KD - Vision

Question 1

What is the role of the optic nerve in the visual system?

Answer 1

It carries retinal ganglion cell axons, and there is partial decussation at the optic chiasm, where temporal visual fields cross

Question 2

Where do retinal ganglion cell axons terminate, and what are the functions of these terminations?

Answer 2

Many terminate at the lateral geniculate nucleus, relaying information to the visual cortex.

Others may terminate at the:

• Pretectal nucleus (reflexive eye movements)
• Suprachiasmatic nucleus (sleep-wake cycle)

Question 3

What are the three layers of the eyeball and what is it filled with?

Answer 3

• Sclera
• Choroid
• Retina

Filled with vitreous humour

Question 4

What is the role of the cornea and lens in refraction?

Answer 4

Cornea and lens are highly transparent

Refraction of light

• Cornea provides most of refraction
• Lens is adjustable => accommodation

Question 5

Explain the process of accommodation in the eye

Answer 5

Accommodation – changing your optical power to focus near and far

Zonal fibres connect the lens to the muscle

• If the muscle is contracted, the fibres are loosened and the lens becomes rounder
• If the muscle is relaxed, the fibres are tightened and the lens becomes stretched

Question 6

Define emmetropia, myopia, and hyperopia in terms of refractive errors

Answer 6

Emmetropia
The refractive state of an eye in which parallel rays of light entering the eye are focused on the retina

• Creating an image that is perceived as crisp and in focus

Myopia
A condition in which an image of a distant object becomes focused in front of the retina

• Making distant objects appear out of focus

Hyperopia
A condition in which an image of a distant object becomes focused behind the retina

• Making objects up close appear out of focus

Question 7

Which retinal ganglion cells cross the contralateral hemisphere?

Answer 7

Nasal ganglion cells

Question 8

Where is visual acuity highest in the retina?

Answer 8

The fovea centralis

• A small depression within the neurosensory retina

Question 9

What are 2 features of the retina?

Answer 9

• Part of the CNS
• Forms from diencephalon (optic vesicle)

Question 10

What are the five neuronal types in the retina?

Answer 10

• Photoreceptors
• Bipolar cells
• Ganglion cells
• Amacrine cells
• Horizontal cells

Question 11

Where is light collected?

Answer 11

At the rods and cones

Question 12

What do photoreceptors detect?

Answer 12

Electromagnetic spectrum

Question 13

What are 5 features of Photoreceptors?

Answer 13

• Adjacent to retina-pigmented epithelium (RPE)
• Outer segment: discs containing pigment for light detection
• Photopigment is regenerated in RPE
• Discs are turned over (“shed”)
• Light detected at outer segment leads to hyperpolarisation => graded responses possible

Question 14

How is light detected in cones (in dark and in light)?

Answer 14

In dark
cGMP channels are open

• Influx of Na+
• Efflux of K+
• Depolarisation
• Sets resting membrane potential

In light
cGMP channels are closed

• Reduced Na+ influx
• Efflux of K+
• Hyperpolarisation

Question 15

Explain the process of phototransduction and the role of opsins

Answer 15

• Opsins sit within the membrane of a disc
• Retinal is bound to opsins
• Opsins tune sensitivity to particular wavelength

Light absorbtion => conformational change => activation of transducin => activation of cGMP hydrolysis

Question 16

Transducin Mechanism (7)

Answer 16

1. Light is absorbed by Rhodopsin
2. Activates transducin g-protein
3. Activates Phospho diesterase (PDE)
4. PDE causes cGMP → GMP
5. Closes cGMP Ca2+/Na+ gated channel
6. Na+ infliux decreases
7. Hyperpolarisation

Question 17

How can the transducin mechanism be terminated?

Answer 17

[Ca2+] decreases (channels closed)

• guanylate cyclase activity increases (cGMP restored)
• rhodopsin kinase gets activated, phosphorylates rhodopsin => arrestin binding/ transducin displacement