the eye Flashcards
where are the photoreceptors found
back of retina
cones for
perceive colour under norma lighting conditions
rods for
allow to see in dim light but not perception of colour
more of these than cones
fovea
no rods but many cones
5 million
provides highest acuity vision, and thus is at the centre of our gaze
retina is thinnest here
when light hits photoreceptors (cascade)
it interacts with photopigment
propagates the signal to bipolar cells which connect photoreceptors to ganglion cells
which leave the eye in a large cluster at the optic disc
after leaving the retina the ganglion cell fibres are called the optic nerve
(which carries visual into to visual cortex)
optic disc
no photoreceptors
so the retina cannot process visual info here - natural blind spot
origin of blood vessels and optic nerve, cannot sense light (no photoreceptors) lets blood vessels in and optic nerve in and out
other two types of cells in the retina
horizzontal and amacrine cells
horizontal cells
receive input from multiple photorepceotrs
integrate signalling from different populations of photorecpeot cells
make adjustments to the signals that will be sent to bipolar cells
and regulate acticivty in photoprepctos cells themsleves
Horizontal cells – input from and output to photoreceptors, output to bipolar cells
amacrine cells
reveice signals from bipolar cells and are involved in the regulation and integration of activity in bipolar and ganglion cells
Amacrine cells – input from bipolar cells, influence ganglion cells, bipolar cells and other amacrine cells
layers of nerve cells
ganglion cell layer
inner plexiform layer
inner nuclear layer
outer plexiform layer
outer nuclear layer
layer of photoreceptors
pigmented epithelium
bipolar cells
connect photoreceptors to ganglion cells
ganglion cells
output from retina
membranous discs in photoreceptors contain
light sensitive photopigments that absorb light
duplicity theory
- can’t have high sensitivity and high resolution in single receptor
- thus separate systems for monochrome and colour
rods structure
greater number of discs
higher photopigment conc
1000 times more sensitive to light than cones
vision in low light (scotopic)
low visual acuity /resolution
~92 million rods in each human retina
cones structure
fewer discs
used during daylight (photooptic)
enable colour vision
lower sensitivity
high visual acuity/resolutions
~5 million cones in the human retina
mesopic conditions
intermediate light conditions
both rods and cones used
central retina
low convergence and high resolution
peripheral retina
high convergence and low resolution
rod photopigment
rhodopsin
cone photopigments
three varieties of opsin (S, M and L
retinal ganglion photopigments
melanopsin
relative absorbance of human photo pigments
lower wave length to higher wave length
S cones melanopsin Rods M cones L cones
phototransduction in the dark
rods are depolarised
due to influx of Na+
known as the dark current
maintained by cGMP (cyclic guanosine monophosphate)
phototransduction in the light
cGMP levels are decreased
Na+ channels close
Na+ influx is prevented
Rods are hyper polarised
refraction
Refraction occurs because the speed of light differs between mediums e.g. slower through water than air. The greater the difference in speed in the two media, the greater the angle of refraction. Refraction occurs towards a line that is perpendicular to the border.
absorption
transfer of light energy to a particle or surface
occurs in some retinal cells
Refraction
light
Electromagnetic light travels in straight lines in a vacuum, known as rays, until it interacts with atoms and molecules
PUPIL
lets light inside the eye
iris
contains schinter muscles to control size
cornea
glassy transparant covering the pupil and iris
sclera
continuous with cornea forms the wall of the eyeball- protects it by coping with pressure
extaocular muslces
move the eyeball (controlled by cranial nerve 3 ocular motor nerve and cranial nerve 4 tropclial nerve and abducens cranial nerve 6
conjunctiva
membrane underneath eyelids that attached to the sclera (gets infected)
optic enrve
carries axons from retina to brain
strabismus
imbalance in the extracoular muscles
estropia- convergence of eyes (cross eyes)
exotropia - divergence of eyes wall-eyed.
macula
region of retina for central vision, devoid of large blood vessels to improve vision quality because nothing distorts vision
aqueous humour
watery fluid that provides nutrients to the cornea and lens
normally produced by the ciliary body and absorbed by the canal of Schlemm (between cornea and sclera)
contains nutrients e.g. glucose
the lens
is suspended by zonal fibres (ligaments) which are attached to the ciliary muscles
vitreous humour
a viscous fluid, keeps the eyeball spherical
disorders of the aqueous humour
cateracts
glaucoma
cateracts
clouding of the lens
can occur due to changes in the composition of the aqueous honour (lack of nutrients)
increased with age, smoking, diabetes mellitus
treament- artificial lens
glaucoma
slowed uptake of aqueous humour to the canal of scheme leads to elevated intracellular pressure
causes compression of the optic nerve and reduced bloody supply to the retina
leads to a progressive loss f vision from periphery inwards
image formation
light rays have to focus on the retina
achieved by refractive powers of the cornea- 80% of toal refraction
the lens- 20% of total refraction
higher the refractive index the slower the light will travel through
refraction by the cornea
light arrives at the cornea though air but the cornea is mostly water
light travels more slowly through water than air– refraction occurs
focal distance
distance from the refractive substance (the cornea) to where the parallel light rays converge.
light rays from distant objects
are almost parallel by the time they reach the eye
light rays from nearer objects
are not parallel, therefore require additional focussing power to focus them on the retina
far point
ciliary muslces relax, zonal fibres stretch, lens flattens
near point
ciliary mscles contract, less tension in zonal fibres, lens beomces more pserhical
how to increase refractive power
rounding of the lends
ir has natural elasticity, so if not stretched it will become more spherical
hyperopia
far sightedness, can’t see up close
resolved with a convex lens in front (can straighten out the light rays giving enough refractive power)
myopia
eyeball too long, short-sightedness cannot see far away light rays are too parallel so converge before the retina
too much refractive power for size of eye ball
resolve by making eyes more divergent
emmetropic
is a state in which the eye is relaxed and focused on an object more than 6 meters or 20 feet away. The light rays coming from that object are essentially parallel, and the rays are focused on the retina without effort.
pupillary light reflex
Iris regulates light levelsPupillary constrictor (smooth circular muscle)Pupillary dilator (smooth radial muscle)
Enables us to adjust to changes in light intensity
Consensual light reflex i.e. occurs in both eyes even if only one is stimulated
globe luxation
eye ball pop out
3 wall layers
fibrous layer (contains connective tissue, white ‘sclera’, outermost part
vasucular
inner- transparent cornea window at front into eye
walls of middle vascular layer contain
posterior choroid (membrane that supplies all layers with blood)
anterior layer
ciliary body\ring of muscles tissue that surrounds lens
retina’s 2 layers
1) outer layer- pigmented, absorbs light so doesn’t scatter around the eye ball
2) inner neural layer- contains photoreceptors
bipolar cells
ganglion cells
bipolar- syanpses at both ends- 1 end synapses with photoreceptor and other with ganglion cell
ganglion cell
goes on to form optic nerve cranial nerve 2)
carries input to thalamus
and then visual cortex
cones
sit near retina centre
detect fine details and colour
1) red 2) green 3) blue
only really reach their activation threshold in bright conditions
fewer compared to rods
each get their own personal ganglion cell
v detailed colour vision
carries inputs to thalamus and then visual cortex
rods
more numerous more light sensitive only register scale of black and white edges of the retina (rule peripheral vision) lots connect to one ganglion cell so brain can't tell which rods are activated can't give detail images general shapes of objects
after image
bright lights can means photoreceptors can keep sending action potentials even after image is switched off
cones can get tired- can receive some bright stimulus for too long– stop responding
how is light reflected from objects only 3 meters from the eyes, focussed on the retina
the cornea enables the majority of the refraction and the fattened lens enables the remainder of the refraction that focusses the light onto the retina (generally when objects are less than 9 meters away)
cornea always provides the majority o the refractive power
how does the lens become more spherical
This is achieved by contraction of the ciliary muscle which leads to slackening of the zonal fibres/suspensory ligament, reducing the pull on the lens and enabling it to assume its naturally spherical shape.
