Optics of vision

Question 1

Define focal length

Answer 1

The power of a lens. The thicker the lens, the shorter the focal length, the stronger the power.

Question 2

Describe accomodation in relation to the eye

Answer 2

The ability of the lens to change power through ciliary muscles. (Horses lack this ability).

Question 3

What are the steps in phototransduction activation?

Answer 3

1. Rhodopsin is the photopigment found in rods. It is composed of retinal and opsin which are bound together
2. Photon hits rhodopsin and causes the isomerisation of retinal from cis to trans. This splits the bond between opsin and retinal
3. Free opsin activates transducin, which is a G protein
4. Transducin activates PDE which decreases intracellular cGMP
5. Causes ion channels to close and the cell to become hyperpolarised

Question 4

What will light cause when it hits the ganglion receptive field?

Answer 4

1. Light hits centre - hyperpolarisation of centre cone which increases inhibition by horizontal cells which will stimulate ganglion cells
2. Light hits around - centre cone depolarisation enhanced by removal of inhibition by horizontal cells, which will cause inhibition of ganglion cells

Question 5

What are the 3 central visual pathways?

Answer 5

1. Recognises objects - consists of simple, complex and hypercomplex cells with different receptive fields
2. Localises objects - Moves eyes to object of interest in visual field
3. Proprioception - linked to vestibular apparatus. Provides information by which head movement is sensed

Question 6

How are objects localised?

Answer 6

Question 7

Describe the pupillary light reflex

Answer 7

1. Shine light into eye
2. Picked up by the retina and stimulates the optic nerve
3. 50% of innervation goes to ipsilateral side of brain, 50% to contralateral
4. Optic nerve goes to parasympathetic nucleus of CN III
5. Stimulates oculomotor nerve (CN III) which goes to the parasympathetic ganglion
6. This causes constriction of the pupil
7. Cross over at the optic chiasm and pretectal nucleus causes constriction of both pupils

Question 8

Describe the central pathway of the dazzle reflex

Answer 8

Light -> retina -> rostral colliculi via CN II -> response via CN VII