Visual System

outermost connective tissue layer

cornea and sclera

transparent, most densely innervated tissues for pain fibers

avascular - nutrition via diffusion

constant removal of water from surfaces

cornea

covers posterior 4/5’s of the eye

largely collagen and elastic fibers arranged in irregular bundles

higher water content than the cornea

sclera

avascular, biconvex structure lying posterior to the iris

suspending from ciliary body by suspensory ligaments

thickness controlled by ciliary body mm

lens

middle layer of the eye, vascular

uvea

thin layer of connective tissue with a specialized network of large capillaries term choriocapillaris

choroid

circumferential structure attached to lens by suspensatory ligaments, and containing ciliary muscle for lens accommodation

ciliary body

pigmented disc with a central aperture (the pupil)

smooth muscle fibers regulating size of pupil and amount of light reaching the retina

ring-shaped sphincter pupillae lies around the margin

iris

delicate, transparent tissue covers the inner surface of the posterior wall of the globe

retina

what produces aqueous humor that then gets pushed into the anterior and posterior chambers

ciliary body

gelatinous mass betwene lens and retina with slow equilibration

vitreous humor

a single layer of cells whose basal surface is in contact with choriocapillaris and apical surface is in contact with rod and cone outer segments

retinal pigment epithelium

retinal pigment epithelium fx

absorption of excess light

storage of vitamin A for use by photoreceptors in visual transduction

light sensitive neurons of the retina

photoreceptors: rods and cones

rods

low levels of light, responsible for night vision (scotopic)

only see black and white at low levels of illumination

cones

specialized for acuity

no color at low illumination

contain 3 visual pigments (blue green red) and stimulation of these three results in ability to see color (photopic)

equal stimulation results in white light

dendrites receive impulse transmissions from rods and cones, and the axon of the bipolar cell synapses with a ganglion cell

bipolar cell

modify visual data from photoreceptors to enhance borders and contours and increase contrast

horizontal and amacrine cells

extend through all portions of retina

fx like oligodendrocytes and provide metabolic support for retinal cells

muller cells

dendrites receive impulse from bipolar cells and the axons join to form the optic nerve

ganglion cells

rods are entirely absent from __ and increase in number toward the _

central fovea ; peripheral retina

cons are densely packed in the _ and decrease in number rapidly in all peripheral directions

central retina

what represents central vision

macula lutea

small yellow circular area at posterior pole of globe in direct line with visual axis

fovea centralis

shallow depression in the center of the macula lutea

area for sharpest vision and most acute color discrimination

only contains cones

shape accommodation of lens on far point

flat lens

shape accommodation of lens on near point

fat lens

ametropia

structural changes to eye results in refractive errors

hyperopia

unable to focus on near objects (farsighted)

eyeball is too short

myopia

unable to bring distant objects into clear focus (nearsighted)

cornea too curvy or eyeball too long

how to correct ametropias

corrected with appropriate lenses

papilledema

build up of fluid at optic disc (papilla)

• optic disc is invested in all three meningeal layers

caused by increased intracranial pressure

bilateral occurs over hours to weeks

how to treat papilledema

diuretics, steroids, removal of excess CSF by lumbar puncture

the optic disc lacks, and what exits the optic disc

photoreceptors (blind spot)

ophthalmic blood vessels enter and exit at this site

glaucoma

aqueous humor drains from the eye at corneoscleral jx

blockage of site results in glaucoma

visual field

what is visible to an individual when eyes are fixed on an object in direct line of vision

predominantly binocular vision with a monocular segment at the extreme peripheral portion of visual field that can only be seen by one eye

visual field divisions

fixation point - center of visual field w image transmitted to the fovea of both retinas

line drawn vertically through point divides into nasal and temporal halves

line drawn horizontally divides into upper and lower quadrants

retinal field divisions

nasal and temporal halves, upper and lower quadrants

retinal field and visual field relationship

inverse

(visual defects defined in terms of visual field lost)

how are visual defects defined

in terms of visual field

light passes through what parts of the eye

transparent cornea, anterior compartment, through the lens and vitreous body and through all the retinal layers

what happens with excess light

absorbed by melanin granules in RPE and the light bends back toward photoreceptor outer segments

light gets converted into electrochemical signals in visual transduction

sends signal to optic disc exit

what retinal ganglion cell axons form the optic nerve

the optic chiasm, where there is a partial decussation

what does the optic chiasm partial decussation result in

left field of vision projects to right side of brain and vice versa

80% of fibers go where

laterally and caudally to synapse in LGN of the thalamus

20% of the fibers go where

brachium of superior colliculus to terminate in

• diencephalic or mesencephalic nuclei

• pretectal area for visual reflexes

• superior colliculus for coordination of head and eye movements in response to visual stimuli

• suprachiasmatic nucleus of hypothalamus for influencing visceral functions entrained to a day/night cycle

what does the pretectal area do

visual reflexes

what does the superior colliculus do

coordination of head and eye movements in response to visual stimuli

what does the suprachiasmatic nucleus of hypothalamus do

influence visceral functions entrained to a day/night cycle

what is the geniculocalcarine tract

optic radiations, axons of LGN leave to carry info from visual field (upper and lower) and from macula

which visual association cortice pathway is responsible for high resolution form vision and object recognition

ventral pathway from striate cortex to temporal lobe

which visual association cortice is responsible for spatial aspects of vision, like analysis of motion and understanding the positional relationships between objects in the visual field

dorsal pathway from striate cortex to parietal lobe

pupillary light reflex purpose

protect the retina from excessive light

afferent limb of pupillary light reflex

retinal relays from photoreceptors to bipolar cells to ganglion cells

enter brachium of SC and terminate in pretectal before reaching LGN

associational limb of pupillary light reflex

cells in pretectal nucleus send axons to bilateral EW nuclei

decussates in posterior commissure

efferent limb of pupillary light reflex

cells in EW nuclei send axons via CN III to postgang neurons in ciliary ganglion

ciliary ganglion innervate sphincter pupillae muscles and result in bilateral constriction

lesion of afferent limb (optic nerve)

light shone into affected eye shows no response in either eye

light shown in healthy eye shows pupillary constriction in both eyes

lesion of efferent limb (oculomotor nerve)

affected eye shows no direct or consensual response to light reflex

healthy eye shows direct and consensual response to pupillary light reflex

pupil of affected eye is larger than healthy eye pupil due to unopposed action of pupillary dilator muscle

near reflex results in what

ocular convergence

lens thickening

pupillary constriction

argyll robertson pupil

pupil that is small, irregular and fixed to light (no pupillary light reflex)

constricts in response to accommodation (near reflex present)

ARP-PRA (accommodation reflex present, pupillary reflex absent)