Neurology: Vision

Visual Pathways

1. Optic nerve axons arise from retinal ganglion cells 2. Optic nerves from each eye unite to form optic chiasm 3. Optic Tract leaves the optic chiasm and enters the LGN (lateral geniculate nucleus) 4. From LGN, information travels to optic radiations to the primary visual cortex

Macula

5mm in diameter. Responsible for high-resolution, color vision

Lateral Geniculate Nucleus (LGN)

6 layers, number 1 to 6 from ventral to dorsal in which the optic tract fibers terminate in a precise retinotopic pattern. -LGN receives information about the contralateral visual field

Visual Field of Healthy Eye

90º to 60º from nasally to center 60º above and 75º below center Each eye has a horizontal range of 155º and vertical field of 135º

Zonule Fibers

Alter the shape of the lens to accommodate to near --> far objects *ciliary muscles attach here

Visual Field

Amount of space viewed by the retina when the eye is fixated straight ahead. Forms an image on the retina that is inverted & reversed.

Dorsal Stream (where)

Analyzes form, with specific regions identifying color, faces, letters, and other visual stimuli. (location, movement, spatial, transformations, spatial relations)

Ventral Stream (what)

Analyzes motion and spatial relationships between objects, and between the body and visual stimuli. (Color, texture, pictorial detail, shape, and size)

geniculostriate pathway

Axons from the neurons in the LGN to the primary visual cortex path

Monocular

Both eyes are used separately, field of view is increased, but depth perception is limited. Eyes are positioned on opposite sides of head, so you can see 2 objects at once

blood supply to visual cortex

Branches of MCA and PCA

Binocular

Central portion of an image seen by both eyes when looking straight forward (eyes are centered in head). Allows for depth perception

Glaucoma

Chronic elevated eye pressure causes optic nerve atrophy and loss of peripheral vision

Vitreous Humor

Clear gel-like substance that is composed of mostly water and gives the eye its shape

Cataract

Clouding of the lends and decrease in acuity. Vision is hazy, particularly in glaring light -Does not affect field of vision -there is no scotoma (a distorted, empty or dark area)

Macular Degeneration

Deterioration of the macula, the central area of the retina -Most prevalent eye disease -central scotoma -Peripheral or side vision remains unaffected

Ptosis

Drooping of eyelid. Muscles that raise the eyelid (levator and superior tarsal muscles) are not strong enough to lift it. More common in elderly when muscles deteriorate

Damage to CN3

Ex. Damage to right CN3. No change in right eye (dilated) but left eye will constrict with light

Damage to CN2

Ex. Right CN 2 damaged. Causes no information to anywhere with right eye. left eye will constrict with light, right won't change

Meyers Loop

Fibers that don't pass directly backward to the occipital lobe, and form a broad sheet covering much of the posterior and inferior horns of the ventricle. Fibers representing the superior visual quadrants

Fovea

In the center of the macula, rich with cones, and specialized with visual acuity. Information from fovea is represented by about half of the fibers in the optic nerve and half the cells in the 1 degree visual cortex

Corneal Pathology

Injury or damage resulting in distorted or clouded image and increase in glare sensitivity. -clear detail is not discernable -field of vision is normal

Optic Disk

Lacks photoreceptors, interneurons, and ganglion cells. It is where the ganglion cell axons leave the eye to form the optic nerve.

Diabetic Retinopathy

Leaking of retinal blood vessels may occur in advanced or long-term diabetes; affects the macula or entire retina and vitreous. -Likelihood of retinopathy and cataracts increase with the length of time a person has diabetes, along with consistency and level of blood glucose control

Bitemporal Hemianopia

Lesion at Optic Chiasm. Causes outer blindness in both eyes

Contralateral Homonymous Hemianopia

Lesion at either at optic tract or anterior radiation orUpper bank of calcarine fissure. Blindness Outter of one eyes and inner of other

Monocular visual loss

Lesion at optic nerve. Blind in one eye

Contralateral superior quadrantanopia

Lesion or disruption of the optic radiations in the temporal lobe (Loop of Meyer) and lower bank. Blindness in upper outer in one eye and upper inner outer of other

Contralateral inferior quadrantanopia

Lesion to superior optic radiation and or upper bank. Blindness in lower outer eye and lower inner eye

Retina

Light-sensitive layer

LGN (lateral Geniculate Nucleus)

Majority of axons in optic tract synapses in LGN and project to primary visual cortex --> retino-ginculo-striate pathway

Extrageniculate pathway

Minority of optic tract fibers bypass the LGN forming the extrageniculate visual pathway. -project mainly on the pretectal area and superior colliculus in the midbrain

Strabismus

Misalignment of eyes. Estimated 4% of the population ha sit. Known for direction of the eye misalignment -3 Cranial nerves (III, IV, VI) responsible for eye movement can be weak or palsied and cause strabismus

Anterior Calcarine Fissure

Peripheral regions of the contralateral visual fields are represented here

Visual Cortex

Primary visual cortex lies on the banks of the Calcarine fissure in the occipital lobe. Upper portions of the optic radiations project to the superior banks of the Calcarine Fissure. Inferior optic radiations terminate on the lower bank.

Sclera

Protective layer, forms the sheath of the optic nerve. Anterior 1/6 is the conrea --> transparent, let light in

Left Visual Hemifield

Seen in left Nasal retina and right temporal retina

Right Visual Hemifield

Seen in right Nasal Retina and Left Temporal retina

Monocular scotoma

Spot in the eye is blind. Lesion is at the scotoma

Optic Radians

Superior visual field projects onto the inferior optic radiations. Inferior visual field projects onto the superior optic radiations

Lens

Transparent, biconvex structure that reflects light to be focused on retina

Visual Field Image

Upper visual field projects onto lower retina. Lower visual field projects on upper retina. right visual field projects on left hemiretina, etc.

Phototransduction

When no light - cGMP levels are high (sodium/potasium open) = higher threshold (less negative). Membrane potentials are at -40 mV. Increase NT When light, cGMP levels are low, closes sodium channels, allows potassium to go out and lowers the threshold to -70mV. Decrease NT

Uvea

Where blood vessels and nerves travel. 3 Areas: Choroid Ciliary Body Iris

Pupil

alters the amount of light reaching the retina

Retina

developed from the diencephalon, is CNS derivative. Optic nerve is a CNS structure

Scotoma (blind spot)

no photoreceptors, blind to any object whose images falls on this part of the retina

Occipital Pole

the region of the fovea is represented near the occipital pole