BMS2-9 Cranial Nerves - CN II, Visual Pathways, Visual Reflexes PDF

CN II, Visual Pathways, Visual Reflexes Dr. A. Aktar Optic Part of Retina Prof. Dr. H. Selçuk Sürücü Optic Nerve (CN II) • Fibers of optic n. are axons of cells in ganglionic layer of retina • Fibers converge on optic disc & exit eye about 3 or 4 mm to nasal side of center as the optic n. • Fi...

CN II, Visual Pathways, Visual Reflexes Dr. A. Aktar Optic Part of Retina Prof. Dr. H. Selçuk Sürücü Optic Nerve (CN II) • Fibers of optic n. are axons of cells in ganglionic layer of retina • Fibers converge on optic disc & exit eye about 3 or 4 mm to nasal side of center as the optic n. • Fibers of optic nerve are myelinated • Myelin sheaths are formed from oligodendrocytes rather than Schwann cells → optic nerve is comparable to a tract within CNS • Optic n. leaves orbital cavity thru optic canal • Unites w/ optic n. of opposite side to form optic chiasma Optic Chiasm • Chiasma: Point of contact • From Greek χιάζω 'to mark with an X', after the Greek letter 'Χ', chi) • Situated at junction of anterior wall & floor of 3rd ventricle • Fibers from nasal (medial) half of each retina, including nasal half of macula cross midline & enter optic tract of opposite side • Fibers from temporal (lateral) half of each retina, including temporal half of macula, pass posteriorly in optic tract of same side Optic Tract • Emerges from optic chiasm • Passes posterolaterally around cerebral peduncle • (Most) fibers terminate by synapsing w/ neurons in lateral geniculate body (small projection from post. part of thalamus) LGN • Some fibers project to: pretectal area (light reflex) superior colliculus (gaze reflex) of midbrain Lateral Geniculate Body • Small, oval swelling projecting from thalamus • Consists of 6 layers of cells, on which axons of optic tract synapse • Tertiary neurons for optic n. Optic Radiations – Geniculocalcarine Tracts • Axons of neurons within lateral geniculate body • The tract passes posteriorly & terminates in visual cortex (area 17) Visual Cortex • Primary visual cortex - area 17: A.k.a striate cortex a.k.a Brodmann area 17 • Occupies upper & lower lips of calcarine sulcus on medial surface of occipital lobe • Visual association cortex (areas 18 and 19) : Responsible for recognition of objects & perception of color Direct and Consensual Light Reflexes • Light shone into one eye → pupils of both eyes (normally) constrict ➔ Constriction of pupil on which the light is shone: direct light reflex ➔ Constriction of opposite pupil - even though no light fell on it - consensual light reflex Direct and Consensual Light Reflexes • Afferent impulses travel through optic n., optic chiasm & optic tract • Small number of fibers leave optic tract & synapse on neurons in olivary pretectal nucleus, which lies close to superior colliculus (mid-brain) Efferent: Pretectal neurons send axons to parasympathetic nuclei (Edinger- Westphal nuclei) of CN III on both sides ➔ Fibers synapse & PS nerves travel thru CN III to ciliary ganglion in orbit ➔ Postganglionic PS fibers pass through short ciliary nerves to eyeball & constrictor pupillae m. of iris • Both pupils constrict in consensual light reflex bcs pretectal nucleus sends fibers to PS nuclei on both sides • Fibers that cross median plane do so close to cerebral aqueduct in posterior commissure Direct and Consensual Light Reflexes Direct and Consensual Light Reflexes Accomodation Reflex • Eyes directed from a distant to a near object → Medial recti muscles contract → Ocular axes converge • Lens thickens to increase its refractive power by contraction of ciliary m. • Pupils constrict to restrict light waves to thickest central part of lens • Afferent impulses: travel thru optic n., optic chiasm, optic tract, LGB & optic radiation to visual cortex • Visual cortex connected to eye field of frontal cortex • From frontal cortex , cortical fibers descend thru internal capsule to oculomotor nuclei in midbrain → oculomotor nerve travels to medial recti muscles • Some descending cortical fibers synapse w/PS nuclei (Edinger-Westphal nuclei) of CN III on both sides • Here, fibers synapse & PS nerves travel thru CN III to ciliary ganglion in orbit • Postganglionic PS fibers pass thru short ciliary n.n. to ciliary m. & constrictor pupillae m. of iris Accommodation - Convergance Reflex When an object approaches your eyes, to have the image at the fovea centralis • Afferent pathway: Optic n. (visual pathway) • Afferent center : Visual cortex (Brodmann 17-18-19) • Efferent center: Oculomotor n. (Edinger-Westphal nucleus + Motor nucleus) • Efferent pathway: Oculomotor n. Result 1) Accomodation: Lens thickens - ciliary m. contracts 2) Pupillary constriction: Miosis- sphincter pupilla contracts 3) Convergence: Medial rectus contracts - eyes converge towards each other Corneal Reflex • Light touching of cornea or conjunctiva → blinking of eyelids • Afferent impulses from cornea or conjunctiva travel through ophthalmic n. to sensory nucleus of trigeminal n. • Internuncial neurons connect w/ motor nucleus of facial n. on both sides thru medial longitudinal fasciculus • Facial n. & its branches supply orbicularis oculi m., which closes eyelids Conjugate Horizontal Gaze • Eyes must move in conjugate, parallel gaze to ensure image projects to same spot of each retina & prevent diplopia (double vision) ➔ The 2 eyes demonstrate conjugate movement to R or L • Muscles used in horizontal gaze: Lateral rectus m. (CN VI) for abduction of eye & medial rectus m. (CN III) for adduction • E.g. To look to R w/ both eyes: R eye must abduct using R lateral rectus m. + L eye must adduct using L medial rectus m. Conjugate Horizontal Gaze – Neural Wiring • Cortical control center for horizontal gaze: Frontal eye fields (area 8), in each frontal lobe • Axons of frontal eye fields project to brainstem → decussate to contralateral paramedian pontine reticular formation (PPRF) in pons • Brainstem control center for ipsilateral horizontal gaze: PPRF • Abducens nuc. embedded in PPRF • Oculomotor nuc. (in midbrain) & abducens nuc. (in pons) interconnected by medial longitudinal fasciculus (MLF) • Interneurons from PPRF project to abducens nuc. • Another set of neurons in abducens nuc. send fibers that cross midline & ascend in contralateral MLF to contralateral oculomotor nucleus → results in adduction of contralateral eye Visual Body Reflexes • Automatic scanning movements of eyes & head made when reading • Automatic movement of eyes, head & neck toward source of visual stimuli • Protective closing of eyes & raising of arm for protection • Visual impulses follow optic n., optic chiasm, & optic tracts to superior colliculi (reflex gaze) • Impulses relayed to tectospinal & tectobulbar (tectonuclear) tracts & to neurons of anterior gray columns of spinal cord & cranial motor nuclei • Tectum (Latin: roof) dorsal part of midbrain Projections to Suprachiasmatic Nucleus • Retinohypotahlamic tract: Circadian rythms & neuroendocrine function Clinical Correlates • Destruction of macula: Central scotoma • Anopsia • Lesions of optic n. cause anopsia plus loss of sensory limb of light reflex • Compression of optic chiasm: Pituatiary tumour or menengioma → Loss of peripheral vision in both temporal fields • Lesions past chiasm produce contralateral defects • Lesions of visual radiations more common – occlusion of posterior cerebral artery • Combination of blindness with intact pupillary reflexes : Cortical blindness • Some causes of lesions: optic neuritis, central retinal artery occlusion, internal carotid a. aneursym, pituitary tumour, craniopharyngioma, middle or posterior cerebral artery occlusion