Visual System

Question 1

5 Types of cells that make up the retina:

Answer 1

1 - Photoreceptor cells (rods & cones) - light energy –> electrical potentials: outer (light sensitive photopigments) & inner segment (give rise to synaptic terminals contact bipolar/horizontal cells
2 - Horizontal Cells -&
3- Amacrine cells –> both help to integrate & regulate visual info as it is passed along
4 - Bipolar cells - distal (contact with PR cells) & proximal (contact w ganglion cells)
5 - Ganglion cells - receive info from bipolar cells & send to brain; axons form the optic nerve

Question 2

Accommodated

Answer 2

Contraction of ciliary muscles - reduce the tensions of zonule fibres
Lens is rounder thus more refraction of light
Good for near vision

Question 3

Unaccommodated

Answer 3

Relaxation of ciliary muscles - zonule fibers pulled tight
Lens pulled thinner & flatter - less refraction of light
Good for far vision

Question 4

Phototransduction

Answer 4

The cascade of reactions that occurs within the PRs outer segments following light exposure that ultimately leads to hyperpolarization of PRs membrane potential

Question 5

In the dark

Answer 5

• cGMP levels in outer segment of PR membrane are high
• cGMP binds to Na+ permeable channels in the membrane, keeping them open, NA & other cations influx, depolarising the cell (~-40mv)

Question 6

In the light:

Answer 6

• Absorption of photons leads to a decrease in cGMP, closing cation channels
• Receptor hyperpolarization

Question 7

Series of biochemical changes leading to the reduction in cGMP levels: (in light)

Answer 7

• Photon is absorbed by photopigment in receptor disks
• photopigment consist of Opsin that interacts with smaller molecule known as 11-cis retinol (interacts w photon directly)
• Absorption of photon - conformational change in 11-cis retinol –> all trans isomer
• Change - activates transducin (G protein) –> activates phosphodiesterase (PDE) –> hydrolyzes cGMP reducing concentration in outer segment & closer of channels

Question 8

Photoreceptor distribution in the retina

Answer 8

Relative density of two types of cells changes as you move from temporal –> nasal side of retina

• low levels of cone in peripheral sides of retina
• center (Folvea) - highest level of cones & absence of rods - clearest level of vision
• Rod concentration increases near center but falls as it approaches folvea then rises again

Question 9

Macula & Peripheral Degeneration

Answer 9

Macula degeneration - degeneration of area around fovea: lose central vision
Peripheral degeneration - lose sight of peripheries

Question 9

Macula & Peripheral Degeneration

Answer 9

Macula degeneration - degeneration of area around fovea: lose central vision
Peripheral degeneration - lose sight of peripheries

Question 10

receptive field centre responses of retinal ganglion cells

Answer 10

1 - On-center off surround bipolar cells: G-protein metabotropic glutamate receptor (mGluR6) (inhibited by glutamate) cause cells to hyperpolarise in response to glutamate (when light falls in center and not surround AP will fire)
2 - Off-center on surround bipolar cells: Ionotropic receptors (AMPA & kainate) - cause cells to depolarise in response to glutamate released from PR terminals

Question 11

PR cells in light:

Answer 11

PR (center) - becomes hyperpolarized

• does not release glutamate (usually inhibits on-center cells)
• on-center cells now depolarised
• Release glutamate onto on-center ganglion
• increased frequency of firing - there is light

Off-center cells (usually excited by glutamate) become hyperpolarized

• do not release glutamate on off-center ganglion cell
• ceases to fire - there is no light in surround
• -> able to perceive contrast

Question 12

PR cells in dark:

Answer 12

In dark PR cells depolarised - therefore releases glutamate
On-center (hyperpolarized by glutamate) therefore inhibits
- does not release glutamate onto on-center ganglion cells
- not fire APs - no light

off-center (depolarized by glutamate)

• release glutamate onto off-center glanglion cells
• fires APs –> there is no light

Question 13

Central Visual Pathways:

Answer 13

Retina made up of:
1 - Nasal hemiretina: receiving visual images from temporal aspects of visual field - Fibers from NH along pathway of optic nerve cross to contralateral side of brain
2 - Temporal Hemiretina: receiving visual images from central/nasal part of visual field - fibers from TH continue ipsilaterally on same side of brain
–> axons from ganglion cells from both hemiretinas meet at optic disc & bundle together = optic nerve

Question 14

The point at which the optic nerve crosses over

Answer 14

Optic Chiasm

Question 15

Optic Nerve –> Brain Pathway

Answer 15

Fibres project to thalamus:
- Lateral geniculate nucleus (90% fibres): –> primary visual vortex (occipital lobe) visual processing
10% fibres - branch of before thalamus & go to brain stem (superior colluculus): Mediates reflexive eye, head & neck movement in response to visual stimuli

Question 16

Which part of the eye has the largest retinotopic representation in the visual cortex?

Answer 16

Fovea - 50% of cortical representation

Question 17

Examples of factors that can decrease acuity

Answer 17

1 - age-related macular degeneration
2 - detached retina
3 - Amblyopia(lazy eye) - decreased eyesight abnormal visual development
4 - Brain injuries
5 - Stroke

Question 18

Three physiological processes contributing to increased light sensitivity in the retina in darkness:

Answer 18

1 - Dilation of the pupil
2 - Synaptic adaptation of retinal neurons
3 - Increase in concentration of rhodopsin (in rods of retina) available in outer segments

Question 19

Name each photopigment

Answer 19

1 - blue (tritonopic) - short wavelength cones
2 - green (deuteronopic) - medium wavelength cones
3 - red (protonopic) - long wavelength cones