19: Vision 2

Question 1

What is refractive power?

Answer 1

Ability of a lens to “bend’ light.

Reciprocal of focal length in meters

Question 2

What is refractive power measured in?

Answer 2

Diopters

Question 3

What is the average refractive power for the “relaxed” eye

Answer 3

60 diopters

focal point at about 1.7m

Question 4

What is emmetropia?

Answer 4

Normal sight

far away object is clear

Question 5

What is myopia?

Answer 5

Nearsightedness (eyeball too long)

Question 6

What is hypermetropia?

Answer 6

Farsightedness (eyeball too short)

Question 7

What is astigmatism (3)

Answer 7

• Curvature of cornea (usually) is aspherical
• Different amount of refraction in different planes
• Corrected with a cylindrical lens

Question 8

Describe presbyopia (3)

Answer 8

• Lens loses elasticity, accommodation falls from about 15 diopters in children, to 2 diopters at around 45-50 years, to essentially 0 at 70 years
• In consequence, near point recedes from 10cm to 20 years to 80cm at 60 years
- Corrective lens needed (convex) to restore near vision

Question 9

What occurs when a person has a cataract?

Answer 9

• Lens becomes opaque, especially with age
• Lens can be surgically removed and a plastic lens installed
However, ability to accommodate is lost post-surgery i.e. prebyopia

Question 10

What are the steps for the near response?

Answer 10

1. Accommodation
   • Contraction/relaxation of ciliary muscle to alter lens shape and change refractive
   2. Constriction of pupil
      • Improved depth of focus, fewer optical abberrations
   3. Convergence of eyes
      Objects remain in register on corresponding parts of the two retinae

Question 11

What occurs for accommodation for distant vision?

Answer 11

Little parasympathetic activity, ciliary muscle relaxed, zonular fibres taut, lens flatten

Question 12

What occurs for accomodation for near vision?

Answer 12

Parasympathetic activation of ciliary muscle increases, ciliary muscle contracts, zonular fibres relaxed, lens becomes more spherical

Question 13

Describe the features of rods

Answer 13

• 120 million per retina
• Few in fovea
• Function in low light (night vision)
• Don’t report colour information

Question 14

Describe the features of cones

Answer 14

• 8 million per retina
• Mainly in fovea
• Require relatively high light levels (day and twilight vision)
• 3 types, each most sensitive to red, green, blue light

Question 15

What are the two components of photoreceptors?

Answer 15

A membrane spanning protein, an ‘opsin’ (rods and cones differ)

A chromophore, retinal (which is the same for all photoreceptors)

Question 16

What photopigment is present in rods?

Answer 16

Rhodopsin

Question 17

What photopigments are present in cones?

Answer 17

Either S(blue), M(green), or L(red) Photopsin

Question 18

Describe phototransduction in the dark

signal transduction of the visual system

Answer 18

• No light
• Retinal non-activated (inactive 11-cis isoform)
• Lots of cGMP
• cGMP-gated channels open
• Much Na influx (dark current)
• Photoreceptor depolarised (~~ 35mV)
• Lots of glutamate released onto bipolar cells
- Na+ carrying positive charge into the photoreceptor in the dark
NOT firing action potentials –> graded changes in action potentials

Question 19

Describe phototransduction in the light

Answer 19

• Light energy
• Retinal changed (to active all-trans isoform)
• G protein activates cGMP phosphodiesterase
• cGMP phosphodiesterase breaks down cGMP
• Less cGMP, cGMP-gated channels close
• Less Na influx
• Photoreceptor hyperpolarised (~ -60mV)
Less glutamate released onto bipolar cells

Question 20

What are two ways that a person can be colourblind?

Answer 20

Inherited (congenital) or

Acquired (due to disease)