Vision 1 - Refraction, Accommodation & Transduction Flashcards
what is refraction?
when light rays bend to form a sharp image on the retina
what is accommodation?
focusing on near by objects by changing how much we bend the light rays
what parts of the eyes are transparent?
the lens, cornea, AH, VH
why do we have to change how much we bend rays when we view an object up close?
when we look at an object in the distance, it is mainly parallel rays which hit the cornea so we don’t have to bend them that much
but when we are up close we are recieving divergent rays which need to bent more to form an image on the retina
what is the difference in bending power between the lens and the cornea?
the cornea has more bending power of 45D
but the lens has the capacity to change it bending power 15D
how does the lens change its bending power?
constriction of the cilliaris muscle causes lax of the suspensory ligaments causing the lens to budge and thicken
a thicker lens = more powerful
what 3 things occur simultaneously and comprise accommodation?
lens changes shape (becomes thicker and more spherical)
pupil constricts
eyes converge
does the pupil constrict or dilate in a dark room and why?
dimly lit room means the sympathetic system will take over for the eyes
this will result in dilation of the pupil
the dilator pupillae muscle is also innervated by a sympathetic plexus
what does it mean when eyes converge?
when we are looking at a close object, we have to turn our eyes in to look at the object (converge)
what/which muscle(s) is responsible for our eyes converging?
medial rectus muscle in both eyes acting at the same time
what muscle of the eye is more thick - the medial or lateral rectus and why?
the medial rectus because we spend more of our time looking up close
what is refractory errors?
the space between the retina and lens is either too long or too short (the eyeball is too short or too long)
what condition occurs when the space between the retina and lens is too short?
myopia - short sightedness
what are the different refractory errors?
myopia - short sighted
hypertrophic - long sighted
presbyopia - long sightedness of old age
astigmatism - non-shperical curvature of the cornea
what condition occurs if the eyeball is too long?
myopia because this results in the space between the cornea and the retina being too short
what condition occurs if the space between the retina and the lens is too long?
hypertrophic - this is due to the eyeball being too short
what type of lenses are used to correct myopia and hypertrophic?
myopia = biconcave lenses hypertropia = biconvex lenses
describe the pathophysiology of myopia.
the bending power of the cornea and lens is too much
when images are brought close, because of the short distance between cornea and retina, the lens and cornea form an image on the retina without needing to increase curvature of the lens
describe the pathophysiology of hypertrophic.
the image of a distant object is formed behind the retina
the person starts to use their accommodative power to bend the lens so that the image can form on the retina
they use up their accommodative power more and more until they don’t have any left
this results in them not having the accommodative power required to see up close objects
what is the most common cause of convergent squint in young children?
hypertropia
what is astigmatism?
close and distant objects appear hazy
describe he pathophysiology of astigmatism.
the cornea on the surface of the lens isn’t even
therefore the surface has different curvatures
the bending of light rays along one axis will never be the same as that of another axis
results in hazy images being formed
how is astigmatism corrected?
cylindrical glasses (curved only in one axis) special contacts = toric lenses laser eye surgery
what is the condition of long sightedness due to old age?
presbyopia
what causes presbyopia?
with age the elens becomes less mobile/elastic - the collagen fibres become rigid
therefore when the ciliary muscle contracts it is not as capable s before to change shape so the lens doesn’t thicken for accommodation
how is presbyopia corrected?
biconvex glasses
what is phototransduction?
the conversion of light energy to an electrochemical response by photoreceptors (cones and rods)
hat are the visual pigments in rods and cones?
rods = rhodopsin cones = opsins S, M and L
what type of protein is rhodopsin?
integral transmembrane helical protein
what is required in regenerating the bleached pigment?
vitamin A
a rest, are the photoreceptors of the eyes (rods and cones) kept depolarised or hyper polarised and how?
depolarised due to open Na/Ca channels
sympathetic stimulation = dilation of pupils at rest
what causes hyperpolarisation of the photoreceptor cells?
Na channels close which cause relative hyperpolarisation
this causes flux of Ca ions to the synapse = hyperpolarisation
what does vitamin A deficiency cause and why?
night blindness
required for regeneration of the visual pigment which stimulates phototransduction
what conditions is vitamin A deficiency found in?
malnutrition
malabsorption i.e. coeliac & sprue
what are the signs of vitamin A deficiency?
Bitot’s spots (triangle of spots in sclera
corneal ulceration
corneal melting leading to opacification of the eye
