How We See and visual pathway Flashcards
everything you see with one eye is your…
visual field
how is your visual field tested?
by confrontation test or automated perimetry
where do the fibres from the eye pass through the optic nerve to?
the optic chiasma
what happens when there is right optic nerve is damaged
blindness in one eye
what happens when optic chiasma disrupted in the middle?
bitemporal hemianopia
what happens when right optic tract is damaged?
contralateral homonymous hemianopia
what happens when optic radiation is damaged?
contralateral homonymous hemianopia
what does the optic tract contain?
fibres from the (lateral) temporal half of the ipsilateral eye and the crossed-over nasal fibres from the contralateral eye.
where does the fibres from the optic tract synapse?
at the LGB of the thalamus
where does the optic radiation go after the fibres synapse at the LGB of the thalamus?
passes behind the Internal capsule (retro-lentiform fibres) to reach the Primary Visual Cortex in the Occipital lobe (Area 17)
what are the 2 factors that influence the action of individual EOMs?
The muscles are attached along the orbital axis and not the optical axis, so they pull on the eyeball at an angle. This is a muscle has more than one action
The Oblique muscles are attached to the posterior part of the sclera, so they pull the posterior part of the eyeball up/down & the anterior part moves in the opposite direction
when LR has abducted the eye, SR causes
elevation
when LR has abducted the eye, IR causes
depression
when MR has adducted the eye, SR causes
intorsion
when MR has adducted the eye, IR causes
extorsion
when eyeball is adducted SO causes
depression
when eyeball is adducted IO cuases
elevation
when eyeball is abducted SO causes
intorsion
when eyeball is abducted IO causes
extorsion
what is the sympathetic innervation of the eye?
Remember the thoracolumbar outflow of the sympathetic
Remember the sympathetic chain and cervical ganglia
Remember that in the head and neck – postganglionic sympathetic fibres travel along with blood vessels
what is this the definition of?
Light rays bend to form a sharp image on the retina
refraction
what is this a definition of?
we can focus on far off or near objects by changing how much we bend the light rays
accomodation
what is this a definition of?
sometimes there is a mismatch between how much we bend light rays
refracting errors
what is refraction?
Bending of light when it passes from one optical medium to another
what is significant about the cornea, AH, lens and VH being transparent?
to allow light to fall on the retina
how does a sharp image form on the retina?
Light waves from an object bend at the cornea, bend some more at the lens to form a clear image on the retina
what happens to the eye when it focuses on an object thats closer
the eye needs more bending power to focus on an object –> The lens becomes thicker & hence more powerful, and a clear image is formed on the retina again.
what part of the eye is the most powerful bender of light?
cornea?
what part of the eye has the capacity to change its bending power?
lens
what happens for accomodation to occur?
Lens changes shape (becomes thicker & more spherical)
Pupil constricts
Eyes converge
what is accommodation?
The changes occurring in both eyes as it changes focus from a distant to a close object
how do the lens thicken?
Ciliary muscle contracts making the ciliary body bulge
Space in the middle decreases
Suspensory ligaments become lax
Lens is no longer under stretch
LENS BECOMES THICKER
how do the pupils constrict?
To sharpen focus the pupil constricts to allow only a few rays (those from the object) to pass through.
Pupillary constrictor (sphincter pupillae) is a concentric muscle around the border of the pupil which gets parasympathetic innervation.
how do the eyes converge?
When focussing on an object up close, our eyes have to turn in to look at the object—> convergence.
We use our medial rectus muscles of both eyes to converge. (IIIn of both sides)
is the MR or the LR thicker?
the MR is thicker because we spend a lot of time looking at cclose things so it works more which makes it thicker
what are results as refractive errors?
myopia
hyperopia
astigmatism
presbyopia
what is myopia?
short sightedness
what is hyperopia?
long sightedness
what is astigmatism?
non-spherical curvature of the cornea
what is presbyopia?
long sightedness of old age
whats it called to have perfect vision?
emmetrope
what is shortsightedness?
close object looks clear, distant objects appear hazy
what is the cause of myopia?
common cause- eyeball too long
why cant people with myopia see far away?
when the cornea + lens bend rays of light, they make the image form IN FRONT OF the retina. So far off objects not seen clearly.
When the object is brought closer, divergent rays from the object reach the cornea; and then this “bending power” comes to use. The image is formed on the retina without needing to increase curvature of lens (i.e. without using the accommodative power
symptoms of myopia
Headaches, Complain of not being able to see blackboard/ distant objects
Infants & preverbal children —> divergent squint
Toddlers —> loss of interest in sports/people. More interest in books, pictures.
how is myopia corrected?
Bending power needs to be decreased.
Biconcave lenses
Spectacles
Contact lenses
Laser eye surgery
what is longsightedness?
close objects are hazy and distant objects appear clear
what causes hyperopia?
Eyeball too short or cornea + lens too flat.
So the image of a distant object is formed BEHIND THE RETINA.
what happens to they eye in people with hyperopia?
The person then automatically starts to use his accommodative power and makes the lens thicker. This causes the image to form on the retina.
So he is using his lens power to see far off things (that he should normally be seeing without using any power).
When seeing closer objects, he uses more and more power until ultimately, his power is all used up!
symptoms of hyperopia
eyestrain after reading/ working on the computer in a young individual.
Convergent squint in children/ toddlers – needs immediate correction with glasses/lenses to preserve vision in both eyes and prevent a “lazy eye”.
how to correct hyperopia
Biconvex glasses alleviates use of glasses for focussing distant objects and ‘rests’ the accomodative power
Contact lenses
Laser Eye surgery
what is astigmatism?
Close and distant objects appear hazy
what occurs in the eye of someone with astigmatism?
the bending of light rays along one axis will never be the same as that of the other axis.
So image formed is always hazy, whatever the distance of the object.
how to correct astigmatism?
special glasses – called cylindrical glasses (which are curved in only one axis).
Laser eye surgery can also be used to correct the defect.
Need special contact lenses called toric lenses.
how do people get presbyopia?
With age the lens gets less mobile/elastic.
So when the ciliary muscle contracts, it is not as capable as before to change shape.
So seeing near objects/ reading the newspaper starts to become difficult needs glasses to read.
how to correct presbyopia
Correction is by using biconvex “reading glasses”
what is visible light?
electromagnetic wave
what does the energy in light waves need to do?
stimulate photoreceptor cells on the retina
what is phototransduction?
conversion of light energy to an electrochemical response by the photoreceptors (rods and cones)
what do the phototransduced rods and cones need to activate?
the optic nerve cells
what are contained in the rods and cones?
contains stacks of discs/lamellae- which have photoreceptors
what is lamellae made up of?
cell membrane
visual pigment rhodopsin in rods
cone opsins S, M and L in cones
what is the result of all- trans retinal not fitting into the opsin?
rhodopsin splits. This results in BLEACHIN~G of the visual purple
when light falls on 11-cis retinal, what is its isomer?
all-trans retinal
how does bleaching of the visual pigment result in phototransduction?
phototransduction cascade
what role does vitamen A play in the visual pigment
visual pigment regeneration
explain the phototransduction cascade
Unlike other cells in the body, the photoreceptor cells are, at rest (in the dark), kept in a depolarised state by open Na+/Ca+ channels
activated rhodopsin
Na channel closes
relative hyperpolarisation of photoreceptor cell
hyperpolarisation transmitted by a flux of ca ions to the synapse with bupolar cell
stimualtes ultimately the retinal cell
what would happen if someone is vitamen A deficient
Since Vitamin – A is supplied through the diet, any condition that affects vitamin A absorption will affect vision – (night)blindness. Vitamin A is also essential for healthy epithelium. So conjunctiva and corneal epithelium are also abnormal.
Vitamin A deficiency can occur in conditions such as malnutrition, malabsorption syndromes such as coeliac disease, sprue.
what can be signs of vitamin A deficiency
Bitot’s spots in conjunctiva are sometimes the first indication of Vitamin A deficiency
Corneal ulceration ( green colour is dye to show up extent of ulcer)
Corneal melting
Which leads to future opacification of the cornea
