: • The superior branch, the smaller, supplies the superior rectus and the levator palpebrae superioris. • The inferior branch supplies the inferior rectus, medial rectus and inferior obliquus. • From the nerve for the inferior obliquus, a short thick branch is given to the ciliary ganglion. This branch is the preganglionic fibers of the parasympathetic nerve which arises from the cells of the Edinger-Westphal nucleus.

18. Clinical significance: • Paralysis of the oculomotor nerve leads, when complete, to (1) ptosis, or dropping of the upper eyelid; (2) external strabismus, on account of the unopposed action of the lateral rectus and superior obliquus; (3) dilatation of the pupil and loss of power of accommodation of exposure to light, when parasympathetic nerve fiber was damaged at the same time.

19. The ciliary ganglion is a parasympathetic ganglion. • It is situated between the optic nerve and the lateral rectus near the apex of the orbit. • The preganglionic fibers (from the E-W nucleus) of the parasympathetic nerve are relayed in the ganglion. • The postganglionic fibers arising from the ciliary ganglion supply the sphincter pupillae and ciliary muscles.

20. Ⅳ. Trochlear Nerve • It is a motor nerve and supplies the superior obliquus. • It arise from the trochlear nucleus, emerges from the superior medullary velum immediately behind the inferior colliculus. The nerve winds forwards round the cerebral peduncle, then passes forwards in the lateral wall of the cavernous sinus. Through the superior orbital fissure, the trochlear nerve passes medially and finally enters the superior obliquus.

22. Ⅴ. Trigeminal Nerve Ⅰ) Fibrous omponents and nervous roots: • It is a mixed nerve. It includes two components i.e. general somatic sensory fibers and special visceral motor fibers. • It is attached to the side of the pons by two roots. • The sensory root is made up of the central processes of neurons which are located in the trigeminal ganglion.

23. The trigeminal ganglion is a semilunar dilatation of the nerve, it occupies the trigeminal impression near the apex of the petrous part of the temporal bone and is situated in the side of the cavernous sinus. • The nerve cells of the ganglion are pseudounipolar neurons, their peripheral processes enter each of the three divisions of the trigeminal nerve, their central processes form the sensory root to enter the pons and end in the pontine nucleus of trigeminal nerve and the spinal nucleus of trigeminal nerve.

24. The three divisions of the trigeminal nerve are: • the ophthalmic nerve, • the maxillary nerve, • the mandibular nerve. • The general somatic sensory fibers of the three nerves are distributed to the eye, the skin of the face, the mucous membranes of the oral cavity, nasal cavity and the paranasal sinuses as well as the teeth and the meninges, and conduct the pain, thermal and crude touch sensation.

26. The distribution in the face is limited by the fissures of the eye and the mouth. • The smaller motor root contains the special visceral motor fibers which arise from the motor nucleus of trigeminal nerve and only take part in the third division of the trigeminal nerve to supply the masticatory muscles, mylohyoid, anterior belly of digastric, tensor veli palatini and tensor tympani etc.

27. Ⅱ) Branches and distribution 1. The ophthalmic nerve • It passes forwards within the lateral wall of the cavernous sinus; just before entering the superior orbital fissure, it divides into three branches, and is distributed to the cerebral dura mater, orbit, eyeball, lacrimal gland, conjunctiva, and partial mucous membrane of nose and the skin of the forehead, upper eyelid and the back of nose.

28. 1) The lacrimal nerve • It supplies the lacrimal gland and the skin of the upper eyelid. • The parasympathetic fibers coming from facial nerve go in the nerve by the communicating branch of the zygomatic nerve and control the secretion of the lacrimal gland.

30. 2) The frontal nerve • It divides into a supratrochlear and supraorbital branches. • They are distributed to the skin of forehead and anterior part of the scalp.

31. 3) The nasociliary nerve • It distributes to the eyeball, the ethmoidal and sphenoidal sinuses, and to the skin of the eyelids, the lacrimal sac, and the skin of the nasal back and mucous membrane of the nasal cavity.

32. 2. The maxillary nerve • It passes horizontally forwards within the lower part of the lateral wall of the cavernous sinus, leaves the skull through the foramen rotundum. It then crosses the upper part of the pterygopalatine fossa, and enters the orbit through the inferior orbital fissure. It is now named the infraorbital nerve. • The maxillary nerve supplies the cerebral dura mater, the skin between the fissures of the eye and the mouth and the mucous membranes of the nasal and oral cavities.

34. 1) The infraorbital nerve • It is the terminal branch of the maxillary nerve. • It supplies the alae nasi, the lower eyelid and the upper lip. 2) The pterygopalatine nerves • It supplies the mucous membrane of the nasal cavity, palate and the pharynx. 3) The zygomatic nerve • It is distributed to the skin over part of the cheek and temple. • The communication with the lacrimal nerve conveys parasympathetic postganglionic fibers from the pterygopalatine ganglion to the lacrimal gland.

35. 4) The superior alveolar nerves • It includes the anterior, middle and posterior branches. These branches communicate freely with each other and form the superior dental plexus to supply the maxillary sinus, the upper gum and teeth.

36. 3. The mandibular nerve • It is a mixed nerve and the largest division of the trigeminal nerve. • It emerges almost immediately through the foramen ovale. • The motor fibers supply the masticatory muscles, tensor tympani and tensor veli palatini. The sensory fibers are distributed to the cerebral dura mater, mandibular teeth and gum, the mucous membrane of the floor of the mouth and the anterior two-thirds of the tongue as well as the skin over the mandible, part of the external ear and temporal area.

38. 1) The auriculotemporal nerve • It generally arises by two roots, which encircle the middle meningeal artery. It supplies the skin of the anterior surface of the auricle, the temporal region and the parotid gland. 2) The buccal nerve • It supplies the skin over the buccinator and the mucus membrane lining its inner surface. 3) The lingual nerve • It passes anteriorly to the inferior alveolar nerve, and forwards between the ramus of mandible and the medial pterygoid. After receiving the chorda tympani of the facial nerve the lingual nerve is connected to the submandibular ganglion by two or three branches.

39. The branches of the lingual nerve supply the mucous membrane of the floor of mouth, and mucous membrane of the anterior two-thirds of the tongue. It conducts the common sense of the mucous membrane. • The special visceral sensory fibers (taste fibers) of chorda tympani nerve distributed with the lingual nerve to the anterior 2/3 of tongue. • The general visceral motor fibers of chorda tympani nerve connect the submandibular ganglion, and the postganglionic fibers control the secretion of the submandibular and subligual glands.

40. 4) The inferior alveolar nerve • It is a mixed nerve. It enters the mandibular foramen, passes through the mandibular canal below the teeth as far as the mental foramen. The terminal branch is mental nerve. • It supplies all the lower teeth and gums, the skin and mucous membrane of the lower lip, mylohyoid and the anterior belly of the digastric muscles.

41. 5) The nerve of maticatory muscles • It is a motor nerve, whose branches supply the masseter, temporalis, lateral pterygoid and medial pterygoid. • A lesion of the whole trigeminal nerve causes anaesthesia of the corresponding anterior half skin of the scalp, and the face, the cornea and conjunctiva, and the mucous membrane of the nose, mouth and anterior two-thirds of the tongue. Paralysis and atrophy of the masticatory muscles result in the mandible to thrust over to the paralyzed side when the mouth is opened.

42. Ⅵ Abducent Nerve • It is a somatic motor nerve and supplies the lateral rectus only. • It arises from the abducent nucleus and passes forwards through the pons, and emerges from the medial part of the bulbopontine sulcus. After leaving the surface of the brain stem, the abducent nerve runs upward and laterally, crosses the apex of the petrous part of the temporal bone. It then traverses the cavernous sinus, lateral to the internal carotid artery, and enters the orbital cavity though the superior orbital fissure and finally sinks into the medial surface of the lateral rectus. • The result of the damage of this nerve is the paralysis of the lateral rectus, medial squint.