The retina Flashcards
What is contained in the inner nuclear layer of the retina?
- Cell bodies of the bipolar cells
What is the pigment in ganglion cells?
Melonpsin
When do rods become saturated?
- In low light levels
- Hyperpolarisation as light intensity increases until light intensity has no affect
- All Na channels are closed
What is the role of the pigmented epithelium layer or the retina?
- Catches the light which has made it past the photoreceptors
- Prevents it from bouncing back into the eye and causing loss of resolution
What does cGMP do?
Binds to Na channels and stimulates them to open during phototransduction
What are the 2 types of bipolar cells?
ON (in light)
- Hyperpolarised in the dark
OFF (in light)
- Depolarised in the dark
What is contained in the outer plexiform layer of the retina?
- Synapses between the bipolar cells and the photoreceptors
- Contains horizontal processing cells
What is contained in the ganglion cell layer of the retina?
Contains the cell bodies of the primary afferent neurons which send output from the retina and make up the optic nerve
What is contained in the photoreceptor outer segment layer of the retina?
- Recptors of the photoreceptors
- Where signal transduction takes place
What is the structure of photoreceptors in the foeva?
- No rods
- 5 million cones
- Processes colour images in high resolution
Why is it important to have a regular daylight cycle?
- Enters the circadian rhythm
- Changes (shift work) can cause cognitive defects
- Abnormal light cycles can cause depression and impair learning
Why does the central retina have a high resolution?
- Low convergence
- One cone to one ganglion cell
- Everything sensed is passed on
- Can detect which part of the retinal field
- In the light
What is the differences between rods and cones?
Rods
- Longer
- More sensitive (have more discs to absorb photons)
- Higher photopigment concentration
- High sensitivity
- Low resolution
- Monochrome
Cones
- Shorter
- Low sensitivity
- High resolution
- Colour vision
What is the photopigment in rod cells?
Rhodopsin
What are amacrine and horizontal cells?
- Processing cells
- Inhibitory
Provide feedback:
Amacrine - to the bipolar cells and ganglion cells
Horizontal - to the photoreceptors and the bipolar cells
Why does the peripheral retina have a low resolution?
- High convergence
- Many photoreceptors to on ganglion cell
- Can’t detect what part of the retinal field
- In the dark ( if only activate a couple of photoreceptors, still get a response)
What is contained in the inner plexiform layer of the retina?
- Synapses between the ganglion cells and the bipolar cells
- Contains amacrine processing cells
What is the process of phototransduction in the light?
- Rhodopsin activated by light in the rods
- Confirmational change
- Acts as a GPCR
- Activates the G protein transductin
- Alpha subunit activates phosphodiesterase
- cGMP –> GMP by the enzyme phosphodiesterase(PDE)
- No dark current (no Na in)
- Rods hyperpolarised
- No neurotransmitter released
Why does the eyes of some animals look bright in the dark?
- They have a reflective layer at the back of the eye instead of a pigmented epithelium
- Increasing sensitivity to low light but compromises activity
What is the process of light adaptation?
- Increased breakdown of cGMP –> GMP by PDE
- Ca2+ can’t enter
- Less inhibition of gunaylyl cyclase
- More cGMP produced
- More channels open
What is the process of dark adaptation?
Feedback loop:
- Ca2+ enters and inhibits guanylyl cyclase
- Decrease levels of cGMP
- Less Ca2+ in
- Less breakdown of guanylyl cyclase
- Increase cGMP
- Increase Ca2+ in
Cycle restarts
What is contained in the outer nuclear layer of the retina?
Cell bodies of the photoreceptors
What does the duplicate theory allow?
- High sensitivity AND low resolution
- Can’t have this in a single photoreceptor
- Seperate systems for monochrome and colour
What enzyme produces cGMP in photoreceptors?
Guanylyl cyclase
What is the process of phototransduction in the dark?
In the dark:
- Rods depolarised ( -30mV) due to the ‘dark current’ where Na in
(cGMP opens the channels)
- Opens voltage gated Ca2 channels
- Ca2 floods in
- Vesicles release neurotransmitter
What happens to the distribution of rods and cones as you move away from the foeva?
- Rapid decrease of cones
- Rapid increase, then steady decrease in rods
What is the structure of the retina?
1) Ganglion cell layer (closest to vitrous humour)
2) Inner plexiform layer
3) Inner nuclear layer
4) Outer plexiform layer
5) Outer nuclear layer
6) Layer of photoreceptor outer segments
7) Pigmented epithelium
