Sensory Systems - Vision

Question 1

What are the compartments of the eye?

Answer 1

Vitreous Humour
Aqueous Humour
Cornea

Iris divides the eye into two compartments (in front and behind)

Question 2

What is the vitreous humour?

Answer 2

Fills compartment behind the iris and lens

Question 3

What is the aqueous humour?

Answer 3

Fills the compartment in front of the iris and lens

Question 4

What does the aqueous humour do?

Answer 4

Nourishes the cornea since this section does not have a blood supply (provides glucose)

Question 5

What is Glaucoma?

Answer 5

Increased pressure due to decreased removal of the aqueous humour which can lead to blindness

Question 6

What are the 2 types of glaucoma?

Answer 6

Angle closure - where iris adheres to cornea and blocks reabsorption of aqueous humour

Open angle - Sclerosis of veins

Question 7

What is the treatment for glaucoma?

Answer 7

Surgery
Beta blockers - decrease aqueous humour production
Prostaglandin analogues to increase blood drainage.

Question 8

What are some cells found in the retina?

Answer 8

Cells connect via chemical synapses
Ganglion cells
Horizontal and amacrine cells

Question 9

What do the ganglion cells in the retina do?

Answer 9

generate action potentials (not photoreceptors)

Question 10

What do the horizontal and amacrine cells in the retina do?

Answer 10

Modulate transmission of information and enable lateral transmission

Question 11

What is the structure of the retina?

Answer 11

Peripheral retina
Macula (central retina)
Fovea (specialised part with only cones)
Blind spot (optic nerve leaves the eye)

Question 12

What is the relationship between cone cells and the fovea?

Answer 12

Increasing proportion of cone cells closer to the fovea

Question 13

What are the photoreceptors of the eye?

Answer 13

Rods and Cones

Question 14

What are rod cells?

Answer 14

1 type of rod cell
Rhodopsin (photosynthetic pigment)
Intracellular disks
Very sensitive to light 
Responsible for viewing in black and white.

Question 15

Why are rod cells so light sensitive?

Answer 15

1 photon is able to generate a signal

Question 16

What is the purpose of the intracellular disks in rod cells?

Answer 16

Increase surface area to increase the density of rhodopsin

Question 17

What are cone cells?

Answer 17

3 types of cone (green, red, blue)
Iodopsin (photosynthetic pigment)
Need high light intensity to help see in colour
Responsible for coloured vision

Question 18

Why is a high light intensity required for cone cells to be activated and help us see in colour?

Answer 18

Infoldings of the membrane mean that there is a decreased surface area leading to a lower density of iodopsin so more light is required to activate cells.

Question 19

What are the properties of the peripheral retina?

Answer 19

Highly sensitive to light
Mostly rod cells
Many rod cells connect to 1 ganglion cell
Low visual acuity

Question 20

What is retinal convergence?

Answer 20

Where multiple photoreceptor cells converge onto one ganglion cell.
This reduces visual acuity

Question 21

What are the properties of the fovea?

Answer 21

Low sensitivity to light
All cones - less photopigment
1 cone per ganglion cell
high visual acuity

Question 22

How does phototransduction occur in rod cells whilst in the dark?

Answer 22

Rhodopsin is inactive
Cyclic GMP binds intracellularly to gated cation channels opening them.
Channels open leading to Na influx and depolarisation
Em = -30mV
Depolarisation causes glutamate release at synapse with bipolar cell
Bipolar cell receives signal in absence of a stimulus.

Question 23

What is the structure of Rhodopsin?

Answer 23

Retinal (chromophore responding to light)

Opsin (GPCR)

Question 24

How does phototransduction occur in rod cells when exposed to light?

Answer 24

Exposure to light causes hyperpolarisation of rod cell:

1. Retinal absorbes light and activates opsin (GPCR)
2. opsin activates phosphodiesterase (PDE)
3. PDE breaks down cGMP
4. less cGMP binds so gated channels closed and therefore no ion influx
5. Causes hyper polarisation.

Question 25

What is the direct pathway of signal transmission through the retina?

Answer 25

Photoreceptor => bipolar cell => ganglion cell => visual cortex

Question 26

What do photoreceptors do when exposed to light?

Answer 26

ALWAYS hyperpolarise

Question 27

What are ON or OFF cells?

Answer 27

Subtypes of bipolar and ganglion cells that either depolarise or hyperpolarise in light.

Question 28

How do ON bipolar cells respond to light?

Answer 28

ON cells are depolarised by light: Light - Rod cell is hyperpolarised - so less glutamate is released causing the cell the depolarise Dark - Rod cell is depolarised - Increase glutamate causes cell to hyperpolarise

Question 29

How do OFF bipolar cells respond to light?

Answer 29

OFF cells are hyperpolarised by light: Lights - Rod cell hyperpolarised - Less glutamate release causes hyperpolarisation Dark - Rod cell depolarised - Increased glutamate release causes cell to depolarise

Question 30

What is responsible for the different and opposite electrical responses to light?

Answer 30

Different glutamate receptors in ON and OFF cells. ON cells - Inhibitory glutamate receptors causing hyperpolarisation in ON cells OFF cells - Excitatory Glutamate receptors causing depolarisation in OFF cells.

Question 31

What is the indirect pathway of signal transmission through the retina?

Answer 31

Each bipolar/ganglion cell has a receptive field Central part - direct connections from photoreceptor Surround part - Indirect connections via amcrine and horizontal cells.

Question 32

What is the receptive field of a bipolar/ganglion cell?

Answer 32

Region of retina that influencesthe cell.

Question 33

What response does the indirect pathway have on bipolar/ganglion cells?

Answer 33

Opposite response to the direct pathway.

Question 34

What is the purpose of the receptive field in bipolar/ganglion cells?

Answer 34

Important for contrast at image borders.

Question 35

What is the optic chiasm?

Answer 35

Where the optic tracts cross over

Question 36

What is the right optic tract?

Answer 36

Visualises the temporal side of the right optic nerve and nasal side of the left optic nerve.

Question 37

What is the left optic tract?

Answer 37

Visualises the temporal side of the left optic nerve and nasal side of the right optic nerve.

Question 38

What happens after transection of the left optic nerve?

Answer 38

Loss of sight in the left eye periphery.

Question 39

What happens after transection of the left optic tract?

Answer 39

Loss of sight in the right visual hemisfield

Question 40

What happens after transection of the optic chiasm?

Answer 40

Lose of sight in the periphery.