Lecture 15- Sensory processing: the visual system Flashcards
what mechanisms help protect eyes from injury? 4
eyeball is sheltered by bony socket
eyelids protect eye from environment
eyelashes trap fine airborne debris
tears lubricate, cleanse and have bactericidal chemicals
what are the 3 tissue layers that the spherical fluid filled structure is enclosed by?
sclera/cornea
choroid/ ciliary body/ iris
retina
what is the sclera? what does it look like?
tough outer layer of connective tissue (visible white part of the eye)
what is the cornea? function
anterior (transparent outer layer) allows passage of light rays
what is the choroid? where is it? what does it contain? what does it form 3?
middle layer underneath sclera
contains blood vessels that nourish retina
contains dark pigment (melanin)
forms ciliary body, suspensory ligaments and iris
what is the retina? what does it consist of?
innermost layer under choroid
consists of outer pigment cells, photoreceptors (rods and cones) and axons of visual nerve fibres
the interior consists of two fluid filled cavities separated by the lens, what are they?
posterior cavity
anterior cavity
where is the posterior cavity? what is it?
area between lens and retina
vitreous humor (gelatin consistency)
where is the anterior cavity? what is it?
area between cornea and lens
aqueous humor (watery, NOT gelatin like)
what is the function of the iris? whta does it contain?
controls the amount of light entering the eye
contains two sets of smooth muscle (circular and radial)
what is the function of the pupil?
opening through which light enters the eye
function of lens
focuses light
what is tapetum lucidum?
many vertebrates (ex. dogs) have less melanin but have an addition reflective layer (tapetum lucidum)
there isn’t as much melanin therefore as light enters the pupil there is not enough melanin to absorb therefore the light bounces around in eye and can cause blurriness because the light might not hit the photoreceptors
why do we change our pupil size?
to control the amount of light entering the eye
controls the depth of field
what is small arperture?
constricted pupil
can see things in focus that are close and far at the same time
what is large aperture?
open pupil
like portrait mode. can focus on object thats up close but not on the object thats far away at the same time
what is the circular muscle of iris? what does it do? what nervous system stimulation does it use?
encircles the pupil of the iris
constrict the pupil (in bright light)
parasympathetic stimulation
what is the radial muscle of iris? what does it do? what nervous system stimulation does it use?
dilates the pupil
sympathetic stimulation
how does the circulatory muscle of iris and radial muscle of iris work?
work antagonistically against each other
what happens when parallel light rays strike a lens whose surface is relatively flat? where does the focal point fall? what does the image look like?
the focal point will fall on the retina, thus the object is already in focus
what happens when light rays strike a lens at an oblique (side) angle? where does the focal point fall? what does the image look like?
focal point will fall behind the retina and the object image becomes fuzzy and out of focus
what happens when light rays are really close? what does the lens do? where does the focal point fall?
the lens becomes more rounded (stronger)
rounding the lens shortens its focal length therefore the focal length goes onto retina
what is accommodation (process of focusing)?
changing the strength of a lens to change focus from far to near
what is the natural shape of the lens? what is a weaker shape of a lens?
natural shape: rounded (strong)
weak: ciliary muscle pulls the lens to a flatter weaker shape
explain the lens area of the eye?
lens is attached to the ciliary muscle by suspensory ligaments
what happens when ciliary muscle is relaxed?
the ligaments pull on and flatten the lens making the lens bigger
what happens when ciliary muscle contracts?
releases tension on the ligaments and the lens becomes more rounded (strong position)
how does accomodation get around?
via parasympathetic nervous system [Acetylcholine (neurotransmitters)]
what does ACh do in the eye in respect to accomodation?
causes contraction of ciliary muscle and constriction of the ring
(leads to increasing lens curvature for close objects)
what happens to the lens if we decrease parasympathetic tone in respect to accommodation?
relaxes the muscle causing a flatter lens for distant objects
what happens with sympathetic nervous system in accomodation?
nothing, it does not play a major role
what is myopia? explain.
where does the focal point fall?
what lens do we use to correct it?
near- sightedness (cant see far away)
focal point falls in front of the retina
use concave lens to correct it (
what is hyperopia? explain.
where does the focal point fall?
what lens do we use to correct it?
far- sightedness (cant see up close)
focal point falls behind the retina
corrected with a convex lens
what happens when light enters the eye?
directly stimulates photoreceptors in the retina
it is able to directly stimulate because the fovea pushes back the other materials in the eye
what happens when light reaches the retina
light hits photoreceptors (cones and rods) in the fovea
signal is transmitted to bipolar cells
then converge (come together) to ganglion cells
transmits signal to fibres of the optic nerve
thalamus
visual cortex
compare and contrast rods and cones? more or less in retina? type of vision? sensitivity? convergence onto ganglion cells? location?
rods: more in retina, greyscale vision, high sensitivity (night vision, low acuity), more convergence onto ganglion cells, located in peripheral retina
cones: less in retina, color vision, lower sensitivity (high acuity, day vision), less convergence onto ganglion cells, located in the fovea
what is the macula?
an area with a high concentration of cones, thus high resolution of images
where is the fovea located? what is density of cones? convergence?
in the macula
highest density of cones
very little convergence of sensory neurons at the fovea
what is the optic disc?
blind spot of the eye, because thats where optic nerves converge
what are photoreceptor molecules (rhodopsin molecules) made out of?
retinal and opsin
what happens when retinal gets struck by light?
shape changes from cis retinal to trans retinal
what happens when in darkness?
rhodopsin is inactive
levels of cGMP are high
cGMP binds to Na+/ Ca++ channels to open them
Na+/ Ca++ channels and K+ channels are open
cells are constantly depolarized by entry of Na+ and Ca++
continuously release transmitter onto bipolar cells
what happens when in light?
light causes less neurotransmitter to be released
light bleaches rhodopsin and activates it
this activates transducin
this activates PD (an enzyme that degrades cGMP
decrease in cGMP causes Na+/ Ca++ channels to close
cell becomes hyperpolarized
causes less transmitter to be released onto bipolar cells
when viewing an image how does light enter the eye?
comes into eye at an angle and 50% of it has to cross over inside the brain slide38
what is amblyopia? how do we cure it?
lazy eye
in young kids we patch the good eye, forcing the brain to use the information from the weaker eye
(cant do this with adults… we dont have enough plasticity in our brains)
