Visual System Flashcards
what are the layer of the eyeball
fibrous layer
vascular layer
retina
what is the fibrous layer comprised of
-sclera
-cornea
what is the vascular layer comprised of
-iris
-ciliary body
-choroid
what is the retina comprised of
-pigmented layer
-neural layer
what does the iris contain
-blood vessels
-pigment cells (melanocytes)
-2 layers of smooth muscle
what does contraction of smooth muscle in the iris do
changes the diameter of the pupil
what is the pupil
its the central opening of the iris that is black in colour
what does the pupil do
it regulates the amount of light entering the eye
what does the pupil do
it regulates the amount of light entering the eye
what happens to the pupil during close vision and bright light
pupils constrict
what happens to the pupil during during distant vision and dim light
pupils dilate
what is the cornea
its a thin transparent epithelium that is devoid of blood vessels and covers the iris
what is the choroid like
its vascular accounting for 85% of total blood flow in the eye and it has a relatively low oxygen content
what does the choroid do
-supplies the outer retina with nutrients
-maintains the temperature and volume of the eye
what is the choroid covered with
sclera
the choroid is attached to what
the outermost layer of the retina
what is the sclera
its the white of the eye
what does the sclera do
-provides structural support
-provides attachment of extrinsic muscles
what does the sclera fuse with posteriorly
the sheath of the optic nerve
what is scleral/ conjunctival icterus
its yellowing of the sclera due to build up of bilirubin
what secrets aqueous humor into the anterior chamber
ciliary epithelium
what does aqueous humor do
-it keeps the anterior chamber of the eye pressurized at 20mmHg
-it maintains eye shape and relative positions of the components
-it provides oxygen, nutrients and metabolic waste removal to the lens and the cornea
-for refraction
what drains the anterior chamber
the canal of schlemm ( scleral venous sinus)
where does the canal of schlemm empty into
extraocular veins
how often is the aqueous humor replaced
every 90mins
what is a glaucoma and what causes it
its a disease that damages the optic nerve and can cause vision loss and blindness.
its caused by fluid build up in the anterior chamber because of clogging of the canal of schlemm resulting in poor drainage of the aqueous humor
what does vitreous humor do
-maintains intraocular pressure
- refraction
-phagocytosis
what is the lens
its an onion like structure with closely packed columnar cells arranged in concentric shells and encased by a thin and transparent capsule
cells of the lens have a high conc of what
alpha crystallins
what do alpha crystallins do
they increase the lens density and enhance its focusing
whats a disease of the lens
cataracts
the ciliary muscle has smooth muscle oriented in what 3 directions
-longitudinal
-radial
-circular
ciliary muscle has the greatest density of what
mitochondria
whats the innervation of the ciliary muscle
parasympathetic innervation (ACh) through oculomotor nerve
how is the ciliary muscle attached to the lens
through inelastic zonular fibers
what does the ciliary muscle do
provides accommodation
what are tears
they are plasma ultrafiltrate produced from the lacrimal gland
what do tears do
-bathe the cornea in a thin layer 10 micrometers thick
- reduce friction
-remove debris
-allow oxygen to diffuse from air to corneal cells
-have antibodies and lysozymes for infection
-their superficial oil layer slows evaporation for good optical surface
what is the flow of tears
lacrimal gland> lacrimal duct> superior/ inferior lacrimal canal> lacrimal sac> nasolacrimal duct> nasal cavity
do newborns shed tears
no, because their lacrimal glands are still developing but by 2 weeks they produce a few tears to keep their eyes moist
when are full tears produced by newborns
2 months
what does the optic disc/ blind spot lack
photoreceptors (so any image that falls on this region wont be seen)
what comes together in the optic disc/blind spot
optic nerves
why dont we notice the blind spot in every day to day observation of the world
because the brain fills the blind spot with an extrapolation of the image surrounding the blind spot
what does the retina consist of
-single cell thick pigmented epithelium
-photoreceptors
-neural layers
what does the pigmented layer of the retina contain
it contains melanin
what does the melanin in the pigmented layer of the retina do
absorbs stray light rays
what does the pigmented layer of the retina do
it provides essential nutrition and waste removal for the photoreceptors
why is visual acuity reduced in people with albinism
this is because of light scattering, due to no/little melanin in the pigmented layer to absorb the excess light and the light is reflected onto the retina degrading the visual image
how many rods and cones are in the photoreceptor layer of the retina
-120 million rods
-6 million cones
what is the center of the posterior portion of the retina
macula lutea
whats the fovea centralis
its the depression in the center of the macula lutea
what does the fovea centralis form
the center of the visual axis
what is the neuronal layer of the retina
its a forward extension of the brain
what cells does the neuronal layer of the retina consist of
-ganglion
-bipolar
-horizontal
-amacrine
what do bipolar cells in the neuronal layer of the retina do
they take the electrical info from the photoreceptors cells and pass it along to other retinal cells
what do horizontal cells in the neuronal layer of the retina do
-they receive input from photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors
-they also help integrate and regulate the input from multiple photoreceptors cells, increasing the visual acuity
what do amacrine cells of the neuronal layer of the retina do
they receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells and other amacrine cells
what do ganglion cells of neuronal layer of the retina do
they contribute axons to the optic nerve
what is an inverted retina
its where the photoreceptors face away from the light causing the incoming light to travel through the front of the retina to reach the photoreceptors
what does an inverted retina allow for
-absorption of excess light by the pigment epithelium, preventing reflection that may distort the image
-photoreceptors to be in close contact with the choroid which supplies the photoreceptors with blood and provides a cooling system for the retina as the light is converted largely into heat
what are the only light sensitive cells in the retina and what is the one exception
rods and cones
some types of ganglion cells are an exception
whats the only source of output from the retina
ganglion cells
what are the only retinal neurons that fire action potentials
ganglion cells
what do the other retinal cells do that don’t fire action potentials
they depolarize or hyperpolarize and release a neurotransmitter (glutamate) ,but they dont fire action potentials
what does the eye have for image formation
-lens system
-variable aperture system i.e. the pupil
-retina that corresponds to the film
what are the 4 refractive interfaces of the lens system
-interface between air and the anterior surface of the cornea
-interface between the posterior surface of the cornea and the aqueous humor
-interface between the aqueous humor and the anterior of the lens
-interface between the posterior surface of te lens and the vitreous humor
what process can increase the refractive power of the lens
accommodation/focusing
what happens to a persons lens as they become older
it gets larger, thicker and less elastic
what is the condition where the lens becomes non-accommodating
presbyopia
as a persons lens change due to old age what will they need
bifocal glasses, with the upper segment focused for far seeing and the lower segment for near seeing
what do you call normal vision
emmetropia
what are some errors of refraction
hyperopia- is farsightedness and its usually due to the eyeball being too short or a lens system that is too weak, causing near objects to look blurry
myopia- is nearsightedness and its due to the eyeball being too long or the lens system having too much refractive power, causing far away objects to look blurry
astigmatism- is due to too great a curvature of the cornea in one plane of the eye, which can cause far away and nearby objects to look blurry or distorted
what are the corrections for errors of refraction
-concave for myopia
-convex for hyperopia
-cylindrical for astigmatism
what does the pupil do
its continuously adjusting for different ambient light levels
what does the pupillary light reflex do
constricts the pupil in response to light
the pupillary light reflex involves connections between what
the retina and neurons in the brain stem that control the sphincter muscles that constrict the pupils
the pupillary light reflex is what
consensual i.e. if you shine a light into only one eye, it will cause the constriction of both pupils
what does the lack of consensual pupillary light reflex show
a sign of serious neurological disorder involving the brain stem
whats visual acuity
its the ability of the eye to distinguish 2 points near each other
what does visual acuity depend on
-spacing of the photoreceptors in the retina
-precision of the eyes refraction
how is distance across the retina described
in terms of degrees of visual angle
what does a photoreceptor comprise of
-synaptic terminals
-inner segment
-outer segement
what is the outer segment of the photoreceptors
its a highly modified cilium
the outer segment of the photoreceptors is the site of what
signal transduction
disk membranes in rods and cones contain what photopigment
R-rhodopsin
C-color pigments i.e. iodopsin
what do rods and cones do on exposure to light
they decompose and excite the nerve fibers leading from the eye
what makes rods more sensitive to light than cones
a higher photopigment conc and greater number of disks
what vision are rods used for
scotopic vision (black and white vision in low light conditions)
what does loss of rod function result in
night blindness
where do rods predominate
in the extrafoveal retinal portions
what is rhodopsin made up of
a purple pigment which consists of the protein opsin that is bound to the yellow photosensitive pigment called retinal
where is retinal derived from
vitamin A
how is rhodopsin incorporated into disks
its first synthesized from inner segments, then it moves across through the stalk and into the outer segments in vesicles
what happens when retinal absorbs photons
it changes from 11-cis retinal to all-trans retinal and causes conformational changes in opsin leading to the activation of transducin a G protein
what vision do cones provide
photopic vision for colour in bright light
what pigment do cons contain
iodopsin
what does iodopsin consist of
retinal combined with a photopigment opsin protein
what do you call someone one who sees with normal colour vision
trichromats
what are the 3 types of cones and what are they sensitive to
S cones- sensitive to blue light
M cones- sensitive to green light
L cones- sensitive to red light
what do cones do
provide colour vision and great visual acuity
what do people with colour blindness lack
one or more of the cones
red-green colour blindness is common in who
males
men acquire abnormal red-green vision if they inherit what
a defective X chromosome from mother
women acquire abnormal red-green vision if they inherit what
abnormal X chromosomes from both parents
whats transduction
its a process by which an environment stimulus causes an electrical response in a sensory receptor cell
describe phototransduction in dark current (in darkness)
-in the dark the rod isnt excited and the receptor potential is approx. -30mv (depolarized)
-the membrane is depolarized because guanylyl cyclase catalyses synthesis of cGMP from GTP, and the cGMP causes gated cation channels to be open causing an influx of Na+
-this then results in glutamate release when the receptors are depolarized
what causes a reduction in glutamate release in phototransduction
when receptors are hyperpolarized
what does glutamate cause in phototransduction in the dark
inhibitory post synaptic potential (IPSP) in bipolar cells of the retina
describe phototransduction in the light ( bleaching)
-when exposed to light there is conformation change of retinal from 11-cis retinal to all-trans retinal and release of opsin
-opsin then activates transducin which in turn activates cGMP PDE (phosphodiesterase)
-PDE catalyses the conversion of cGMP to GMP
-this results in closure of the Na+ channels as cGMP in no longer there and hyperpolarization of the cell occurs (-70mv)
-glutamate release is turned off and this excites the bipolar cell of the retina
what happens when you have been in bright light for a long time (light adaption)
large portions of the photochemicals in both rods and cones will have been reduced to retinal and opsin during the bleaching reaction hence resulting in a reduction of the photosensitive chemicals remaining in the rods and cones, resulting in the reduction of sensitivity of the eye to light
what happens during dark adaptation
-there is a gradual increase in the photoreceptor sensitivity due to rhodopsin regeneration in the dark
neurotransmitter molecules released by rods and cones induce what
graded potentials in bipolar and horizontal cells
what happens when bipolar and amacrine cells transmit excitatory signals to the ganglion cells
the ganglion cells become depolarized and an action potential is sent
where do the right and left optic nerves come together
at the optic chiasm
what happens to fibers from the nasal half of each retina
they cross over to the opposite side in the optic chiasm
where do the optic tracts go
they go and synapse in the lateral geniculate body of the thalamus, then the fibers continue to the visual cortex in the occipital lobe where visual images are perceived
What is legal blindness
Acuity no greater than 20/200 with best correlation
What happens when you look at a bright red circle for a while and then you look at a white background
-You see a blue/green afterimage and this is because the prolonged viewing of the red stimulus causes adaptation in the red selective cones causing them to stop firing and the ganglion cells connected to the red cones decrease firing
-but the blue/green cones are not fatigued so the ganglion cells receiving green light send signals through the red-green channel and the brain will only receive a green signal
How can the red green channel signal white
It must receive signals from both the red and green cones so that they can cancel each other
Genes that encode for the blue pigment are found where
Chromosome 7
What is red green colour blindness
It’s a difficulty in distinguishing red and green
What’s blue-yellow colour blindness
It’s difficulty distinguishing blue and green and yellow and ref
what do you call the condition where you are red-green weak
Protanopia
What do you call the condition where you are blue-yellow weak
Tritanopia
What do you call it when you can’t see any colour
Monochromacy
How many rods synapse with a single bipolar cell
Between 6-600
What effect do the multiple rods synapsing with a single bipolar cell do
Increases light sensitivity of rod vision but the image may be perceived as slightly blurry
How many cones will often synapse with a single bipolar cell
1 cone
What effect does a single cone synapsing with a single bipolar cell do
Causes sharper vision
What do horizontal cells transmit
Inhibitory signals in areas lateral to excited rods and cones
What do the inhibitory signals transmitted by the horizontal cells in areas which are lateral to the excited rods and cones do
- it enhances visual contrast between areas in the retina that are strongly stimulated and adjacent areas with weak stimulation
- allows for differentiation of colour
Where do other axons from the optic chasm pass to
They go to the suprachiasmatic nuclei of the hypothalamus where they form connections that synchronize a variety of endocrine and other circadian rhythms associated with the light-dark cycle
