Chapter 17

center surround mechanism set up by

bipolar and horizontal cells

plexiform layers of neural retina

contain dendrites and axons in each synpatic layer, 1 cell type brings visual infomration in, another type carries information out, and a third forms a lateral interconnections

lesions of optic lobe

contralateral homonymous hemianopsia - due to blockage of psoterior cerebral artery -complete loss of half of visual space -usually get some sort of macular sparing (central visual space) due to size

lesion of optic radiation

contralateral homonymous quadrantanopsia - loss of 1/4 of visual space, lesions of inferior fibers cause homonymous superior quadrantanopsia

bipolar cells

for direct signaling between photoreceptors and ganglion cells (retinal output cells) - come in different types for light turining ON and OFF, also specific for rods or cones

amacrine cells

for lateral communication between bipolar cells and other bipolar/ganglion cells

horizontal cells

for lateral communication between photoreceptors and other photoreceptors/bipolar cells

neural retina - 5 basic cell types invovled in processing visual infomration

have cell bodies in 3 layers- nuclear - outer nuclear, inner nuclear and ganglion cell layer

cortical Processing

in cortex, visual information is processed -2 areas in visual cortex - primary visual cortex (processes contrast information and object orientation) -visual assocation areas (processes form, color, and motion input from primary visual cortex) -complex visual processing extends into other regions - temporal lobe (processes identification of objects), parietal cortex and postcentral gyrus (process spatial location)

modules are composed of small columns

in which the neurons throughout respond best to stimuli with a specific property and conveyed from 1 eye or the other - properties include orientation, motion, and depth -adjacent columns interpret closely related information -columns show ocular dominance (prefer input from 1 eye)

retina

inner layer- 2 layers 1. inner layer - transparent neural layer 2. outer layer- retinal pigment epithelium light receptive part of eye - light passes through trasnparent tissue to strike photoreceptors

retinal processing

intensity of illuminations is NOT as important as the contrast between different areas of receptive fields - the brain searches for borders between dark and light or between different colors or areas of motion - then makes its best guess in interpretting patterns of light

several different types of ganglion cells are arranged in doughnut-shaped receptive fields

on-center set-ups - stimulated by light hitting the center of the field, inhibited by light hitting the periphery (the surround) of the field off-center set-ups have the opposite effects *these responses are due to different receptor types for glutamate in the "on" and "off" fields of each set up

sclera

outer layer - white dense collagenous connective tissue anterior 1/6 transparent -cornea

cell body/axon in outer nuclear layer

outer segment filled with photon sensitive disks of cell membrane- rhodopsin and cone pigments, is the sight of visual trasnduction, photons absorbed here cause a receptor potential that spreads to rest of cells

phototransduction

retinal changes from 11-cis - all-trans retinal hit by light - all trans retinal dissociates from opsin (activates it) - opsin g-protein (transducin) - transducin activates phosphodiesterase - hydrolyzes cGMP - less cGMP leads to closure of sodium channels = hyperpolarization

off center cell

the reverse of this entire scenario can be created by reversing all signals (which we can do with different receptors to the same NT)

lesions of lateral optic chaism

(non-crossing temporal fibers beside chaism) - nasal hemianopsia of ipsilateral eye - retain most of visual field - noticable if unaffected eye is closes - caused by lateral pressure one side of chaism (resutls from aneurysm of Internal carotid artery)

retinal output- important for depth perception

-comparisons between right and left visual perspectives -from optic tract to visual cortex fibers representing corresponding areas of 2 retinas are located near each other - lesion cause comparable visual defect in both eyes

area from fovea (macula) are disproportionately represtned in the visual system

-important area for focusing- need more cells per area -smaller bipolar and ganglion cells -more direct pathways with minimal visual processing

primary visual cortex - still retain retinotopic map of visual space

-inferior visual fields project to cortex superior to calcarine sulcus -superior visual field project to cortex inferior to calcarine sulcus -central fields represented posteriorly and peripheral fields represented anteriorly

a retinotopic map is preserved throughout this visual pathways

-specific cells in retina correspond to specific cells in lateral geniculate nucleus and specific cells in cortex -nerual retina -> optic nerve-> chiasm -> optic tract -> lateral geniculate nucleus -> optic radiations of retrolenticular/sublenticular limbs of internal capsule --> primary visual cortex (calcarine sulcus)

for an on center cell...

1. light hyperpolarizes the cone (or rod) and excites the bipolar cell directly underneath - that bipolar cell then excited its ganglion cell 2. However, neighbor cones also excite horizontal cells, the horizontal cells send processes laterally and inhibit the center bipolar cell - diffuse light excited the central bipolar cell but also inhibits it via nerights = ganglion cell does not get excited = continues its normal tonic rate 3. a small spot of light, however, excites the center cone but not its neighbors - there is no inhibition, so it is free to get really excited and excited the ganglion cell, which fires like crazy 4. a ring of light excites only the neighboring cones- the bipolar cell is strongly inhibited with no excitation, in response to this strong silencing of the bipolar cell, the ganglion cell shuts down as well

photoreceptors really detect...

DARK, depolarize and release more NT when in the dark DARK - cGMP concentration high, cGMP opens sodium cahnnels and influx of sodium results in depolarization

all retinal ganglion cells leave the eye via the optic nerve (CN2)

approx 50% of fibers cross to contralateral side at optic chiasm - nasal half of retina crosses - temporal haf of retina enters i/l optic tract - each optic tract contains fibers from i/l temporal retina and c/l nasal retina (OR c/l visual field of each eye)

macula lutae

area of high concentration of cones

fovea

area of just cone photoreceptors that produce high visual acuity at center of macula - light doesn't have to filter through transparent rertina to reach photoreceptors, all neruons and capillaries are collected arouns the edges of the fovea

1- Retinal Pigment epithelium

attached to choroid, which supplies blood to outer retina and absorbs stray photons - also supports photoreceptor layer (phagocytizes shed disks)

9- nerve fiber layer

axons of ganglion cells converging at optic disk, also route for central retinal branch (of opthalmic artery) to supply blood to inner retina

inner plexiform layer

bipolar cells terminate on - ganglion cells (axons leave the eye as the optic nerve) amacrine cels - spread laterally and interconnect bipolar cells, ganglion cells and other amacrine cells -modulate flow- leave back of eye

lesions of optic nerve

blindness of ipsilateral eye - retain most of visual space due to unaffected eye

glaucoma

caused by blocked drainage of aqueous humor creates pressure that is transmitted through the vitreous humor to the retina - causes retinal damage - loss of peripheral vision

8- ganglion cell layer

cell bodies of output neurons of neural retina, on and off subtypes, axons form optic nerve

4- outer nuclear layer

cell bodies of photoreceptors (4 types)- 1 rod, 3 cones

6- inner nuclear layer

cell bodies of retinal interneurons (bipolar cells)

central visual space information occupies broad middle area - from macula fibers

central representations of space are carried bilaterally so deficits either compromise entire central area, or central visual space spared of visual deficits

primary visual cortex, information is divided into...

component elements (color, orientation, depth and motion) for processing - allows for simultaneous parallel processing -rapid

3- outer limiting membrane

connection between inner segments of photoreceptors cells and support cells of neural retina (muller- specialized glial cells) - actually a row of intercellular junctions - sort of like a blood brain barrier for retina

7- inner plexiform layer

connections between bipolar/amacrine cells (processes spread laterally) and ganglion cells

5- outer plexiform layer

connections between photoreceptors and other retinal cells (bipolar/horizontal) and the processes of horizontal cells that spread laterally, rods and cones synapse on different populations of bipolar cells

primary visual cortex interpsersed among the orientation columns are...

cylindrical assemblies of cells that are sensitive to color - tend not to follow column pattern

Visual deficits

damage to visual pathways cause predictable deficits - visual deficits names for part of visual field lost - not parts of retina compromised - remember:retinal images are inverted and reversed so damage to temporal areas of retina causes nasal field losses damage to superior areas of retina causes inferior field losses

photoreceptors hyperpolarize in repsonse to light

decreased release of glutamate LIGHT induced hydrolysis and cGMP concentration decreaseed, then channels close and cell hyperpolarizes and less transmitter is released

heteronymous

different visual field losses in each eye (not overlapping)- damage anterior to chaism affects i/l eye, damage at chaism causes heteronymous deficits, damage behind chiasm causes homonymous deficits

optic disk

exit of axons of ganglion cells from eye, area of no photoreceptors or other retinal neurons, creates blind spot that is not usually perceived due to filling in at cortical levels

stimulation of on-center with light and on surround without light produces...

greatest activty

stimulation of on-center without light and on surround with light produces

least activity

corrective lenses changes how light enters eye

lens has to do less work to achieve correct focal distance -near sighted focus too far anteriorly -farsighted focus too far posteriorly

quadrantanopsia

loss of 1 quarter of visual field

hemianopsia

loss of half of visual field

organization of primary visual cortex

made up of a series of modules - each module receives information from one area of the contralateral visual field - a very small area for foveal parts of the field - a larger area for peripheral parts

choroid

middle layer- provides a route for blood supply to outer retina - dark layer absorbs stray photons of light (similar to arachnoic and pia layer)

Focus (accommodation)

most focusing of images on retina performed by lens - accommodation - lens is suspended by strands of connective tissue attached to ciliary muscle - contraction of ciliary muscle casues strands to go loose and then lens fattens - view near object - relaxation of ciliary muscle casues strands to get taught and lens to thin -view far objects

divergence

moving or spreading apart or in different directions (single cell synapsing on many cells)

light intensity

neural retina sensitive to range from starlight to bright sunlight (10^12) -to simplify components, cant see both extremes at same time - iris used to restrict light entering eye - sphincter/dilatory muscle for pupil -change diamter of pupil from 8 mm to 1.5 mm in response

stimulation of on-center with light and on-surround with light with produce...

no net effect remember: stimulating the surround has the opposite effect of stimulating the center effects of 2 areas roughly cancel out

every point in the visual field is represented by both on-center and off-center ganglion cells

pattern of illumination of center and surround produces a variety of changes in firing rate

outer plexiform layer

photoreceptor cells - project to first layer of synapses where they terminate on -bipolar cells (which porject to next layer of synapses) -horizontal cells (spread laterally and interconnect photoreceptors, bipolar cells, and other horizontal cells (modulating effect)

different photoreceptors respond to specific wavelengths of light

photoreceptors come in 2 types

Optic radiations

projections of LGN - form a broad sheet of fibers that terminate in the cortex - superior visual space (fibers from inferior retina) information travels in inferior radiations - travels through the temporal lobe -damaged here will cause vision loss - inferior visual space (fibers from superior retina) information travels through superior radiation

type of photoreceptor - ROD

repsonds to low intensity light (starlight) very sensitive to small amount of light, acts slowly, requires lots of convergence of infomration to result in signal - weak signal, convergence causes loss of detail and spataial resolution in dim light is poor

type of photoreceptor- CONES

respond to high intensity light (daylight) sensitive to large amounts of light, acts quickly, less convergence-stronger signals (sharper images, fine spatial details)

development

retina is an outgrowth of diencephalon

2- photoreceptor layer

rods and 3 types of cones - cells have outer segment (rod/cone part), inner segment (mitochondria)

receptive fields of ganglion cells have 2 zones of concentric

roughly circular areas - stimulation of central area (the center) causes an increase or a decrease in the cell's activity -stimulation of peripheral area (the surround) causes an opposite effect on the cell

Eye shape

shape of eye maintained by intraocular pressure -anterior eye filled with watery aqueous humor -like CSF made by choiroid in ciliary body -used for support as well as nutrition for cornea, iris, lens -aqueous humor continually produced - flows from ciliary epithelium-through pupil-canal of schelmm- drains back to venous circulation

pupillary light reflex

shine bright light in one eye and get pupil constriction -ipsi eye = direct pupillary light reflex -contra eye- consensual pupillary light reflex axons from retina project (CN2) to pre-tectal areas --> bilateral projections to edinger-westphal nuclei (cn3) --> cn3 projects back toward eye and synapse in ciliary ganglion --> post ganglionic fibers innervate iris muscles no cortical connections required, lesions of optic nerves and occulomototr nerves can be observed with this response

homonymous

similar visual field losses in both eyes

Lesions

small lesions to specific areas of cortex can result in loss of some visual information without compromising entire sensation ex: may still be able to see colors, but not a horizontal yellow bar in superior visual field

Visual association cortex

some information is distributed to visual association areas for further processing - - these areas of cortex are involved with recognizing more complex patterns, area for putting color and form or motion visual information together - also for being able to recognize specific unique objects (faces houses cars)

Receptive field

specific space that is sensitive or responsive to stimulus - for vision the particular part of the outside world image that falls on a particular region of the retina

secondary pathways involve other nuclei

superior colliculus - thought to play role in visual reflexes and eye movements pre-tectal nucleo- pupillary light reflex - project b/l to edinger-westphal nuclei

convergence

tending toward a common point (many cells synapsing on a single cell)

retinal ganglion cells output to

the thalamus (LGN) and then visual cortex (conscious vision) -superior colliculus (tracking and visual reflexes) -other brainstem structures, including supraoptic and suprachiasmatic nuclei of hypothalamus - ganglion cells are the only projection cells from the retina

axons from retina project (CN2) bilaterally to LGN

then to primary visual cortex->visual association cortex-> superior colliculus -> occulomotor nuclei-> cn3 projects back toward eye and synapse in ciliary ganglion -> post ganglionic fibers innervate iris muscles

10- inner limiting membrane

thin basal lamina between proximal endings of muller cells, vitreous humor (separates vitreous from neural retina)

corneal reflex

touch cornea causes blink reflex - axons project through trigeminal (opthalmic division) -> main sensory nucleu of trigeminal nerve-> interneurons via MLF -> facial mottor nucleus -> facial nerve to orbicularis oculi muscles

anopsia -anopia

visual deficits - used to denote the loss of 1 or more quadrants of visual fields

posterior eye filled with gelatinous...

vitreous humor -formed in development -never replaced- single chamber support most of globe of eye -rests against neural retina and helps to support lens

Near reflex (accomodation reflex)

when focus on near objects 3 things happen - convergence (both eyes center gaze at same place), accomodation (contraction of ciliary muscle to allow lens to get fat -thicken lens to change focal length and focus image on retina), pupillar constriction - reduces aberrations and increased depth of focus - improve optical performance for near objects - requires particiatpion of cerebral cortex