Vision

Question 1

Where is aqueous humour stored?

Answer 1

In the anterior and posterior chambers.

Question 2

What is the function of the aqueous humour?

Answer 2

To provide nutrients to the cornea (since the cornea doesn’t have a blood supply).

Question 3

Describe the course of flow of aqueous humour.

Answer 3

• It is produced by the ciliary body in the anterior chamber.
• It is then secreted into the posterior chamber behind the iris.
• It returns and is reabsorbed in the anterior chamber.

Question 4

What is glaucoma?

Answer 4

Visual impairment due to an increase in pressure, which in turn is due to a decrease in removal (by reabsorption) of aqueous humour.

Question 5

List the types of glaucoma.

Describe the pathophysiology of each type.

Answer 5

1 - Angle-closure (iris adheres to the cornea, blocking reabsorption).

2 - Open-angle (sclerosis of veins draining the eye).

Question 6

List 3 treatments for glaucoma.

Answer 6

1 - Surgery.

2 - Beta blockers (to reduce production of aqueous humour).

3 - Prostaglandin analogues to increase blood drainage of the eye.

Question 7

List the cells of the retina.

Describe the organisation of these cells in the retina.

Answer 7

1 - Photoreceptors at the most superficial surface.

2 - Bipolar cells synapse with photoreceptors, connecting them to:

3 - Ganglion cells, which transmit the signal by merging into the optic nerve.

4 - Horizontal cells synapse at 2 different photoreceptor-bipolar cell synapses.

5 - Amacrine cells synapse with 2 amacrine cell-optic nerve synapses.

Question 8

Which cells of the retina generate action potentials?

Answer 8

Ganglion cells only.

Question 9

What is the function of horizontal and amacrine cells?

Answer 9

They modulate the transmission of information by providing alternate pathways.

Question 10

List the types of photoreceptors.

Answer 10

1 - Rods.

2 - Cones.

Question 11

Where are rods found?

What is the density of rods in this location?

Answer 11

• In intracellular membrane disks.

- They are found in relatively high densities.

Question 12

Where are cones found?

What is the density of rods in this location?

Answer 12

• In infoldings of surface membranes.

- They are found in relatively low densities.

Question 13

How many types of rods exist?

List the photopigment(s) involved in each type.

Answer 13

• 1.

- Rhodopsin.

Question 14

How many types of cones exist?

List the photopigment(s) involved in each type.

Answer 14

• 3:

- Red, green and blue photopigments.

Question 15

List 2 functional differences between rods and cones.

Answer 15

• Rods are very sensitive to light, and therefore contribute to night vision, whereas cones are not very sensitive to light, and contribute to daytime vision.
• Rods are not sensitive to colour, whereas cones are.

Question 16

What causes the blind spot?

Answer 16

It is caused by the the area of the eye where the optic nerve leaves the eye.

Question 17

What is the fovea?

Answer 17

The central part of the retina.

Question 18

Which cell type predominates in the fovea?

Answer 18

Cone cells.

Question 19

Which cell type predominates in the peripheral retina?

Answer 19

Rod cells.

Question 20

Why are rod cells highly sensitive to light?

What is the compensation for this?

Answer 20

• Because many rods connect to one ganglion cell, so more light is transmitted per ganglion cell.
• The compensation is that rods produce low visual acuity.

Question 21

Why do cone cells have a low sensitivity to light?

What is the tradeoff for this?

Answer 21

• Because only one cone cell only connects to one ganglion cell, so less light is transmitted per ganglion cell.
• The tradeoff is that cones produce high visual acuity.

Question 22

List the components of rhodopsin.

Answer 22

1 - Retinal (the chromophore - part of the molecule that responds to light).

2 - An opsin (a GPCR), known as transducin in photoreceptors.

Question 23

Describe the mechanism by which rhodopsin in rod cells responds to light.

Answer 23

• When there is no light, a cGMP-gated ion channel causes Na+ and K+ influx, causing depolarisation of -30mV.
• This results in glutamate release at the synapse.
• When there is light, the retinal is activated, and changes conformation to an all-trans structure.
• This causes the opsin (transducin) to activate phosphodiesterase, which catalyses the breakdown of cGMP to GMP.
• The loss of Na+ and K+ influx causes hyperpolarisation and therefore stops glutamate release.

Question 24

List the types of bipolar and ganglion cells.

What is their state of depolarisation in the presence of light?

What is the implication of this with regards to the effect of glutamate on depolarisation?

Answer 24

1 - On cells (depolarised in light).

2 - Off cells (hyperpolarised in light).

• Since glutamate is being released by photoreceptors in the dark, this must mean that:
• Glutamate hyperpolarises on cells.
• Glutamate depolarises off cells.

Question 25

Which cells have receptive fields?

Define receptive field.

Answer 25

• Bipolar and ganglion cells have receptive fields.
• The receptive field for a particular bipolar or ganglion cell is the region of the retina that has an influence on that cell.

Question 26

List the components of a receptive field.

How do they differ functionally?

Answer 26

1 - Centre, where direct connections with photoreceptors are made.

2 - Surround, where indirect connections with photoreceptors via horizontal and amacrine cells are made.

Question 27

How does the surround component of a receptive field contribute to the overall state of polarisation of a bipolar cell?

How does this compare to the centre component of a receptive field?

Answer 27

• The surround component of a receptive field responds to light in the opposite way to normal (normal being in the centre of a receptive field).

Take, for example, an on cell:

• Normally, an on cell is depolarised in light (when no glutamate is being released) and hyperpolarised in dark (when glutamate is being released).
• If a photoreceptor is in the surround component of a receptive field, its reaction to glutamate switches, so an on cell in the surround component of a receptive field behaves like an off cell.
• In the centre component of a receptive field, photoreceptors behave normally.

Question 28

What is the importance of the switching of responses of on and off cells in the surround component of receptive fields?

Answer 28

It produces contrast at image borders.

Question 29

How are the left and right visual hemifields preserved within the optic nerve?

What happens when the optic nerves merge to form the left and right optic tracts?

Answer 29

• The left visual hemifield is preserved within the right half of each optic nerve and the right visual hemifield is on the left half of each optic nerve.
• When the optic nerves merge, the left visual hemifield is preserved within the right optic tract and the right visual hemifield is preserved within the left optic tract.