Principles of Neuroscience Lecture 6, Vision, the Retina Flashcards

0
Q

What are the two types of photo transducer cells?

A

Rods and Cones

Rods : numerous, night vision, few in fovea, black and white vision, sensitive

Cones : few in number, day vision, concentrated in fovea, colour vision, less sensitive

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1
Q

Describe the basic structures of the eye

A

Cornea :
Sclera : tough material protecting eye
Lens : through which light passes through and is refracted
Vitreous humour: fluid filled cavity inside eye
Retina : contains photo transducer cells, along the back of the eye wall
Fovea : central vision, has many cone cells

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2
Q

Compare the sensitivity of rods and cones

A

Rods more sensitive, cones less

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3
Q

Compare colour vision in rods and cones

A

Rods : black and white

Cones : colour vision

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4
Q

Compare the number of rods and cones

A

Rods : numerous

Cones : few

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5
Q

Compare the localisation of rods and cones in the fovea

A

Rods : very few

Cones : concentrated to the fovea

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6
Q

What does the differential distribution of rods and cones in the fovea mean?

A

Colour vision is localised to the fovea –> colour vision is most clear in central vision
Colours are hard to distinguish in peripheral vision

Night vision is clearest off centre

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7
Q

Describe photo transduction in photoreceptor cells

A

Light hits the rods and cones.

The rods and cones have a protein in their cell membranes called rhodopsin and one of three coneopsins
This protein has Retinal (vitamin A) bound in the cis conformation.

When light hits the retinal, it changes shape to become in the trans conformation.

This initiates a transduction pathway.
A G-protein is activated (ATP - ADP)
2. cGMP –> GMP
3. cGMP no longer a ligand for a sodium channel
4. Hyperpolarisation occurs
5. Hyper polarisation of the cell
6. Hyper polarisation causes neurotransmitter release to stop

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8
Q

Which is the Neurotransmitter released by photo transducer cells?
When is released, and when is it not released?

A

The neurotransmitter is Glutamate.

I is released in the dark, when the cell is depolarised.

When light hits the photoreceptor cells, the sodium channels close and the cell becomes hyperpolarised, and neurotransmitter release stops.

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9
Q

Describe the structure of the retina

A

Pigmented epithelium

The innermost layer of cells in the retina is photoreceptors : rods and cones.

Bipolar cells : 2nd order neurons

Ganglion cells: Magnocllular and Parvocellular : 3rd order neurons, which become the optic nerve

Lateral communication :
Horizontal cells : between photoreceptors
Amacrine cells : fine tuning of signals

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10
Q

Which neurons fire action potentials, and which don’t ?

A
Don't fire action potentials:
Photoreceptors
Bipolar cells
Horizontal cells
Amacrine cells

Do fire action potentials:
Ganglion cells

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11
Q

Describe the properties of the receptive field in the retina

A

When light is shone directly on a cell or group of cells, they respond by increasing the rate of action potential firing.

In the surrounding areas, action potentials are inhibited.

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12
Q

What does the different response in the periphery mean for vision?

A

Edges are encoded

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13
Q

What does the phrase : “the eye is a comparator, not a detector” mean?

A

That the eye does not give us absolute information about the intensity of light, for example, but compares the things we are seeing at once, and distinguishes between them.

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14
Q

Describe the different responses the different ganglion cells can have

A

Transient: incident light causes a burst of action potentials firing which stop after a while

Sustained : the action potentials continue firing for the duration of the stimulus being present.

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15
Q

How many different types of ganglion cell are there, and how do they differ

A

There are 20

Different ganglion cells detect different aspect of the light stimulus.
This information is sent to the brain in parallel, where it is the reconstructed to form an image

16
Q

What are the two types of lateral inhibition?

A

In the horizontal and Amacrine cells

17
Q

Describe lateral inhibition in horizontal cells

A

Horizontal cells receive input from photoreceptors, and output to other photoreceptors.

Horizontal cells release GABA onto neighbouring photoreceptors. This inhibits the other photoreceptors.

18
Q

Describe lateral inhibition in Amacrine cells

A

Fine tuning of information occurs when Amacrine cells perform lateral inhibition

Amacrine cells release Glycine and GABA onto neighbouring bipolar cells

19
Q

What optical factors affect visual acuity?

A
  1. Pupil size
  2. Clarity of visual media
  3. Refractive errors
20
Q

How many types of cone cell are there?

A

3

21
Q

Differentiate between the terms scotopic and photopic. How do they relate to photoreceptor cells?

A

Scotopic: vision in the dark : rods

Photopic: vision in the light : cones

22
Q

How many different types of bipolar cells are there?

A

10

1 rod bipolar cell
9 cone bipolar cells

23
Q

What is the receptive field of ganglion cells?

A

These are the cells that change their firing rate when stimulated by light

24
Q

Ganglion cells convey … information

A

Parallel

25
Q

How do horizontal cells respond to light?

A

By hyper polarising and releasing inhibitory neurotransmitters onto bipolar cells

26
Q

What is retinitis pigmentosa?

A

This is a genetic defect in rhodopsin
Gradual death of photoreceptors
Tunnel vision

27
Q

What are the two broad categories of factors affecting visual acuity?

A
  • Optical factors

- Neural factors

28
Q

Rod and cone cells don’t undergo action potentials. Hw do they signal?

A

Graded changes in membrane potential

29
Q

What is dark current?

A

This is the constant release of neurotransmitters when we are in the dark.

When we transduce light, we stop releasing glutamate onto bipolar cells

30
Q

What does the through pathway refer to?

A

This is the pathway from photoreceptor to bipolar cell and on to the ganglion cells

31
Q

Which neurotransmitter do Ganglion cells release?

A

Glutamate

32
Q

Describe how Ganglion cells respond to stimulation

A

They either increase or decrease their firing rate

This response can be either sustained or transient, depending on the cells type

33
Q

Ganglion cells are especially tuned for …

A

Edges