The Retina

what is the retina

the retina is the light sensitive part of the eye which is responsible for visual transduction

• which is responsible for converting light energy into electrical signal into neurons

describe what photoreceptors are

they are sensory receptors of the retina that detect light and convert it into electrical signals.

• There are two main types: rods and cones.

what are rods and cones

• rods are sensory receptors of the retina which are responsible for black & white (greyscale) vision

• cones are sensory receptors that detect color and are responsible for high acuity vision in bright light conditions.

why are we able to see the EM spectrum

because the wavelength of our photoreceptors transduce into electrical transduction

describe the structure of the retina

• the retina is made up of layers of neuronal cells with photoreceptors at the back of the retina

• Photoreceptors synapse with the bipolar cell which then synapse with retinal ganglion cells which takes the electrical signal from the retinal to the brain via the axon (yellow) which goes across the front of the retina & leave at the optic nerve to take the information onto the brain

name 2 types of neuronal in the retina

1. the horizontal cells

2. amacrine cells

   they are important to lateral processing

describe the structure of the photoreceptor

• it is divided into 3 segment : outer segments, the inner segment, & synaptic terminal

• The outer segment contains the photopigments that absorb light,

• the inner segment contains the cell's metabolic machinery,

• the synaptic terminal transmits signals to bipolar cells.

Approximately how may rods & cones in the retina

1. there is 100 million rods

2. and 6 million cones

why at night are we unable to distinguish colours

At night, we primarily rely on rods for vision, which has a high sensitive to light so are unable to distinguish between color,

• cones have low sensitivity to light because they require bright light for colour to function effectively.

what does the fovea allow

it allows us to see fine detail and colour due to the high density of cones.

what does the peripheral vision allow

it allows us to detect motion and see in low light conditions, as it relies primarily on a higher density of rods , so we are unable to detect colour

why are rods highly convergent and cone not

because up to 100 rods feed into one ganglion cells, allowing for greater sensitivity to light but less detail in visual information.

• cones are less convergent, with one or a few cones connecting to a single ganglion cell, providing high resolution images.

what is the fovea

• the fovea is the area where lens focuses the imageson the retina,

• providing the highest visual acuity and color perception due to a high density of cones.

what is photopigment

• it is the outer segment of the photorecepetor

• it consist of the protein opsin and a chromophore called retinal

what is opsin

it is a GPCR and the retinal is the ligand

what are the 4 photopigmentrs

• There are four types of photopigments in the retina: rhodopsin (found in rods)

• three types in cone which are sensitive to red, green and blue light

what happens when the retinal is activated by light

it changes shape from 11-cis-retinal to all-trans-retinal

• this tiggers a conformational change in opsin, initiating the phototransduction cascade which results in hyperpolarisation of the photoreceptor cells.

describe photoreceptor activity in the dark

1. the outer segment of the photoreceptor contains a cGMP-gated cation channel

2. next the cGMP opens the cation channel which allows sodium ion to move inot the cell down its electrochemical gradient

3. because in the dark cGMP levels are high due to the action of guanylate cyclase which coverts GTP to cGMP, the cGMP-gated cation channels are kept open

4. movement of sodium into the cell then depolarisation (to approx -40mV)

5. depolarisation then causes the opening of the voltage gate calcium channels in the synaptic terminal

6. so, in the dark, the photoreceptors are depolarized, releasing neurotransmitters and maintaining a steady signal

describe photoreceptor activity in the light

1. light causes the 11-cis-retinal to change conformation to all-trans-retinal which causes a conformational change when the G protein is stimulated

2. transducin is also a g protein associated with the receptor inactive (inactive bound to gdp, activated to gtp exchange)

3. activation of transducin causes the activation of phosphodiesterase (PDE), which decreases cGMP levels.

4. This causes cGMP levels to fall, and cGMP-gated channels close the cGMP gated cation channel

5. closure of this means that sodium ion is no longer moving into the cell,

6. leading to hyperpolarization of the membrane potential.

7. Hyperpolarization then causes a closure of the voltage-gated Ca2+ channels, and decreases the amount of neurotransmitter released

8. •Therefore in the light, the photoreceptor is hyperpolarized and neurotransmitter release decreases