Visual Physiology Flashcards
Describe the basic features that are shared by both cone and rod photoreceptors.
- Outer segment containing phospholipid membrane discs and proteins which allow it to detect light
- Inner segment containing nucleus
- “Axon” carrying electrotonic potentials to synaptic terminal
- Glutamate released at synaptic terminal
- Relatively depolarised at rest: -40mV - this is due to Na channels in outer segment
Describe and explain the transduction mechanism that initiates the photoreceptor response to light.
- All rods and cones have opsin photopigment containing 11-cis retinal, which changes to all-trans retinal when it absorbs light
- Activated photopigment triggers a cascade of reactions resulting in a fall in intracellular cGMP
- Fall in cGMP closes Na+ channels and hyperpolarises the photoreceptor
Describe and explain the transduction mechanism that terminates the photoreceptor response to light.
- The opsin photopigment is inactivated - 11-cis retinal is used up
- An enzyme restores the level of intracellular cGMP
- Another molecule of 11-cis retinal is attached to the photopigment in preparation for the next initiation cycle
How do photoreceptor cells adapt to increased levels of light?
- When light intensity increases and then stays constant, the initial hyperpolarisation induced by the light subsides and the membrane potential returns to -40mV
- In effect, the new light level becomes the new baseline level
- Proteins control the rate and sensitivity of the light response - i.e. adaptation
Photoreceptors have very complex biochemistry which enables them to transduce light. What are the 2 main disadvantages of this?
- Very small defects in many of the necessary proteins results in degenerative diseases - e.g. retinitis pigmentosa - more than 60 known genes involved
- Very high metabolic rate - specialised blood supply required - the choroid
Why do the outer segments of the photoreceptors need to point away from the incoming light?
So that they are adjacent to the choroid blood supply, separated by the pigment epithelium
What are the important functions of the retinal pigment epithelium (RPE)?
- Creates suction between outer segments of photoreceptors and choroid
- Suction holds the retina in place
- Creates large surface area for supply of nutrients across blood-retinal barrier
- Take up all-trans retinal and resynthesise 11-cis retinal
- RPE cells are phagocytic - every 10 days the entire outer segment is phagocytosed and replaced
Why is there a high potential for photo-oxidation and other damage in the retinal pigment epithelium and neural retina? What is the consequence of this?
- Electromagnetic radiation from the Sun and high quantities of saturated oxygen in the same place
- Proteins and phospholipids are prone to photo-oxidation, and may become indigestible
- With age the RPE tends to accumulate and secrete indigestible material, leading to the build up of fatty plaques - “drusen”
- Drusen blocks the blood supply from the choroid, leading to photoreceptor death
Name the 4 types of ganglion cell that each cone cell provides input to.
- “Off” cells - excited by decreased illumination
- “On” cells - excited by increased illumination
- Parvocellular cells - specialised for extracting fine detail and colour information
- Magnocellular cells - specialised for detecting fast movement and broad outlines
A parvocellular cell receptive field consists of how many cone cells? What is the consequence of this?
1 cone cell - fine detail (if image is stable)
Visualise a parvocellular receptive field in the fovea, imagining you are looking down on the retina.
If a very bright light hits both the centre and periphery of the retina, what will happen in the retinal ganglion cells?
- Retinal ganglion cells respond to contrast
- If the centre and periphery of the receptive field receive the same stimulus, the excitation and inhibition will cancel out
- Thus the ganglion cells will not respond by firing any action potentials
Describe the differences between parvocellular and magnocellular ganglion cells.
- Magnocellular cells have larger receptive fields
- This allows for increased sensitivity but at the expense of resolution
- At night a parvocellular cell may have a very large receptive field of rods - but still only receives information from a single cone
How does the visual system compare the output of different cones?
- “Red” is compared with “green”
- “Blue” is compared with “yellow”
Contrast the information encoded by retina + LGN and visual cortex cells.
Retina and LGN:
- Contrast - i.e. edges of objects
- Wavelength
Visual cortex:
- The orientation of edges
- The presence of corners etc.
- Direction of motion
- Binocular disparity
