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Kim Dej (45)

Visual System

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Kim Dej

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Laboratory C Laboratory 12: Visual System • Sclera and cornea are continuous connective sheets that form the perimeter of the eyeball o Cornea is site at which light is most refracted • Iris and pigment epithelium are both pigmented to absorb light o Iris restricts light rays that pass pupil to fall on retina, which increases acuity by allowing only image of one object to fall upon any part of the retina • Dioptric system – components of eye which light passes and is refracted o Cornea, aqueous humor of anterior chamber, lens, and vitreous body • Extraocular muscles – medial longitudinal fasciculus carries axons from nuclei of III, IV, VI to each other and also brings vestibular input into the nuclei of the EOM. o Coordination of EOM contraction and conjugate movements  Intranuclear ophthalmoplegia – uncoordinated mvmts of eye due to an interruption of MLF at level btw VI and IV o Reflexive movement – of eyes in response to mvmt of head/body, and consequenct activation of the vestibular system • Intrinsic musculature and autonomic neurons – smooth muscles in iris and ciliary body o Dilator pupillae (of iris) – control size of pupil. Receive sympathetic (adrenergic) innervation from superior cervical ganglion neurons o Sphincter pupillae – constricts pupil and innervated by parasympathetic (cholinergic) fibers from ciliary ganglion  Preganglionic neurons are in Edinger-Westphal nucleus, which is a part of IIIrd nerve complex in MB o Ciliary body –  At rest: there is resting tension on zonular fibers (suspensory ligaments)  it keeps the lens flat so eye focuses on distant objects  Contraction due to parasympathetic afferents: the ZF go slack  allows lens to fatten • Accomodation – rounded lens has greater refractory power to bring near objects into focus • Retina o Outer segments (of rods and cones) – contains light-sensitive visual pigment molecules in stacked membranes  Light absorbed causes closure of Na channels, hence hyperpolarization  The shape of OS gives names to rods and cones o Inner segments – contain metabolic machinery of cell. Synthesis of new visual pigments.  Much mitochondria b/c of extreme energy needed to keep internal Na low despite constant influx o Outer nuclear layer – soma of rods and cones o Outer plexiform layer – synapses of photoreceptors with BP and H cells  Light absorption decreases rate of NT release at photoreceptor axon terminal o Inner nuclear layer – soma of BP, H, and A cells o Inner plexiform layer – synapses of BP and A with RGCs o Ganglion cell layer – soma of RGCs. Axons make up optic nerve. o Nerve fiber layer – axons of RGCs as they run twd optic disk, where they turn at right angles and leave retina as optic nerve • Fovea is surrounded by a region of yellow pigment, the macula lutea • Optic nerve  optic chiasm  optic tract • Terminations of optic tract o Suprachiasmatic nuleus – RGCs express melanopsin and are intrinsically photosensitive. These are impt in regulation of circadian rhythms o LGN – majority or RGCs terminate here. o Pretectum or SC • Lateral geniculate nucleus – immediately lateral to cerebral peduncles o Magnocellular layers (1 and 2), parvocellular layers (3-6), koniocellulars (occuply lightly-stained regions ventral to each of the other layers o Layers 1, 4, 6 are innervated by the contralateral retina (axons that crossed in optic chiasm) o Layers 2, 3, 5 are innervated by ipsilateral retina o Contains an orderly representation of the contralateral visual field  Rostral – temporal/monocular crescent  Caudal 1/3 – fovea  Medial – inferior visual field  Lateral – superior visual field • Small portion of optic tract bypasses LGN and enters brachium of superior colliculus o Brachium is btw MGN and pulvinar (2 other thalamic nuclei) • Superior colliculus o Superficial layers – directly innervated by retina, and respond briskly to visual stimuli o Deeper layers – innervated by cerebral ctx (FEF, PPC). More myelinated, and sends axons to BS structures like CN III, IV, and VI.  Integrate multimodal afferents to control saccades that orient the eyes to visual, auditory, or SS stimuli • LGN  V1 along optic radiations o Meyer’s loops – axons from Lateral half of LGN (represent superior visual field)  inferior horn of lateral ventricle in temporal lobe  caudally to occipital lobe o Retrolenticular axons – axons from Medial half of LGN (represent inferior visual field)  project straight caudally • V1 – burned in both banks of calcarine sulcus o Upper bank – inferior visual field o Lower bank – superior visual field (axons of Meyer’s loop terminate on lower bank of fissure) o Occipital pole  ½ way thru rostrocaudal extent of V1 – fovea [magnification factor] o Has an expanded layer IV o Stria of Gennari – bundle of intracortical axons in layer IV • Lesions o Anterior to chiasm – affect one eye, but produces a loss in only small part of visual field [temporal/monocular crescent – can map by closing and opening each eye in succession] o At chiasm – affect only crossing axons [temporal visual fields on each side are lost] o Posterior to chiasm – deficit in contralateral visual field [hemianopsias]  Partial lesions – losses confined to a quadrant • Meyer’s loops – contralateral superior quadrantanopsia o Macular sparing – lesions that produce hemianopsias often leave macular vision intact b/c occipital pole is vascularized by branches of 2 arteries • Visual reflexes o Obligatory structures  Retina, midbrain tectum (SC or pretectum), oculomotor complex o Pupillary light reflex (a simple reflex) – simple gauge of light intensity. Does not require conscious perception of stimulus  RGCs  pretectum  bilateral innervation of parasympathetic neurons in oculomotor complex [Edinger-Westphal nucleus] • Axons from contralateral pretectum cross midline in posterior commissure  Preganglionic parasympathetics from E-W nucleus  ciliary ganglion  postganglionic fibers (short ciliary fibers)  papillary constrictor  pupils narrow in width  B/c pretectal neurons project bilaterally to E-W nucleus, the pupil of an unstimulated eye constricts when light is shone in the other eye • Direct reflex – constriction of stimulated eye • Consensual reflex – constriction of contralateral eye • (Raymond removed images showing the neural pathway for pupil constriction and the dilation response when different nerves are damaged.) o Accomodation/convergence (complex reflex) – when eyes are moved to a new object of interest  LGN  V1  visual association cortex  Cortex  SC  oculomotor complex  conjugate mvmt of the two eyes  SC  preganglionic neurons of E-W nucleus  papillary constriction and contraction of ciliary muscle to flatten lens  brings object of interest to focus on the fovea Laboratory 13: The Auditory System • Perilymph – resembles CSF, within b
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