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Physiology and Pharmacology 2 > Vision Lecture 11-12 > Flashcards

Vision Lecture 11-12 Flashcards

1
Q

What structure is the eye part of?

A

It is part of the CNS as it is connected to the visual cortex by the optic nerve

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2
Q

What is the function of the pigment epithelium?

A

Dark to avoid the scatter of light

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3
Q

Label the retina:

A

Insert Diagram

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4
Q

What are the types of cells used by the retina?

A

Light sensitive: Rods, Cones Action Potential generating: amacrine and ganglion cells Plexiform layers containing synapses

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5
Q

Describe the light sensitive cells- Rods?

A

Very sensitive to light so are used in dark conditions and are used to see fainter objects. Concentrated in the periphery not the centre - able to see fainter objects which are off centre. Achromatic

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6
Q

Describe the light sensitive cells- Cones?

A

Concentrated in the fovea and used in normal light Allow for colour vision

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7
Q

What is the difference in cilium between rods and cones?

A

Cilium: region where the outer segment is connected to the inner segment

Discs pinch off of cones but not from rods

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8
Q

What are rods and cones sensitive to light?

A

Plasma membrane discs which contain opsin (cones) and rhodopsin (rods)

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9
Q

What range of wavelength and colours can humans see?

A

380nm-750nm

Violet - Indigo - Blue - Green - Yellow - Organe - Red

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10
Q

What types of opsin (photopigment) are there?

A

Red, Green, Blue

Rhodopsin (rods) which is achromatic

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11
Q

What is the link between rhodopsin and GPCRs

A

Rhodopsin is coupled to G alpha T

This has a binding site for 11 cis retinal at residue 296

Activated by light: conforomational change from 11 cis retinal to all trans retinal- This is known as the phototransfuction step

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12
Q

What is the difference in sensitivity of the different photopigments

A

Rods / rhodopsin has a broader spectrum than any individual opsin.

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13
Q

What is the mechanisms for activation?

A

Light is absorbed causing a signalling cascade

GTP binds to G-Potein –> activates cGMP phosphodiesterase –> breakdown of cGMP (produced from the conversion of GTP by guanylyl cyclase) into 5’-GMP (inactive form) –> prevents the gating of the Na+ cGMP-gated channel

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14
Q

Why is cGMP vital?

A

Regulates the opening of sodium channel, therefore controlling membrane potential

Channel is naturally open in dark conditions as cGMP is in high concentrations

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15
Q

Describe the rhodopsin visual cycle

A

Rhodopsin is converted by light energy (Phototransduction) into bathorhodpsin –> lumirhodopsin –> metarhodopsin I –> metarhodospin II –> either opsin or all trans-retinal

Opsin –> Rhodopsin

All trans-retinal –> 11-cis-retinal –> rhodopsin

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16
Q

What is the purpose of this mechanism?

A

Allows for amplification of the signal

reduction in cGMP causes hyperpolarisation due to lack of Na+ entry.

17
Q

What are the mechanisms at action in a rod cell in the dark?

A

cGMP gated channel is open –> Na+ influx which travels through outer segment and down through cilium into inner segment –> Na+ K+ exchanger swaps Na+ for K+

Influx of K+ causes a release of glutamate –> graded potential change

K+ leaves through K+ selective background leakage channels –> current flux –> Na+ enters

18
Q

What is the affect of a light stimulus?

How is this terminated?

A

cGMP gated channel closes –> hyperpolarisation

This leads to decreased glutamate release

This action is terminated by opsin kinase and arrestin which are activated by the photopigment

19
Q

How do the cells adapt to the light response?

A

Ca2+ flows through the Na+ channel

This inhibits guanylyl cyclase moderately preventing an excessive increase in cGMP concentration

If no Ca2+ can enter –> rapid hyperpolarisation

20
Q

Comparison between rods and cones?

  • Light detection
  • Temporal resolution
  • Response and integration times
  • Sensitivity
  • Acuity
A

Rods: single photon detection / Cones: saturate in really intense light

Rods: low temporal resolution (12Hz) / Cones: high (55Hz)

Rods: Fast response, Slow integration / Cones: opposite

Rods: sensitive to scattered light / Cones: direct light

Rods: low acuity, highly convergenet retinal pathway / Cones: high acuity, less convergent retinal pathway

21
Q

Describe ganglion cells?

A

Mediated by horizontal and bipolar cells in the retina

Graded change in membrane potential from horizontal and bipolar cells feed in

Ganglion cells are the first to fire action potentials

22
Q

What types of ganglion cells are there?

A

M-type

P-type

Others

23
Q

Describe M-type Ganglion Cells

A

Large receptive fields detecting gross features

On and off centre

Not wavelength selective

24
Q

Describe P-type ganglion cells

A

Small receptive fields, detect fine features

On and off centre

Wavelength selective

25
Q

Describe other types of ganglion cells

A

Circular receptive fields with a centre and antagonistic surrounding

Process information via 2 parralel pathways

Cause dilation of pupils in response to light

26
Q

What are receptive fields?

A

Areas of sensitivity for light which extra information about size, form and shape of light and dark in a visual field.

Usually circular with centre and antagonistic surround, meaning information is processed in parralel

There are on centre and off centre receptive fields

27
Q

What are on-centre receptive fields / how do they respond?

A

Stimulated by light spot on

Adapt to light by decreasing the firing of A.P bursts

Light not straight on –> prevention of APs

Illuminate centre –> powerful stimulation

Illuminate off centre –> prevents A.Ps but rebound excitation occurs

Diffuse illumination –> normal AP firing

28
Q

What are off centre receptive fields and how do they respond

A

Apply light spot on centre –> prevents AP’s

Apply light spot off centre –> stimulation then adapts

Illuminate centre –> prevent APs but then rebound

Illuminate off centre –> powerful stimulation (continous AP firing)

Diffuse illumination –> normal AP firing

29
Q

How do ganglion cells respond to bipolar cells

A

On and off centre bipolar cells

With receptors on bipolar cells

30
Q

Ganglion cells respond to on centre bipolar cells?

A

mGluR6 receptors

DARK

Increased glutamate release –> binding to receptor –> release of second messenger –> closing of TRPM1 channels –> hyperpolarisation

LIGHT

Decreased glutamate release –> decreased binding to receptor –> decreased second messenger –> TRPM1 channels open –> inward cationic current –> depolarisation

31
Q

How do ganglion cells respond to off centre bipolar cells?

A

AMPA receptors instead of mGluR6

DARK:

Increased glutamate released –> AMPA receptors open –> sodium influx –> depolarisation

LIGHT:

Decreased glutamate release –> AMPA receptors closed –> no sodium influx –> hyperpolarisation

32
Q

What is centre surround inhibition?

A

Light activates seperate cone –> glutamate release –> activation of AMPA receptors at inhibitory interneuron –> glutamate release from horizontal cell –> binds to GABAa receptors on central cone –> decreased glutamte release –> binds to mGluR6 –> open TRPM1 channels –> depolarisation

33
Q

What is the role of inhibition in ganglion cells?

A

Where does the edge of the ganglion cell fall / off centre?

If the edge is exposed to light –> inhibition = -1

This allows for extraction of shape information

This is enhanecd by the overlapping of receptive fields

34
Q

How do rods cooperate with cones?

A

When there is decreasing brightness the electrical synapse opens to allow ionic movement between rods and cones.

This helps image detection because it increases the sensitivity to light

When it is completely dark the synapses close again and only rods are used.

This is regulated by amacrine cells

35
Q

Describe the causes and symptoms of colour blindness

A

Occurs when one pigment (red green or blue) is abnormal or absent

Most common to be affected is green (blue is least)

Causes the loss of the ability to detect colour differences due to lack of contrast

Females are carriers and males affected.

Physiology and Pharmacology 2 (10 decks)

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