Chapter 27 (Monitoring Visual Function)

Question 1

Function of Sclera?

Answer 1

Outer layer of the eye. Tough, strong white connective tissue, which protects and maintains shape of eyeball

Question 2

Function of conjunctiva?

Answer 2

Thin layer or protective cells
Covered by layer of fluid secreted from tear ducts.

Question 3

Function of cornea

Answer 3

Layer under conjunctiva and is continuous with sclera
Focuses light onto retina
Main structure responsible for the refraction of light rays onto the retina

Question 4

Function of vitreous humour?

Answer 4

Helps maintain shape of eye by exerting outward pressure on sclera and surrounding tissue.
Help refract the light rays onto retina.
Supplies oxygen and nutrients to cornea and lens.

Question 5

Function of choroid?

Answer 5

Richly supplied with blood vessels to supply oxygen and nutrients to retina.
A pigmented epithelium that absorbs any light that passes through rods and cones so it’s not reflected back into eye.

Question 6

Function of iris?

Answer 6

Circular tissue - radial and circular muscles that control the size of the pupil.
Radial muscles stimulated by sympathetic nervous system
Circular muscles stimulated by parasympathetic nervous system.

Question 7

Function of aqueous humour?

Answer 7

Helps maintain shape of eye by exerting outward pressure on sclera and surrounding tissue.
Help refract the light rays onto retina.
Supplies oxygen and nutrients to cornea and lens.

Less viscous than vitreous humour

Question 8

Function of pupil?

Answer 8

Centre of iris which allows light to pass through

Question 9

Function of retina?

Answer 9

Inner most layer of eye - sensory area.
Contains photoreceptors: cones, and rods.

Question 10

Function of optic nerve?

Answer 10

Axons of cells in retina leave eyeball at this point. Carries action potentials to the brain.

Question 11

Function of ciliary body?

Answer 11

Contains ciliary muscles which control the shape of the lens.
Controls diameter of suspensory ligaments, so therefore diameter of lens.

Question 12

Function of lens?

Answer 12

Made of stacks of long, transparent cells
Make fine adjustments to the refraction of the light rays so they are sharply focused on retina.

Question 13

Function of suspensory ligaments?

Answer 13

Holds lens in place and connects lens to ciliary muscles.

Question 14

Function of fovea?

Answer 14

Most sensitive part of retina, contains highest conc. of cone cells.
Area with maximum visual acuity.

Question 15

Function of blind spot?

Answer 15

Point at which optic nerve leaves eyeball, contains no photoreceptors to light hitting here isn’t interpreted.

Question 16

Cornea, conjunctiva, lens info?

Answer 16

Don’t contain blood vessels as light has to pass these structures to reach the retina. If there was blood, it would absorb and reflect too much of the light.

Question 17

Definition of visual acuity?

Answer 17

Resolution of an image that is perceived by the brain.

Question 18

Visual acuity in rod cells info?

Answer 18

Low visual acuity

Many rod cells form synapses with the same bipolar cell. - the information is therefore combined.
This helps in the detection of dim light as small responses of many rod cells can be summated to produce a larger response in the brain.

‘The brain interprets an average response from a collection of rod cells not individual responses from individual cells’

Question 19

Visual acuity in cone cells info?

Answer 19

Each bipolar cell receives inputs from only one cell cone cell in the centre of the fovea.
The brain therefore receives information from each cone cell separately.
This produces a high resolution image, ie, high visual acuity.
This is further enhanced by the concentration of cone cells in the fovea where the image that is formed is sharpest and least distorted.

Question 20

What are eye tests?

Answer 20

Carried out to help detect eye diseases before there is any serious effect on sight, to prevent the condition getting worse by early treatment.
Carried out every 2 years by optometrists
Can detect diabetes, eye cancers etc..

Question 21

What do eye tests routinely test for?

Answer 21

Visual acuity, ie, ability to focus on objects
Colour vision
Pupil response to light intensity.

Other tests to be completed:
Eye pressure
field of vision

Question 22

Visual acuity test?

Answer 22

Measured using the Snellen scale
No. rows of letter that get smaller (or pictures for the young)
Quantitative measure of ability to see an in-focus image at certain distance.

1. Cover one eye
2. Read chart top to bottom at distance of 6m/20ft.
3. Then repeat with other eye.

if 20/20 vision, patient can read letters under 1cm in height, at a distance of 20ft.

Appropriate corrective measures, eg, glasses, contact lenses, surgery.

Question 23

Colour vision test info?

Answer 23

(arise from altercations in gene located on X chromosome and is recessive, so males are more likely to be colour blind)

Two types:
Ishihara pseudo-isochromatic colour plate exam
Farnsworth-Munsell 100 hue test.

Question 24

Ishihara pseudo-pseudo-isochromatic colour plate exam info?

Answer 24

Most common
Series of test plates where a matrix of coloured dots are arranged with a number visible on the page.
Usually to test for red-green blindness.

Question 25

Farnsworth-Munsell 100 hue test info?

Answer 25

Involves asking patient to arrange a coloured set of caps in sequence to form a gradual transition of colour between two points. Caps show 4 times 25 hues of different colours Each colour hue at each end is fixed. Use to grade colour discrimination.

Question 26

Optical coherence tomography (OCT) scan info?

Answer 26

Capture image reflections from within eye tissue Produce cross-sectional images at very high resolutions Used to diagnose: AMRD Diabetic retinopathy Macular holes Macular oedema

Question 27

OCT scan procedure

Answer 27

Optical beam of near infra-red light detected at tissue Small proportions of light reflected by retina are captured Limitations: - Most light scatters off at large angles which causes background noise and obscured image - interferometry can reject background noise and build up clear 3D images. - Eyes may be sensitive to bright light for several hours after exam - Can only image tissue to max depth of ~ 1-2mm below surface (but retina is ~ 0.5mm, so ok)

Question 28

Advantages of OCT scan?

Answer 28

Non-invasive Totally painless Only takes ~10-15 minutes Results available instantly.

Question 29

Definition of accommodation

Answer 29

The change in shape of the lens to focus objects from a particular distance Achieved by varying tension on suspensory ligaments which makes the lens more or less convex.

Question 30

How are the light rays bent?

Answer 30

They are focused on the retina by cornea, aqueous humour, lens, and vitreous humour. Majority of refraction occurs at surface of cornea.

Question 31

Focusing on distant object

Answer 31

Light rays enter eyes parallel. Cornea bends light rays Ciliary muscle relaxes, diameter is wider This pulls on suspensory ligaments so they are under tension, pulled taut. Lens is stretched, becomes thinner and less convex. Bends light rays slightly.

Question 32

Focusing on nearby object?

Answer 32

Light rays enter eye widely diverging Cornea bends light rays Ciliary muscle contacts, diameter reduces This loosens tension on SL so they slacken. Lens is under pressure so becomes fatter and more convex Bends light rays strongly

Question 33

Myopia info?

Answer 33

Short sightedness Light focused in front of retina. Can't see distant objects clearly. Causes Eye ball is too long Cornea is too curved Lens is too thick Concave lenses to correct.

Question 34

Hyperopia info?

Answer 34

Long sightedness Light focused behind retina Can't see close up objects clearly Causes Eye ball too short Cornea not curved enough Lens too thin Can be caused by diabetes Convex lenses to correct.

Question 35

Similarities between rod and cone cells?

Answer 35

* Both are photoreceptors * Both detect and respond to light * Both contain molecules that change shape when light hits them * Both contain opsins (specialised proteins) * Both have similar structures NOT FINISHED!!!!

Question 36

Info on bipolar and ganglion cells?

Answer 36

* Specialised neurones * Carry action potential from rod and cone cells to optic nerve * Are unmyelinated (so are transparent) to allow light to pass through them to the rod and cone cells. * Cell bodies of the bipolar and ganglion cells in the fovea are pushed to the side so light hits the photoreceptors without having to pass through other cells first