Vision Lecture 11-12 Flashcards
What structure is the eye part of?
It is part of the CNS as it is connected to the visual cortex by the optic nerve
What is the function of the pigment epithelium?
Dark to avoid the scatter of light
Label the retina:
Insert Diagram
What are the types of cells used by the retina?
Light sensitive: Rods, Cones Action Potential generating: amacrine and ganglion cells Plexiform layers containing synapses
Describe the light sensitive cells- Rods?
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
Describe the light sensitive cells- Cones?
Concentrated in the fovea and used in normal light Allow for colour vision
What is the difference in cilium between rods and cones?
Cilium: region where the outer segment is connected to the inner segment
Discs pinch off of cones but not from rods
What are rods and cones sensitive to light?
Plasma membrane discs which contain opsin (cones) and rhodopsin (rods)
What range of wavelength and colours can humans see?
380nm-750nm
Violet - Indigo - Blue - Green - Yellow - Organe - Red
What types of opsin (photopigment) are there?
Red, Green, Blue
Rhodopsin (rods) which is achromatic
What is the link between rhodopsin and GPCRs
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
What is the difference in sensitivity of the different photopigments
Rods / rhodopsin has a broader spectrum than any individual opsin.
What is the mechanisms for activation?
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
Why is cGMP vital?
Regulates the opening of sodium channel, therefore controlling membrane potential
Channel is naturally open in dark conditions as cGMP is in high concentrations
Describe the rhodopsin visual cycle
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
What is the purpose of this mechanism?
Allows for amplification of the signal
reduction in cGMP causes hyperpolarisation due to lack of Na+ entry.
What are the mechanisms at action in a rod cell in the dark?
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
What is the affect of a light stimulus?
How is this terminated?
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
How do the cells adapt to the light response?
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
Comparison between rods and cones?
- Light detection
- Temporal resolution
- Response and integration times
- Sensitivity
- Acuity
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
Describe ganglion cells?
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
What types of ganglion cells are there?
M-type
P-type
Others
Describe M-type Ganglion Cells
Large receptive fields detecting gross features
On and off centre
Not wavelength selective
Describe P-type ganglion cells
Small receptive fields, detect fine features
On and off centre
Wavelength selective
Describe other types of ganglion cells
Circular receptive fields with a centre and antagonistic surrounding
Process information via 2 parralel pathways
Cause dilation of pupils in response to light
What are receptive fields?
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
What are on-centre receptive fields / how do they respond?
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
What are off centre receptive fields and how do they respond
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
How do ganglion cells respond to bipolar cells
On and off centre bipolar cells
With receptors on bipolar cells
Ganglion cells respond to on centre bipolar cells?
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
How do ganglion cells respond to off centre bipolar cells?
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
What is centre surround inhibition?
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
What is the role of inhibition in ganglion cells?
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
How do rods cooperate with cones?
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
Describe the causes and symptoms of colour blindness
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.
