Vision 1 - Refraction, Accommodation & Transduction

Question 1

what is refraction?

Answer 1

when light rays bend to form a sharp image on the retina

Question 2

what is accommodation?

Answer 2

focusing on near by objects by changing how much we bend the light rays

Question 3

what parts of the eyes are transparent?

Answer 3

the lens, cornea, AH, VH

Question 4

why do we have to change how much we bend rays when we view an object up close?

Answer 4

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

Question 5

what is the difference in bending power between the lens and the cornea?

Answer 5

the cornea has more bending power of 45D

but the lens has the capacity to change it bending power 15D

Question 6

how does the lens change its bending power?

Answer 6

constriction of the cilliaris muscle causes lax of the suspensory ligaments causing the lens to budge and thicken
a thicker lens = more powerful

Question 7

what 3 things occur simultaneously and comprise accommodation?

Answer 7

lens changes shape (becomes thicker and more spherical)
pupil constricts
eyes converge

Question 8

does the pupil constrict or dilate in a dark room and why?

Answer 8

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

Question 9

what does it mean when eyes converge?

Answer 9

when we are looking at a close object, we have to turn our eyes in to look at the object (converge)

Question 10

what/which muscle(s) is responsible for our eyes converging?

Answer 10

medial rectus muscle in both eyes acting at the same time

Question 11

what muscle of the eye is more thick - the medial or lateral rectus and why?

Answer 11

the medial rectus because we spend more of our time looking up close

Question 12

what is refractory errors?

Answer 12

the space between the retina and lens is either too long or too short (the eyeball is too short or too long)

Question 13

what condition occurs when the space between the retina and lens is too short?

Answer 13

myopia - short sightedness

Question 14

what are the different refractory errors?

Answer 14

myopia - short sighted
hypertrophic - long sighted
presbyopia - long sightedness of old age
astigmatism - non-shperical curvature of the cornea

Question 15

what condition occurs if the eyeball is too long?

Answer 15

myopia because this results in the space between the cornea and the retina being too short

Question 16

what condition occurs if the space between the retina and the lens is too long?

Answer 16

hypertrophic - this is due to the eyeball being too short

Question 17

what type of lenses are used to correct myopia and hypertrophic?

Answer 17

myopia = biconcave lenses 
hypertropia = biconvex lenses

Question 18

describe the pathophysiology of myopia.

Answer 18

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

Question 19

describe the pathophysiology of hypertrophic.

Answer 19

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

Question 20

what is the most common cause of convergent squint in young children?

Answer 20

hypertropia

Question 21

what is astigmatism?

Answer 21

close and distant objects appear hazy

Question 22

describe he pathophysiology of astigmatism.

Answer 22

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

Question 23

how is astigmatism corrected?

Answer 23

cylindrical glasses (curved only in one axis)
special contacts = toric lenses 
laser eye surgery

Question 24

what is the condition of long sightedness due to old age?

Answer 24

presbyopia

Question 25

what causes presbyopia?

Answer 25

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

Question 26

how is presbyopia corrected?

Answer 26

biconvex glasses

Question 27

what is phototransduction?

Answer 27

the conversion of light energy to an electrochemical response by photoreceptors (cones and rods)

Question 28

hat are the visual pigments in rods and cones?

Answer 28

rods = rhodopsin 
cones = opsins S, M and L

Question 29

what type of protein is rhodopsin?

Answer 29

integral transmembrane helical protein

Question 30

what is required in regenerating the bleached pigment?

Answer 30

vitamin A

Question 31

a rest, are the photoreceptors of the eyes (rods and cones) kept depolarised or hyper polarised and how?

Answer 31

depolarised due to open Na/Ca channels

sympathetic stimulation = dilation of pupils at rest

Question 32

what causes hyperpolarisation of the photoreceptor cells?

Answer 32

Na channels close which cause relative hyperpolarisation

this causes flux of Ca ions to the synapse = hyperpolarisation

Question 33

what does vitamin A deficiency cause and why?

Answer 33

night blindness

required for regeneration of the visual pigment which stimulates phototransduction

Question 34

what conditions is vitamin A deficiency found in?

Answer 34

malnutrition

malabsorption i.e. coeliac & sprue

Question 35

what are the signs of vitamin A deficiency?

Answer 35

Bitot’s spots (triangle of spots in sclera
corneal ulceration
corneal melting leading to opacification of the eye