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Cranial Nerves II, III, IV & VI

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Cranial Nerves II, III, IV & VI Cranial Nerves II, III, IV & VI Neuroanatomy block-Anatomy-Lecture 13 Editing file Objectives At the end of the lecture, students should be able to: 01 List the cranial nuclei related to occulomotor, trochlear, and abducent nerves in the brain stem. 02 Describe the type and site of each nucleus. 03 Describe the site of emergence and course of these 3 nerves. 04 Describe the important relations of occulomotor, trochlear, and abducent nerves in the orbit. 05 List the orbital muscles supplied by each of these 3 nerves. Color guide 06 Describe the effect of lesion of each of these 3 nerves. ● Only in boys slides in Green ● Only in girls slides in Purple 07 Describe the optic nerve and visual pathway. ● important in Red ● Notes in Grey Extra-Ocular Muscles 7 muscles 1. Levator palpebrae superioris. (Elevates the upper eyelid) 2. Medial rectus.(medial) 3. Lateral rectus.(lateral) 4. Superior rectus.(upward and medially ) 5. Inferior rectus.(downward and medially) 6. Superior oblique.(downward and laterally) 7. Inferior oblique.(upward and laterally) ❖ All muscles of the eye are supplied by the oculomotor nerve, EXCEPT LR6 lateral rectus (by abducens) + SO4 superior oblique (by trochlear) Brain stem (Lateral view) Brain (Ventral view) 3 Oculomotor Nerve ➢ Motor for most of extraocular muscles. ➢ Also carries preganglionic parasympathetic fibers to the pupillary constrictor (pupillary reflex) and ciliary muscles (accomodation). Has two nuclei Accessory nucleus Main oculomotor nucleus (Edinger-Westphal nucleus) ❖ Lies in the midbrain at the ❖ Lies dorsal to the main motor level of superior colliculus, nucleus. ❖ located in the periaqueductal ❖ receives Fibres from the pretectal grey matter. nucleus for the direct and consensual pupillary reflexes, and Corticonuclear fibers for accommodation reflex ❖ Its cells are preganglionic parasympathetic neurons. 4 Oculomotor Nerve ➔ It curves ventrally through the tegmentum and the red nucleus in the midbrain. Axons from the ➔ The nerve emerges on the anterior surface of oculomotor nucleus the midbrain in the interpeduncular fossa accompany the oculomotor nerve fibers to the medial to crus cerebri/cerebral peduncle. orbit, where they terminate in the ciliary ➔ Then it passes forward between posterior ganglion (one of the parasympathetic ganglion of cerebral and superior cerebellar arteries. the head & neck)Postganglionic fibers pass ➔ In the middle cranial fossa it runs in the through the short ciliary nerves to the eyeball, lateral wall of the cavernous sinus, then it where they supply: divides into superior and inferior divisions ❏ Constrictor pupillae muscle of the iris which pass to the orbit through the superior ❏ Ciliary muscle. orbital fissure to the orbit Axons from the Edinger-Westphal nucleus oculomotor nerve ciliary ganglion 5 The results of Lesion Oculomotor Nerve Lateral squint. (since medial is affected and 1 only lateral is working) Oculomotor Nerve Ptosis (drooping of the eyelid). 2 responsible Diplopia (double vision). Diplopia always supplies accompanies squint for 3 Pupillary dilatation Motor to Elevation of upper 4 eyelid (open the eye). ● Levator palpebrae superioris ● Superior rectus muscle Loss of accommodation ● Medial rectus muscle Turning the 5 ● eyeball upward, Ptosis Inferior rectus muscle downwards and ● Inferior oblique muscle medially. The eyeball is fully abducted and 6 depressed (down and out) because of the unopposed activity of lateral rectus and Constriction of the superior oblique. Parasympathetic pupil (pupillary fibers to reflex) ● Constrictor pupillae ● note: The preganglionic parasympathetic fibers run superficially in the ● Ciliary muscles Accommodating nerve and are therefore the first axons to suffer when a nerve is affected reflex of the eyes. by external pressure. Consequently, the first sign of compression of the oculomotor nerve is ipsilateral slowness of the pupillary response to light. 6 Trochlear Nerve (IV) 4th Cranial Nerve ➢ Type: Motor ➢ location: of Small motor nucleus in the periaqueductal grey matter at the level of inferior colliculus of the midbrain. ➢ Course of the nerve: ★ Fibers curve backwards and decussate. ★ The nerve emerges immediately caudal to the inferior colliculus, on the dorsal surface of brainstem. ( it is the only cranial nerve to emerge from the dorsal surface). ★ It passes forward through middle cranial fossa in the lateral wall of the cavernous sinus.(trochlear and oculomotor run in the lateral wall and abducens runs medial to them). ★ The nerve then enters the orbit through the superior orbital fissure Lesion results in: Function: Supplies: 1. diplopia Rotates the eyeball Superior oblique muscle 2. Inability to rotate the eyeball inferolaterally. downwards and laterally. (only one muscle) So, the eye deviates; upward and slightly inward (medially). This person has difficulty in walking downstairs 7 Abducent Nerve (VI) 6th Cranial Nerve ➢ Only one motor nucleus. ➢ Lies in caudal pons in the floor of the 4th ventricle. close to the middle line, in a line with 3rd, 4th & 12th nerves. ➢ Fibers of facial nerve looping around the abducent nucleus forms the facial colliculus. ➢ It emerges from the ventral aspect of brainstem, at the junction of the pons and the pyramid of the medulla oblongata. ➢ It passes through the floor of cavernous sinus, lying below and lateral to the internal carotid artery ➢ Then it enters the orbit through the superior orbital fissure. ➢ It supplies: the lateral rectus muscle which rotates the eyeball laterally ; (abduction). Abducent Nerve Lesion ● medial squint: Inability to direct the affected eye laterally ● nuclear lesion:may also involve the nearby nucleus or axons of the facial nerve, causing paralysis of all facial muscles in the ipsilateral side. # Oculomotor lateral squint| Abducent medial squint 8 Optic Nerve (II) 2nd Cranial Nerve ● Type: sensory ● Function: Vision ● Lesion :visual field defects and loss of visual acuity, a defect of vision is called anopsia. Visual Pathway Optic Optic Optic nerve tract radiation Lateral Optic geniculate Visual chiasm body cortex ”nucleus” 9 Visual Pathway Photoreceptors: Rods & Cones of the retina Three orders neurons pathway : 1 Bipolar cells of retina Ganglion cells of retina. 2 Their axons form the optic nerve Neurons in the lateral geniculate body. 3 Their axons terminate in primary visual cortex. 10 Visual Pathway Optic nerve 01● Axons of retinal ganglion cells converge at the optic disc and pass as the optic nerve. 02● Then the nerve passes posteromedially in the orbit. 03● Then exits through the optic canal to enter the middle cranial fossa to form the optic chiasma. Optic Chiasma ➔ Fibers from the nasal (medial) half of retina decussate in the chiasm and join uncrossed fibers from the temporal (lateral) half of the retina to form the optic tract. ➔ The decussation of nerve fibers in the chiasm results in the right optic tract conveying impulses from the left visual field and vice versa. ➔ The partial crossing of optic nerve fibers in the optic chiasma is a requirement for binocular vision. 11 Visual Pathway Optic Tract ➢ 01 Mainly terminate in the (LGB), lateral geniculate body of the thalamus (3rd order neuron). ● From the lateral geniculate nucleus (third-order neuron), thalamocortical fibres project through the retrolenticular part of the posterior limb of the internal capsule as the optic radiation ● which terminates in the primary visual cortex of the occipital lobe. 02 ➢ A few fibers terminate in pretectal area and superior colliculus. ➔ These fibers are related to light reflexes. Visual Cortex The primary visual cortex is located predominantly on the medial surface of the occipital is extensive, including the whole of the occipital lobe, the lobe in the region above, below and behind the calcarine sulcus. adjacent posterior part of the parietal lobe. This cortex is It’s occupies the upper and lower lips of the calcarine sulcus on involved in interpretation and recognition of objects and the medial surface of occipital lobe .(area 17 of Brodmann's perception of color, depth, motion, and other aspects of classification) vision. # The primary cortex: I saw something, while Association cortex: what did I see (recognition & interpretation) The visual association cortex # Each primary cortex has an association cortex. 12 Visual Defects Bitemporal Contralateral Monocular Blindness Hemianopia Homonymous Hemianopia ● “Loss Of Vision In The Affected Eye “ Compression of the optic Vascular and neoplastic lesions ● Disease of the eyeball (cataract, chiasm by an adjacent of the optic tract, optic radiation intraocular hemorrhage, retinal pituitary tumor or occipital cortex detachment, glaucoma & methyl alcohol poisoning) ● disease of the optic nerve (multiple sclerosis, optic nerve tumors) For your knowledge : •A person may not be able to see objects on their right or left sides (homonymous hemianopsia) if the optic tract or radiation or visual cortex is affected. •Or may have difficulty seeing objects on their outer visual fields (bitemporal hemianopsia) if the optic chiasm is involved. •The pretectal area, or pretectum, is a midbrain structure composed of seven nuclei and comprises part of the subcortical visual system. Pretectal nuclei are bilateral group of highly interconnected nuclei located near the junction of the midbrain and forebrain. 13 Only on the boy’s slides. Retinitis Pigmentosa It is an inherited metabolic disorder of the Definition photoreceptor and retinal pigment epithelial cells. It is due to mutation of a key protein called
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