the visual system Flashcards
the visual system
~ 50% of the cortex is devoted to visual perception
- has the most complex circuitry of all sensory systems
- over 1 million fibers
anatomy of the human eye
the surface of the retina
- has numerous vessels fan out over the surface
- vessels arise from othalamic artery and vein
where does the opthalmic artery and vein enter the eye through?
optic disk
retinal axons leave the eye through
optic disk
the optic disk
- has no photo receptors
- blind spot
macula lutea
- located near the center of the retina
- contains yellow pigment
- high visual acuity
where is acuity the greatest?
center of macula (fovea)
physical properties of light
- electromagnetic radiation visible to eye
- visible light consists of wavelengths 400-700nm
cornea
- the site of most refractive power of the eye
- responsible for the majority of the refractive power of the eye (40D)
refraction of the cornea
- has refractive power of about 40 diopters
focal distance
distance from the refractive surface of the point where parallel light rays converge
greatest amount of refraction is at
the anterior surface of the cornea
corneal refractive power is
constant
lens
- 20 D of refractive power
- changes with accommodation
accommodation by the lens
- lens adds refractive power by changing the shape of the lens
- contraction of ciliary muscles releases tension in zonula
- lens becomes rounded
- greater curvature provides greater refraction
ciliary muscle is innervated by
parasympathetic fibers in CN III
amplitude of accommodation
- the range of accommodation decreases with age
- crystalline lens and ciliary muscles become less elastic
refractive errors
- ametropia
- myopia
- hyperopia
- presbyopia
ametropia
a refractive error is present
emmetropia
normal vision
myopia
near sightedness
hyperopia
far sightedness
correction of myopia and hyperopia
presbyopia
loss of accommodative ability of the lens resulting in difficulties with near tasks
where do the photoreceptor cells lie?
in the back of the retina
- light rays must pass through retina before reaching photopigment molecules to excite
the laminar organization of the retina
(furthest back toward front)
- pigment epithelium
- rods
- cones
- outer plexiform layer
- horizontal cells
- bipolar cells
- amacrine cells
- inner plexiform layer
- ganglion cells
- nerve fiber layer
retinal pigment epithelium
- plays critical roles in maintenance of photoreceptors and photopigments
- has melanin
- absorbs any light not captured bu the photoreceptors
- thus minimizes reflection of light that could blur image
retina: neural circuitry
- light hits photoreceptors
- sends signals to the bipolar cells
- bipolar cells send signal to ganglion cells
- ganglion cells send signal to the brain
photoreceptors include
rods and cones !!
rods
- 100 million
- light sensitive
- found in periphery of retina
- low activation threshold
- achromatic
cones
- 6 million
- are color sensitive
- found mostly in fovea
- high acuity
the outer segments of a rod or cone contain
different photopigments
cones work under _____ light.
rods work under _____ light.
bright, dim
photopigments of rods are
rhodopsin
rhodopsin…
absorbs light in a very narrow range
photopigments of cones are
cone opsins (iodopsins)
3 isoforms of cones include
- short WL
- medium WL
- long WL
short wavelength isoform of the cones:
- absorb the blue or violet color region of visual spectrum
- S or blue cone
~437nm
medium wavelength isoform of the cones:
- absorbs in the green region of visual spectrum
- M or green cone
~533nm
long wavelength isoform of the cones:
- absorbs in the red region of the visual spectrum
- L or red cone
~573nm
rhopsodin is formed by
- cis-retinal (aldehyde of vitamin A) bound to opsin
iodopsin is formed by
- the same cis-retinal bound to isoforms of opsins
melanopsin in
intrinsive photosensitive RGC (ipRGCs)
rods and cones are
not evenly distributed
in fovea there are
only cones
in peripheral retina there are
- rods and cones
- primarily rods
convergence of rods
- high convergence
- high sensitivity
- low spatial resolution
convergence of cones
- low convergence
- low sensitivity
- high spatial resolution
visual acuity of the fovea
- enhanced by displacement of neurons to minimize scattering of light
- 1:1 relationship of cones to bipolar to retinal ganglion cells
- high density of cones
differences between rods and cones
phototransduction
- photoreceptors do no inhibit action potentials
- light activation causes graded change in membrane potential
much of the processing within the retina is mediated by
graded potentials
shining light of photoreceptors leads to
hyperpolarization
in the dark, the receptor is in a
depolarized stated and releases glutamate
cyclic GMP gated channels in outer segment membrane of photoreceptors
- cGMP allow Na and Ca to enter thru ion channel in the dark
- cGMP does not bind in light and Na and Ca cannot enter
phototransduction in rod photo receptors
- light stimulation of rhodopsin leads to activation of a G-protein transduction
- activated G-protein activates cGMP phosphodiesterase (PDE)
- PDE hydrolyzes cGMP reducing concentration
- closure of Na channels
bipolar cells have 2 main groups
- on (light on)
- off (light off)
on-center bipolar cells
- have inhibitory glutamate receptor
(metabotropic mGluR6)
off-center bipolar cells
- have excitatory glutamate receptors (ionotropic glutamate receptors)
both cones and rods photoreceptors release what neurotransmitter?
glutamate
horizontal cells
- play crucial role in creating receptive fields and lateral inhibition
why? because of inhibitory connections
amacrine cells
- 30 types
- mediate lateral interactions between bipolar cell terminals and the dendrites of ganglion cells
glial cells of the retina
- muller cells
- microglia
muller cells
the principal glial cell of the retina
microglia
- enter the retina coincident with the mesenchymal precursor of retinal blood vessels in development
- ubiquitous in retina (found in every layer)
retinal ganglion cells
- 10-15 types
- 1.5 million that form optic nerve (CN II)
- connect eye to brain
% of RGCs in primate retinas
- M cells = 10%
- P cells = 70%
- K cells = 8%
- intrinsically photosensitive RGC (1-2%)
M RGCs
- origin of magnocellular pathway
- broadband, achromatic signal to magnocellular layer of LGN
the cells in magnocellular pathway
- generally have large receptive fields
- fast axonal conduction velocities
- are found in the periphery of the retina
P RGCs
- origin of parvocellular pathway
- convey color signal to the parvocellular layers of LGN
the cells of parvocellular pathway
- small receptive fields
- slow axonal conduction velocities
- mostly found in the fovea (high acuity)
intrinsically photosensitive RGCs (ipRGC)
- 1-3% of RGCs
- contain melanopsin
- axons innervate suprachiasmatic nucleus (SCN)
- contribute to photic entrainment of circadian rhythm
- project to olivary pretectal nucleus
PULPILLARY LIGHT REFLEX
the receptive field
the area of the retina that, upon illumination, enhances or inhibits signaling
RGC receptor fields
center surround organizaiton
- serves to emphasize areas of difference (contrast)
- visual system detects intensity not the absolute amount of light
visual pathways (main)
- retinal signal travels to lateral geniculate nucleus (LGN) of thalamus
- then to primary visual cortex (V1)
lateral geniculate nucleus (LGN)
- major target of RGCs
- receives input form both eyes and relays these messages to the primary visual cortex via optic radiation
central projections of retinal ganglion cells
- optic nerve and optic disc
- RGC exits retina through optic disc
- bundle of all the cells form optic nerve
central visual pathway includes what structures?
- optic nerve
- optic chiasm
- optic tract
- LGN
- optic radiations
- primary visual cortex
lateral geniculate nucleus (LGN)
- 6 layers
- each layer receives input from one eye
- 2 inner layers = magnocellular
- 4 outer layers = parvocellular
koniocellular sublayers of LGN
- ventral
- small as dust
primary visual cortex (V1)
- neurons from LGN carrying visual information synapse with cortical neurons in medial portion of the occipital lobe
- area 17 of Brodmann
- AKA striate cortex
cortical retinoptic map
- info received at adjacent portions of retina remain adjacent in visual cortex
- about 25% of V1 is dedicated to input from fovea
- upper fields are mapped below calcarine fissure and lower fields are above
striate cortex input
- LGN projections to the striate cortex are segregated
- axons from magnocellular layers terminate principally within sublamina (4Ca)
- axons form parvocellular layers terminate principally within sublamina (4Cb)
- axons from koniocellular layers terminate in layers 2 and 3
striate cortex cells
- simple
- complex
simple cells
- elongated receptive fields
- respond best to oriented elongated stimuli
- bars and edges
- on and off regions
- direction selective
complex cells
- respond only to appropriately oriented stimuli
- don’t have stimulatory and inhibitory zones
- direction-selective
- NO on or off regions
- orientation and direction
orientation of columns in V1
????????
distribution of axons from the eye are distributed into patches called
ocular dominance columns
ocular dominance columns respond to
either left or right eye… NOT BOTH
orientation columns respond to lines of
the same angle
hypercolumns contain
orientation columns that respond to complete 180 degrees
cytochrome oxidase blobs respond to
color
extrastriate visual areas form 2 processing pathways
- dorsal pathway
- ventral pathway
dorsal pathway
- leads to parietal lobe
- spatial vision
- neurons selective to speed and direction of movement
ventral pathway
- leads to temporal lobe
- cognition
- selective to color, texture, shape
