CN II-Optic Nerve and Visual System

Question 1

Visual System

Answer 1

• most highly developed of sensory modalities
• optic nerve and retina grow out of diencephalon
• retina encodes visual information and projects that info to brain via myelinated fibers in optic nerve
• optic nerve fibers undergo hemidecussation (part of fibers cross) in optic chiasm and project to primary visual cortex of the occipital lobe via thalamus (lat geniculate body)
• from eye –> optic chiasm = optic nerve
• optic chiasm to lateral geniculate body = optic tract
• lat geniculate body –> 1* visual cortex = optic radiation

Question 2

The Eye

Answer 2

• optic nerve (II) emerges from posterior pole
• sclera: white, fibrous, protective outer layer of eye; over the anterior pole it is transparent and called cornea-light passes through to lens and then retina
• iris: fibers arranged in circular and radial fashion (under control of autonomic nervous system): circular (pupil constriction)-parasympathetic; radial (dilate pupil): sympathetic
• pupil: central aperture
• transition from eye into optic thalamus nerve = optic disc/blind spot
• ciliary body: contains ciliary muscle (parasympathetic innervation), alters shape of lens
• choroid: dark pigmented inner surface of sclera that absorbs light and reduces reflection
• retina: inner surface of choroid, photoreceptive

Question 3

Diabetic Retinopathy

Answer 3

• eye filled with vitreous fluid

- diabetics who aren’t under good control –> leaky vessels in eye –> min bleeds –> see floaters in vision

Question 4

Retina

Answer 4

• photoreceptors: transduce light to electrical impulses by a series of photochemical reactions and physicochemical changes
• two varieties: rods-20x more numerous, very sensitive to light; cones-responsible for color
• highest acuity and high spatial and temporal resolution in cones
• more cones at fovea
• photoreceptor (rods and cones) –> bipolar cells –> optic nerve

Question 5

Visual Field

Answer 5

• visual field consists of four quadrants
• image formed is inverted both vertically and laterally
• objects that lie in left half of visual field form an image on the right half of right eye and the right half of the left eye

Question 6

Visual Pathways

Answer 6

• retinal ganglion axon travel in optic nerve and converge in optic chiasma
• axons from medial portions of retina decussate at chiasma and pass into contralateral optic tract
• axons from lateral portions of retina remain ipsilateral through chiasma
• axons then travel to thalamus, a relatively small number of axons bypass thalamus and go to pretectal region and superior colliculi for mediation of pupillary light reflex

Question 7

Thalamocortical Neurons (3rd Order)

Answer 7

-project to primary visual cortex of occipital lobe: primarily on medial aspect of the occipital lobe above and below calcarine sulcus

Question 8

Visual Association Cortex

Answer 8

• surrounding primary visual cortex
• interpretation of visual images
• recognition
• depth perception
• color vision

Question 9

Visual Field Disorders

Answer 9

• monocular blindness: cataract, intraocular hemorrhage, retinal detachment
• bitemporal hemianopia: compression of optic chiasma by pituitary tumor
• homonymous hemianopia: vascular or neoplastic lesions of the optic tract, optic radiation

Question 10

Meyer’s Loop

Answer 10

• medial is parietal lobe goes to upper bank of calcarine fissure and receives images from lower visual field
• temporal is meyer’s loop white matter tracts that go to lower bank of calcarine fissure and receives vision from upper visual quadrant from opposite eye
• look at drown out schematics

Question 11

Optic Chiasm Pressure

Answer 11

• pituitary gland lies under optic chiasm –> tumor here would put pressure on optic chiasm at central fibers = tunnel vision
• bitemporal hemianopia

Question 12

L Optic Nerve Disruption

Answer 12

• monocular anopia

- blindness in left eye

Question 13

Lesion on L optic Tract

Answer 13

• affects right side of both eyes

- right homonymous heminaopia