Structure and Function of the Eye (B)

Question 1

Where is the lacrimal gland located?

Answer 1

Superio-laterally to the orbit

Question 2

Describe the innervation of the cornea.

Answer 2

The cornea is very sensitive and it is innervated by the ophthalmic branch of the trigeminal nerve (CN V)

Question 3

What two things regulate how much light reaches the retina?

Answer 3

Pupil Pigmented Uvea uvea is pigmented so that the iris (part of uvea) can block out excessive light to retina

Question 4

What is the term given to perfect focusing ability?

Answer 4

Emmetropia Adequate correlation between axial length and refractive power, parallel light rays fall on the retina

Question 5

What is the technical term for long-sightedness? Why does it happen?

Answer 5

Hypermetropia/hyperopia same thing Focal point behind the retina Caused by: short globe (axial hyperopia, more common) Insufficient refractive power (refractive hyperopia)

Question 6

How can long-sightedness be corrected and what is it caused by?

Answer 6

Convex lenses It is usually caused by having short eyeballs It is occasionally caused by a flat corneal surface

Question 7

What is the technical term for short-sightedness? Why does it happen?

Answer 7

Myopia Focal point is anterior to retina. Can be caused by: -long globe ie high axial length (axial myopia, more common) - XS refractive power (refractive myopia)

Question 8

How can short-sightedness be corrected? (what sort of lens)

Answer 8

Concave lenses

Question 9

What is astigmatism?

Answer 9

The cornea is oval rather than round This means that the refractive power varies in different planes (in some planes you will be hypermetropic, and in others you would be myopic)

Question 10

Which nerve is responsible for accommodation?

Answer 10

Oculomotor Nerve (CN III)

Question 11

What is the near response triad?

Answer 11

1.Pupillary Miosis (constriction of sphincter pupillae) – increases the depth of field 2.Convergence – the medial recti of both eyes contract 3. Accommodation – ciliary muscles contract which makes the zonules relax, this makes the lens rounder and increases its refractive power

Question 12

What is the term given to naturally occurring loss of accommodation with age? What is the treatment

Answer 12

Presbyopia Reading glasses with convex lenses. This INCREASES refractive power which moves the focal point anteriorly (normally done by accommodation but function is lost). More deets on types of glasses see slides

Question 13

What is the name of visible part of the optic nerve

Answer 13

optic disc. NB When looking at the blood vessels next to the optic disc, the thick ones are the venules and the thin ones are arterioles

Question 14

Describe the difference in perfusion between the outer and inner parts of the retina.

Answer 14

Inner 2/3 of the retina = retinal arteries Outer 1/3 of the retina = choroidal vasculature

Question 15

What part of the retina is responsible for central vision?

Answer 15

Fovea (it has the highest concentration of cones)

Question 16

What is peripheral vision responsible for?

Answer 16

Shape, movement, night vision

Question 17

Describe the structure of the retina.

Answer 17

The structure is counter-intuitive because the cones and rods are actually ‘behind’ the ganglion cells when the light shines in (see slide). From deep to superficial of eyeball: inner layer- retinal ganglion cells (3rd order neurons). Their axons then go off and become part of optic nerve Middle layer- bipolar cells (2nd order neurons) Outer layer: photoreceptors ie rods/cones (1st order neurons) Next to photoreceptors is retinal pigment epithelium

Question 18

What is the function of the retinal pigment epithelium?

Answer 18

Transports nutrients from the choroid to the photo-receptor cells and removes metabolic waste from the retina back out

Question 19

Describe how the fovea appears on a cross-section of the macula.

Answer 19

It appears as a foveal pit due to the absence of overlying retinal ganglion cells

Question 20

Describe the recycling of photopigments.

Answer 20

Photopigments are synthesised in the inner photo-receptor segment and then are transported to the outer segment discs The distal discs with deactivated photo-pigments are shredded from the tips and phagocytosed by retinal pigment epithelial cells The deactivated photo-pigments are regenerated inside the retinal pigment epithelial cells and are then transported back to the photo-receptors

Question 21

What is the rod photopigment?

Answer 21

Rhodopsin It is a G-protein coupled receptor system

Question 22

What is another term for night vision?

Answer 22

Scotopic vision

Question 23

What is another term for day-time vision?

Answer 23

Photopic vision

Question 24

Describe the distribution of rods and cones across the retina.

Answer 24

Rods have the highest density just outside the macula They decrease in density the further you move away from the macula There are NO rods in the macula Cones are MAINLY found in the macula The highest density of cones is in the fovea no photoreceptors near the blindspot (optic disc)

Question 25

Where is the physiological blind spot located? Why is there a blindspot?

Answer 25

20 degrees temporal to central vision This is because there are no photoreceptors (rods/cones) at the optic disc

Question 26

What are the three types of cone photopigment and which colours do they respond maximally to?

Answer 26

S-cone – short wavelength – BLUE M-cone – medium wavelength – GREEN L-cone – long wavelength – RED

Question 27

What is the most common colour vision deficiency and what is it caused by?

Answer 27

Deuteranomaly It is caused by the shifting of the M-cone (green light) towards the L-cone (red) caused by mutation so loss of green peak, cannot distinguish difference between green and red.

Question 28

What is the term given to shifted peaks?

Answer 28

Anomalous trichomatism

Question 29

What test is used to diagnose colour blindness?

Answer 29

Ishihara Test

Question 30

Describe how your eyes adapt to darkness

Answer 30

Dark adaptation: Light sensitivity increases. (biphasic process, cones 7mins, rods 30 mins) Pupil adaptation eg dilation (minor)

Question 31

What happens during adaptation to brighter environment

Answer 31

Light adaptation: -photopigment bleaching the pigments become almost transparent. Transparent objects do not ABSORB light, they transmit it. Therefore, less light absorbed by photopigments - neuro-adaptation (increased tolerance to light) -inhibiting rod and cone function (increasing threshold intensity, ie greater intensity needed to generate a response, ie less sensitive.) Pupil adaptation (minor) eg constriction

Question 32

What part of the eye is responsible for central vision and where is it located? What is the centre of this region called?

Answer 32

Macula- located near the centre of the retina, temporal to the optic nerve/optic disc Macula is also called macula lutea because of the yellow patch when you look at it down the scope. Fovea is the centre of the macula. Fovea of the macula is the most sensitive part of the retina (highest cone concentration but low rod conc). It gives us ability for detailed vision for tasks like reading

Question 33

Compare central and peripheral vision and explain the reasons behind this. -detail -recognising colour -recognising motion -day/night vision

Answer 33

Detail: Central > peripheral Colour: Central > peripheral Motion: Peripheral>central Night vision: Peripheral> central Day Vision: Central>peripheral Navigation vision: mainly peripheral- patient may fail to navigate if peripheral vision is lost. Reason is due to more cones in the fovea which is responsible for central vision and more rods elsewhere. Look at word file comparing cones with rods table but basically the above list applies for the difference between cones and rods.

Question 34

State the test for central and peripheral vision

Answer 34

Central - visual acuity assessment (acuity= sharpness) Peripheral- visual field assessment

Question 35

What scan can be used to assess the macula and fovea

Answer 35

OCT scan Optical coherence tomography

Question 36

Compare cones and rods

Answer 36

Outer segment (part that absorbs light and contains photosensitive pigment): Rods > cones Speed of response to light: Cones>rods Sensitivity to light: Rods>cones so better for night vision (this is NOT acuity of vision) Acuity/resolution: Cones>rods Location: Rods= more in periphery. Cones= more in macula/fovea Colour: only cones Abundance: more rods than cones You still see colour in peripheral vision because you still have cones outside of macula, just not much

Question 37

What are cone and rod vision also called

Answer 37

Scotopic vision (rod)= means dim light vision Photopic vision (cone)= means bright light vision

Question 38

2 most common colour blindness problems?

Answer 38

Deuteranomaly (less sensitive to green, most common colour blindness because the green curve shifts towards red) and protanomaly (less sensitive to red as red curve shifts towards green) Deuter= second Proto= first look at the absorption curves of red green and blue in the slides. Red is furthest right then green then blue on left.

Question 39

What is the index of refraction?

Answer 39

every medium has this. Speed of light in vacuum (c)/speed of light in new medium (Vm) denominator is always smaller. This value has no units. Greatest possible value is 1.

Question 40

PLEASE LOOK AT PDF TO REVIEW GCSE LEVEL OPTICS

Answer 40

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Question 41

What is the general term used to describe mismatch between axial length and refractive power such that the light rays dont fall on retina? (eg myopia, hyeropia, astigmatism, presbyopia)

Answer 41

Ametropia is the general term for all of these refractive disorders

Question 42

Symptoms of hyperopia and myopia

Answer 42

look at slides

Question 43

What is amblyopia

Answer 43

this is also called lazy eye. If one of your eyes is weaker than the other (either myopic or hyeropic or due to other pathological reasons.) overtime, your brain will start to prefer the stronger eye over the weak one and so the neuronal connections to the weak eye will be tuned down. If this is left untreated, amblyopia can develop and basically you wont be able to see well even with glasses because the brain has switched off the neuronal connections to the weaker eye. To fix this, children may be forced to use the weaker eye more by covering up the strong eye.

Question 44

What is astigmatism

Answer 44

multiple focal points rather than one due to the refractive media not being spherical. Eg if cornea is not circular dome shaped but rugby ball dome shaped then it will refract light differently at different points. (see word file for better explanation)

Question 45

Astigmatism symptoms and treatment?

Answer 45

Symptoms – asthenopic symptoms ( headache , eyepain) – blurred vision – distortion of vision – head tilting and turning Treatment – Regular astigmatism: cylinder lenses with or without spherical lenses(convex or concave), Sx (surgery) – Irregular astigmatism : rigid Contact lens , surgery