Week 5: Vision part I : The Retina

Question 1

What is the optic nerve?

Answer 1

The initial segment of the nerve that connects the eye to the rest of the brain

Question 2

What is the optic disc? What is another name for it?

Answer 2

The point in which the nerve exits the eye. Blind spot because it does not have neurons unlike the rest of the retina. So we are blind in that part of the space that falls onto the optic disc.

Question 3

What is the retina?

Answer 3

Refers to the nervous tissue or thin neuronal layer that covers the back surface of the eye.

Question 4

What is the Iris?

Answer 4

Coloured area of the eye

Question 5

What is the pupil?

Answer 5

The hole left by the iris in the centre of the eye

Question 6

What is the cornea?

Answer 6

The surface of the eye. If we touch our eye with our finger, we will be touching the cornea.

Question 7

What is the function of the lens?

Answer 7

It distorts the light that enters the eye to focus it on the retina.

Question 8

What is the fovea?

Answer 8

The area of highest concentration of neurons.

Question 9

What forms the optic nerve?

Answer 9

Retinal ganglion cells (neurons) that send their axons to the brain

Question 10

Why do blindpsots not affect our visual capacity much?

Answer 10

We have 2 eyes and both our eyes receive light from overlapping parts of space. So the blindspot of the left eye is covered by the right eye, and vice versa. So the blindspot has little bearing on our visual capacities.

Question 11

Describe the layers of the retina from the point light enters?

Answer 11

ganglion cells –> bipolar cells & horizontal cells –> photoreceptors (rods & cones)

Question 12

Describe the synapsing sequence in the retina.

Answer 12

photoreceptors synapse onto bipolar & horizontal cells
bipolar cells synapse onto ganglion cells
ganglion cells are the output of the retina, sending axons into the brain through the optic nerve.
Hence the optic nerve is made of axons of retinal ganglion cells (RGCs)

Question 13

What is the morphological specialisation of the fovea?

Answer 13

Tissue is pushed to the sides, allowing light to go straight to the photo receptors in the area. Having light cross the ganglion, bipolar, horizontal, amacrine layers before reaching the photoreceptors is not ideal as the tissue may distort the light, affecting vision.

Question 14

Describe the distribution of rods and cones at the fovea?

Answer 14

cones – centred on the fovea (high density)

rods – absent at the fovea. but parafoveal area has highest density of rods.

Question 15

What is the difference in luminosity for rods and cones?

Answer 15

Cones - low sensitvity. daytime, activity reflects changes in luminosity. night, completely quiet.
Rods - hypersensitivity. modulate activity at night (low luminosity) based on small changes in luminosity. quiet during the day as they are all saturated.

Question 16

Describe the density of cones at the fovea.

Answer 16

High in density and tightly packed

Larger than those in periphery

Question 17

The different in size of cones at the fovea is ____ than the periphery. What does this explain?

Answer 17

larger. possibly explains why we have such high resolution at the fovea but so poor resolution at the periphery.

Question 18

What is phototransduction?

Answer 18

The conversion of light energy to electrochemical energy (cell depolarisation) in neurons.

Question 19

Why is the outer segment of the photoreceptor layer composed to many tightly packed membranes, stacked on top of each other? (3)

Answer 19

1) To give more space for rhopsodin molecules
2) To increase likelihood of catching a photon
3) To increase density of retinal per square millimeter

Question 20

What is the rhopsodin molecule?

Answer 20

a protein and photopigment found in the outer segment of photoreceptors (rods & cones). It contains retinal molecule.

Question 21

What happens when a light photon hits a retinal molecule of a rhodopsin?

Answer 21

Retinal molecule absorbs the energy of the photon, and this energy makes the molecule twist slightly from cis to trans. The whole rhodopsin molecule changes configuration to accommodate this newly shaped retinal molecule inside. Change in configuration triggers a cascade of events that lead to vision.

Question 22

How is the phototransduction signal amplified?

Answer 22

For every rhodopsin activated, there will be 800 transducin molecules cleaved and 800 phosphodiasterase activated. This leads to 4800 conversions of cGMP to GMP. Leads to closure of 200 ion channels. Basically 1 photoreceptor closes hundreds of ion channels.

Question 23

In the photoreceptor, increases in light leads to a decreased response and vice versa. What kind of response is this?

Answer 23

OFF response

Question 24

What is an ON response?

Answer 24

Increase in response to increases in light.

Question 25

What is a receptive field?

Answer 25

A receptive field of a sensory neuron is the region of space of which a stimulus will trigger a response in the neuron.

Question 26

What neurons have receptive fields?

Answer 26

All neurons in visual system have receptive fields, not just photoreceptors.

Question 27

What is centre-surround organisation?

Answer 27

Centre has one response property and surround has an opposite response property.