Physiology - The Retina Flashcards
Describe the pathway of signal transmission between cells in the retina
photoreceptors (rods and cones, first order neurones) →
bipolar cells (second order neurons) →
ganglion cells (third order neurons) →
axons of ganglion cells merge to form the optic nerve
Where are horizontal cells located & what is their role
- Between photoreceptors & bipolar cells
- Receives input from photoreceptors &
projects to other photoreceptors & bipolar cells
Where are amacrine cells located & what is the role
- Between bipolar and ganglion cells
- Recieves input from bipolar cells &
projects to other bipolar cells, amacrine & ganglion cells
Why are the photoreceptors located at the back of the retina
They need to be in contact with the pigmented epithelium
What are the two types of photoreceptors
Rod cells
Cone cells
What is the role of photoreceptors
Converts electromagnetic radiation to neural signals (transduction)
How are photoreceptors different from other neurons
Photoreceptors are depolarised at rest
Describe the steps that occur in photoreceptors in response to light exposure
Light exposure =>
Light stimulation of opsins (GPCR) =>
Activation of G protein =>
Activation of cGMP PDE =>
cGMP PDE causes cGMP breakdown =>
cGMP-gated Na channel closure =>
Decreased influx of Na but increased efflux of K =>
Decreased charge inside the cell =>
Membrane hyperpolarisation =>
Reduced glutamate (neurotransmitter) release
Rod cells
- ______ convergence
- __________ sensitivity
- ________ visual acuity
- Are _____chromatic
- Are used in ______light
Rod cells
- high convergence
- increased sensitivity
- decreased visual acuity
- Are achromatic
- Are used in dim light
Cone cells
- ______ convergence
- __________ sensitivity
- ________ visual acuity
- Are _____chromatic
- Are used in ______light
- low convergence
- decreased sensitivity
- increased visual acuity
- Are chromatic (detect colour)
- Are used in normal daylight
High vs low convergence
High convergence - Multiple rod cells input to a single ganglion
Low convergence - Few cone cells input to a single ganglion
What are the three types of cone cells
Short wave cone - blue
Middle wave cone - green
Long wave cone - red
How do cone cells differ from each other
Different opsin (GPCR) allows them to have specific wavelength sensitivities for different colours
Where are rod vs cone cells located
Many more rod cells than cone cells in the retina
Cones cells are most abundant in the fovea
Visual acuity definition
The ability to distinguish two nearby points, determined largely by photoreceptor spacing and refractive power
- Increased spacing (like cone cells) => increased acuity
- Decreased spacing (like rod cells) => decreased acuity BUT increased sensitivity
What does the ‘dark current’ mean with reference to phototransduction
cGMP-gated Na+ channels are open in the dark & closed in the light
So when it is dark there is a current of Na+ into the cell (depolarisation)
What is Rhodopsin and what is it composed of
- It is the light sensitive part of the rod cells
- It contains Retinal (Vit A) & Opsin (GPCReceptor)
