10/26 Eye Movement Part 1

Question 1

• Structure of Eye

Answer 1

o two large parts: anterior and posterior divided along ora serrata (point where retina ends on each side eye must maintain round shape. layer on outside (scelera) is continuout with dura. sclera becomes transparent and has no water so it is clear next layer is choroid (vascular) provides nutrients to neural retina. anterior chamber main purpose is to focus light optic nerve contains axons of neurons in neural retina in brain there is no neural retina in the back of your eye—there is no light perception (blind spot)

Question 2

• Main component of anterior segment

Answer 2

o main component of anterior: cornea, lens, iris lens is positioned behind ciliary muscle and is suspended by zonule fibers (attached to ciliary muscle). Lens changes shape to ensure light is directed at fovea. iris controls pupil size (center of ringed musculature). can increase focal length

Question 3

• Accommodation

Answer 3

o Mechanism: near–contraction of circular portion of ciliary muscle (oculomotor system), reduced diameter of musclerelaxation of zonular fibers that attach lens to muscle. Increase in convexity and refractive power of lens. Distant—relaxation of circular ciliary muscle, increased diameter increases tautness of zonular fibers, flattening of central portion of lens, decrease in refractive power.
o Refractive errors: presbyopia—aging lens capsule less elastic difficulty with near.

Question 4

• Binocular vision

Answer 4

o Binocular vision is the area of the visual field where there is overlap between the left and right eye. Monocular vision is the periphery. In the center of the binocular visual field is the. Foveal region and on either side of that is blind spot

Question 5

• Effects of lens on visual field map on retina

Answer 5

o Input from the superior visual field transmits to the ventral retina (inferior to dorsal) and temporal (outer) stimulus goes to nasal (inner) retina and vice versa. Cerebral cortex fills in the blind spots

Question 6

• Computerized perimetry to examine visual field

Answer 6

o A person looks into a machine and sees a series of bright flashes. Clicks a button every time they see light this maps where they can and cannot see. Optic nerve head is closer to the nose but the visual field is on outside. There is no light perception on the optic nerve heads (lateral visual field)

Question 7

• Major arteries and locations relative to internal carotid

Answer 7

o the first branch of ICA is opthalmic artery which travels along the nerve then branches so the central retinal artery branches off and goes into the nerve continues then forms ciliary arteries to form the choroid.

Question 8

• Fundus view – superior/inferior branches central retinal artery

Answer 8

o The optic nerve splits to form superior and inferior branches. Mascula in the middle contains fovea and pit. There are no veins because we do not want light scattered here. Central retinal arteries form superior and inferior branches.

Question 9

• Central retinal artery occlusion + Visual field

Answer 9

o Sudden painless loss of vision. Irreversible after 90min, may save some vision. Risks: age (50-70), arteriosclerosis (carotid artery disease—clots can travel to ICA)

Question 10

• Monocular altitudinal scotoma + Visual field

Answer 10

o Defect of a single branch of the optic nerve. Superior lesion leads to loss of sight in the ventral retina.

Question 11

• Amaurosis fugax

Answer 11

o Transient monocular vision loss due to transient occlusion of the central retinal artery or one of its branches. Temp loss, dimming, fogging, blurring. Sign of stroke or blindness. Causes: vascular disease internal carotid or ophthalmic artery, vasospasm. Closed angle glaucoma, intraocular hemorrhage, inflammation of optic nerve

Question 12

• Cavernous sinus thrombosis

Answer 12

o superior and inferior opthalmic veins and central all converge and deposit spent blood in cavernous sinus Infections can end up in cavernous sinus easily. lots of CN end up with severe problems (loss of vision, bulging of the eye, changes in CN function. Thrombosis is caused by these infections. Associated symptoms with CN III-VI and impaired venous drainage from orbit and eye (bulging). Retinal hemorrhages, decreased acuity, blindness. CT/MRI

Question 13

• Central retinal vein occlusion

Answer 13

o Blurring or loss of vision, blood and fluid leak into retina, floaters (blood in vitreous body). Risks: glaucoma, diabetes, vascular disease, high bp.

Question 14

• Structure of retina

Answer 14

o cell layer (ganglion form optic nerve. Internuclear layer contains horizontal, bipolar, and amacrine cells. allows communication and processing. bipolar is commuting with ganglion and photoreceptors outernuclear layer—cell bodies and photoreceptors (cones and rods) we have melanin in pigmented epithelium. supports function of photoreceptors. if they become detached, they die (detached retina) can tack back down with laser

Question 15

• Photoreceptors

Answer 15

o Rods and cones. Outer segment, inner segment, cell body, synaptic region. outer segment has machinery. rods have disks which sit inside extracellular membrane and has visual pigments which absorb light
o Rods: scotopic (dim), 1 pigment, no discrimination, high sensitivity, slow response, fast dark adaption, saturation
o Cones: photopic (bright), 3 pigments, discriminatory (color), low sensitivity, fast response, slow dark adaptation, no saturation (can keep responding to high light)

Question 16

• Photoreceptor distribution

Answer 16

o Macula= 5000µm, 650,000 cones (most)
o Fovea=1500µm, 100,000 cones
o Foveola=350µm, 25,000 cones
o The foveal line is packed with cone cells, very few rods. The ability to detect detail falls off dramatically due to loss of cones in the periphery but peripheral sight is better in the dark because of all the rods.

Question 17

• Visual pigments / color vision abnormalities

Answer 17

o Rods and cones have the same protein—11-cis retinal (vitamin a) which is able to absorb light. When it does so, it changes shape and pushes retinal protein apart and activates downstream cascade. Amino acids surrounding retinal binding pocket determine absorption characteristics of pigment. Rod—496. Cone—419, 531, 599.
o Dichromatic vision is severe red/green color deficiency. Loss of red cone.
o Protanopia—loss of L pigment (sex linked, x chromosome, 1% male)
o Deuteranopia—loss of M pigment (sex linked, x, 1%male)
o Tritanopia—loss of S pigment (chromosome 7, 1 in 10,000)
o Protoanomaly and deuteranomaly-mild colorblindness, skewed AA

Question 18

• Phototransduction

Answer 18

o Light stimulation of rhodopsin leads to activation of transducin. Activated G-protein activates cGMP phosphodiesterase (PDE). PDE hydrolyzes cGMP reducing concentration. This leads to closure of cGMP gated ion channels.
o Photoreceptors hyperpolarize in response to light, light reduces cGMP levels in photoreceptor, transmitter release decreases with light, combined inputs from long, middle, and short wavelength cone pigments are required to see color.

Question 19

• Inherited retinal diseases and treatment approaches

Answer 19

o Leber congenital amaurosis cone/rod dystrophy affects guanylate cyclase, cone dystrophy effects GCAP, retinitis pigmentosa causes rod cells to die. distributed in peripheral retina. loses peripheral first then macula until degeneration.this person will have tunnel vision. Recessive retinitis pigmentosa leads to closed channels and effects PDE.
o Treatment: nutrition, vitamins, growth factors, gene therapy, cell therapy, retinal prostheses (bypass neural retina and send signals upstream)

Question 20

Rodent dichromatic vision, acuity, depth of field, and comparison between pigmented and albino vision.

Answer 20

• Dichromatic vision
o 1% of photoreceptors are cones. 88% green 12% blue/uv. 12x less sensitive to red. Brightness is more important than color
• Visual acuity
o Pigmented rat 20/600, albino 20/1200. No fovea.
• Depth of field
o Lens cannot change shape—no accommodation. Field=7cm—infinity.
o Large visual field poor binocular depth. Their binocular overlap is above them. Use motion parallax to estimate depth (head bobbing)
• Comparison of albino and pigmented rat vision
o Albinos have lower acuity and more impaired vision. More susceptible to light damage. Fewer rods than pigmented rats and slow to adapt.