Cranial Nerves PDF

Cranial nerves By Dr. Shilan Hussein Cranial nerves, like spinal nerves, contain sensory or motor fibers or combination of these fiber types. named cranial nerves because they emerge through foramina or fissures in the cranium and are covered by tubular sheaths derived from the cranial meninges. The...

Cranial nerves By Dr. Shilan Hussein Cranial nerves, like spinal nerves, contain sensory or motor fibers or combination of these fiber types. named cranial nerves because they emerge through foramina or fissures in the cranium and are covered by tubular sheaths derived from the cranial meninges. The cranial nerves are named as follows: I. Olfactory II. Optic III. Oculomotor IV. Trochlear 5 V. Trigeminal 6 VI. Abducent 7 VII. Facial 8 VIII. Vestibulocochlear a IX. Glossopharyngeal X. Vagus XI. Accessory 12 XII. Hypoglossa I 2 8th The olfactory, optic, and vestibulocochlear nerves are entirely sensory; 6 11 12 4 3 the oculomotor, trochlear, abducent, accessory, and hypoglossal nerves are entirely motor; and the remaining nerves are mixed. 570 1975C Nerves 1st Olfactory dec sensory Olfactory Nerves The olfactory nerves arise from olfactory receptor nerve cells in the olfactory mucous membrane. The olfactory mucous membrane is situated in the upper part of the nasal cavity above the level of the superior concha. Bundles of these olfactory nerve fibers pass through the openings of the cribriform plate of the ethmoid bone to enter the olfactory bulb in the cranial cavity. The olfactory bulb is connected to the olfactory area of the cerebral cortex by the olfactory tract. This nerve is responsible for smelling (Olfaction). F Testing the Integrity of the Olfactory Nerve The olfactory nerve can be tested by applying substances with different odors to each nostril. Fractures of the anterior cranial fossa or cerebral tumors of the frontal lobes may produce lesions of the olfactory nerves, with consequent loss of the sense of smell (anosmia). lossofthesenseofsmell 2ndOptic Nerve sensors Optic Nerve The optic nerve is composed of the axons of the cells of the ganglionic layer of the retina. The optic nerve emerges from the back of the eyeball and leaves the orbital cavity through the optic canal seen to enter the cranial cavity. The optic nerve then unites with the optic nerve of the opposite side to form the optic chiasma. In the chiasma, the fibers from the medial half (nasal) of each retina cross the midline and enter the optic tract of the opposite side, whereas the fibers from the lateral (temporal) half of each retina pass in the optic tract of the same side. canal veyptic I Optic tract Emerges from the optic chiasma Each optic tract winds around the midbrain.and divides in to medial and lateral root 2 E 1 Medial smaller root runs to the z superior brachium, superior L reflex) and to colliculus nuclei (light the pretectal nuclei in the tegmentum (for accomodation) Lateral larger root terminate in LGB of thalamus (visual fibers) 3 Lateral geniculate body (LGB) It’s a thalamic nucleus from it to optic radiation and then to occipital cortex. Most of the fibers of the optic tract terminate by synapsing with nerve cells in the lateral geniculate body. A few fibers pass to the pretectal nucleus and the superior colliculus and are concerned with light reflexes. The axons of the nerve cells of the lateral geniculate body pass posteriorly as the optic radiation and terminate in the visual cortex of the cerebral hemisphere (Occipital lobe). o Testing the Integrity of the Optic Nerve When examining the optic disc, it should be remembered that the intracranial subarachnoid space extends forward around the optic nerve to the back of the eyeball. The retinal artery and vein run in the optic nerve and cross the subarachnoid space of the nerve sheath. A rise in intracranial pressure in the subarachnoid space will compress the thin walls of the retinal vein , resulting in congestion of the retinal veins, edema of the retina, and bulging of the optic disc (papilledema) 3rd Oculomotor Nerve The oculomotor nerve emerges on the anterior surface of the midbrain, it divides into a superior and an inferior ramus, which enter the orbital cavity through a the superior orbital fissure. The oculomotor nerve supplies the following: The extrinsic muscles of the eye: the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique. The intrinsic muscles of the eye: the constrictor pupillae of the iris and the ciliary muscles. e Drooping of the upper eyelid (ptosis) occurs because of paralysis of the levator palpebrae superioris muscle. The pupil is widely dilated because of the paralysis of the sphincter pupillae and the unopposed action of the dilator pupilla. e ptosis 4th Trochlear Nerve in The trochlear nerve leaves the midbrain, enters the orbit through the superior orbital fissure. The trochlear nerve supplies: The superior oblique muscle of the eyeball (extrinsic muscle SO4). The trochlear nerve is entirely motor and assists in turning the eye downward and laterally. In fourth nerve paralysis the patient complains of double vision. e It This is because the superior oblique is paralyzed and the eye turns medially as the inferior rectus pulls the eye downward. son ath