Applied Anatomy and Physiology of the Eye Flashcards
what is the epithelium of the cornea?
stratified squamous non-keratinised
what are the layers of the cornea?
epithelium, Bowman’s membrane (basement membrane of corneal epithelium), storma, descent’s layer (thick basement membrane), endothelium (single squamous layer)
what is the storma made of and why is this important for the function of the cornea?
regularly arranged collagen- fibres in a uniform diameter and placed equal distance from each other, no blood vessels as tendency to leak and this would make fibres non-uniform and so not transparent –> this is all important to allow light to pass through and maintain transparency
what are the 3 important factors for maintaining cornea transparency?
histologically= regular arrangement of collagen in storma, no blood vessels and the endothelium layer has a pump that actively keeps the aqueous humour out
what are corneal opacities?
fibroblast migration to heal wounds causes opacity causing loss of corneal transparency which dimities vision but can be rectified with corneal transplant
why is the asvascularity of the cornea a benefit to surgeons performing a graft surgery?
the cornea is an immune-privileged site so there is a lesser change of foreign antigens from the corneal graft being recognised by the recipient so lesser chance of graft rejection
what is the structure of the retina?
10 layers- outermost= pigment epithelial layer, next layer= layer of rods and cones which are modified neurones which are light sensitive cells with photoreceptors
what is the vascular supply of the retina?
the choroid supplies the outer layers
what is a retinal detachment?
when the retina is formed the space between the 2 layers of the optic cup is obliterated but this can become detached again due to fluid in the space- detachment is between the pigment epithelial layer and the other 9 layers
what is the fovea centralis?
it is the central part of the retina and is packed with cones and mulch thinner than the rest of the retina so is the spot of maximum visual acuity
what are the functions of the tear film in the eye?
keeps the cornea moist and prevents drying, washes away any foreign antibodies, kill microbes, smooths outer surface of cornea for smooth surface for refraction
what are the 3 structures forming the tear film?
mucinous layer overlying the corneal epithelium, aqueous layer secreted by the lacrimal gland and oily layer- secreted by the meibomian glands and most superficial
what is refraction?
bending of light when it passes from one optical medium to another due to the change in speed - light rays bend to form a sharp image on the retina
what happens when light passes through a convex lens?
they converge to a point called as the focal point of that lens
what happens when light passes through a concave lens?
they diverge and appear to come from a focal point behind the lens
what are the structures in the pathway of light through the eye which are refractive media?
cornea, AH, lens, VH
describe the refractive index of these structures.
the cornea is the most powerful “bender” of light (45D) but the lens (15D) has the capacity to change its “bending power”
what is D?
dioptre= unit of the amount of refraction
describe the passage of light from an object.
bend at the cornea, bend some more at the lens to form a clear, inverted image on the retina
what needs to happen when an object is closer to the eye?
the eye needs more bending power to focus an object so the lens becomes thicker and hence more powerful so that a clear image is formed on the retina again
what is accommodation?
we can focus far off or near objects by changing how much we bend the light rays
what are the 3 things that happen simultaneously and comprise accommodation?
lens changes shape (thicker and more spherical), pupil constricts and eyes converge
how does the lens thicken?
the ciliary body contracts due to PSNS from the IIIn and bulges out so that the space between the ciliary body and lens decreases so the suspensory ligaments become lax and the lens becomes thicker as no longer stretched= can focus up close
how does the pupil constrict?
the constrictor papillae contracts through PSNS allowing sharp focus on the close up object as only a few rays of light can pass through
how do the eyes converge?
our eyes need to turn to look at the object up close and so the medial rectus muscles of both eyes contract to converge the eyes
what is emmetropia?
perfect vision
what is myopia?
short-sightedness which is caused by a difference in the curvature of the lens (more curved) or the size of the eyeball (most commonly= eyeball too long) causing rays to be focused in front of the retina
what are the consequences of myopia?
the rays focusing in front of the retina means that when they reach the retina they are out of focus again causing blurry vision of far-off objects but objects closeup an be focussed quite easily
what are the symptoms of myopia?
headaches, complaining of not being able to see far away objects, loss of interest in children during class
how is myopia corrected?
bending power needs to be decreased so given biconcave lenses (-) - spectacles, contact lenses or laser eye surgery
what is hypertropia?
long-sightedness which is caused by eyeball being too short or the cornea or lens too flat and the rays are focusses behind the retina and so the image formed on the retina is blurry - can see far off objects clearly but nearby objects are blurry
what are the symptoms of hypertropia?
eyestrain after reading/work on computer, convergent squint in children
how is hypertropia corrected?
biconvex (+) glasses, contact lenses or laser eye surgery which rest the accommodative power
what is astigmatism?
non-spherical curvature of the cornea (or lens) which causes hazy picture as different curvatures causes bending on different axis
how is astigmatism corrected?
special cylindrical glasses, laser eye surgery can also be used, special contact lenses called toric lenses
what is presbyopia?
long-sightedness of old age as lens gets less mobile/elastic so when ciliary muscles contracts it is not as capable as before to change shape ie need reading glasses
describe transduction of light through the eye.
cascade of reactions within the rod/cone hyperpolaries and the impulse is transmitted, becomes an AP in the optic nerve cells
what is the visual field?
everything you see with one eye including the periphery
how is the visual field tested?
confrontation test= sit across from patient and both close the opposite eyes and say when you see the finger
automated perimetry= more accurate map of visual field
describe the pathway of fibres from the eye.
through the optic nerve to the optic chiasma where the medial nasal fibres from both eyes pass to the opposite side so that the optic tracts contain fibres from the (lateral) temporal half of the ipsilateral eye and the crossed-over nasal fibres from the contralateral eye, they hen synapse at the LGB of the thalamus and then the optic radiation passes behind the IC to reach the primary visual cortex in the occipital lobe (area 17) and the right visual cortex sees the left half of the visual field and vice versa
