HMO202 W1 Role of the retinal layers in visual processing

What are the retinal layers?

Internal limiting membrane
Nerve fibre layer
Ganglion cell layer
Inner plexiform layer - synapse between bipolar axons and dendrites of amacrine cells
Inner nuclear layer
Outer nuclear layer
External limiting membrane
Rods and cones
RPE

What are the four stages of visual processing?

Photoreception by photoreceptors
Transmission of signal to bipolar cells
Transmission to ganglion cells
Transmission along the optic nerve
At each stage there are horizontal and amacrine cells regulating transmission
We can use OCT when assessing damage and disease in these visual processors

What are the retinal layers?

Internal limiting membrane
Nerve fibre layer
Ganglion cell layer
Inner plexiform layer - synapse between bipolar axons and dendrites of amacrine cells
Inner nuclear layer
Outer nuclear layer
External limiting membrane
Rods and cones
RPE

What are the four stages of visual processing?

Photoreception by photoreceptors
Transmission of signal to bipolar cells
Transmission to ganglion cells
Transmission along the optic nerve
At each stage there are horizontal and amacrine cells regulating transmission
We can use OCT when assessing damage and disease in these visual processors

Explain the transmission of light

Photoreceptors -> Bipolar cells -> Ganglion cells

What are neurotransmitters?

Chemical messengers that transmit messages from a nerve cell across the synapse to a target cell

Can be an exciter (glutamate and acetylcholine) or inhibitor (GABA and glycine)

It is believed that 80% of amacrine cells release inhibitor substances

What is a receptive field?

Area in which stimulation leads to a response of a particular sensory neuron

Why are there two parallel channels for cones and one for rods?

Two streams allows for visual processing to on centre and off centre responses to stimuli

What stimulates on and off channels?

Brighter stimuli stimulates on channels
Darker stimuli stimulates off channels

The maximum response in a ganglion cell with an on-centre/off surround receptive field is achieved when...

The entire centre of the receptive field is illuminated whilst keeping the surround dark

What are photoreceptors?

Light cells that converge light through photo transduction

Distinguish between rods and cones

Rods

• dim light (scotopic vision)

• More abundant

• Highly sensitive to light

• Absent in fovea

• Rhodopsin pigment in outer segment

• Increase as you move away from macula Cones

• bright light (phototopic light)

• smaller

• wider

• more sensitive to colour

• Humans have three different cone types (trichromatic) (blue, red and green)

• Present in fovea

What is spectral sensitivity?

Refers to relative efficacy of detection of light by cones

When are photoreceptors depolarised?

In darkness

When are photoreceptors hyperpolarise?

When illuminated

Explain production of glutamate

Constantly releasing glutamate in the dark

Describe neurotransmitter receptors on bipolar cells

Two types of receptors:
- Ionotropic glutamic receptors (off bipolar cells)
Form an ion channel
Depolarizes

-Metabotropic glutamic receptor (on bipolar cells)
indirectly linked to ion channels to signal transduction of a g protein
Hyperpolarises

What are the functions of horizontal cells

Provide lateral inhibition through a feedback channel which assists in the modulation of photoreceptor responses

Inhibits responses under high light

Summation of light under low light

Maximizes contrast

Colour opponency

What are the two types of horizontal cells?

Luminosity types ( responds to changes in intensity)
Chromaticity type (responds to changes in wavelength)

When do horizontal cells release glutamate

Under low light levels, as it does not need to inhibit, just to detect

What is the role of retinal horizontal cells

Minimising contrast

What are amacrine cells?

Responsible for the modulation of brightness and motion perception

How is simultaneous contrast achieved

Achieved by lateral inhibition where a dark boundary inhibits a light area / dark area

How can we improve image resolution

Perceive contrast, should be able to process brighter areas of image to darker image

How does lateral inhibition work?q

Reduces activity of neighbouring neurons to increase contrast
Needs to occur simultaneously
Can do so with centre surround responses

What are off ganglion cells?

Off centre, on surround

What are on ganglion cells?

On centre, off surround

Where are ganglion cells located?

Nerve fibre layer towards optic nerve head in arcuate pattern

What are the broad classifications of ganglion cells

Midget (P) (Smaller receptive field, use for perception of colours)

Parasol ganglion cells (M) (Large, respond optimally to rapid changes in stimuli)

S cone

What are the three kinds of support cells?

Astrocytes
Muller cells
Microglia

What is the function of astrocytes?

Envelops ganglion cell axons, supporting relationship to blood vessels of the nerve fibre layer

Contain glycogen

Provide nutritive surface

Assists in ionic homeostasis

What is the function of Muller cells?

Clearing metabolic waste

Glutamate re synthesis

Synthesize retinoic acid from retinol

Can take up extracellular potassium and redistributing

In the healthy retina, it is involves in retinal glucose metabolism

Regulates retinal blood flow and contribute to the regulation of blood neuron barrier

What is the function of microglia

Roles in retinal homeostasis

Can assist in recovery from injury and disease

Will contribute to phagocytosis for degenerating retina neurons