Physiology Of Sight Flashcards
(40 cards)
What information do the rods in the eyes pass to the CNS?
They tell the CNS about the absence or presence of photons without regard to wavelength
Where does the transduction of light energy into a receptor potential occur in the photoreceptors?
The outer segments of both rods and cones
Describe the plasma membrane in the outer segment of the cones.
Folds back and forth in a pleated fashion
Describe the plasma membrane in the outer segment of the rods.
The pleats formed pinch off from the plasma membrane to form discs - around 1000 stacked discs piled in each outer segment
How are the outer segments of the rods renewed?
One to three new discs are added to the base each hour, while old discs are court off the tip and phagocytosed by pigment epithelial cells
What is contained in the inner segment of the photoreceptors?
Mitochondria
Golgi complex
Nucleus
Describe the proximal end of the photoreceptors.
Expands into a bulblike synaptic terminal filled with synaptic vesicles
What is the first step in signal transduction?
Absorption of light by a photopigment
What is a photopigment?
A coloured protein that undergoes structural changes when it absorbs light in the outer segment
What is the photopigment found in rods?
Rhodopsin
What are the photopigments in the outer section of the cones?
Three different types - one for each colour
- colour vision comes from selective activation of these pigments
What do all photopigments in the eye have in common?
Their two constituent parts
- Opsin (glycoprotein)
- retinal (pigment)
Out of retinal and Opsin, which part of photopigments varies depending on cone colour or rod?
Opsin (4 different types)
- small variations in amino acid sequence of the different opsins permit the rods and cones to absorb different wavelengths of light
Describe retinal.
Vitamin A derivative formed from carotene
This is the light absorbing party of all visual photopigments
What is the shape of retinal in darkness?
Retinal has a bent (cis-retinal) shape which fits into the opsin portion of the photoreceptor
What happens to cis-retinal when it absorbs a photon of light?
It undergoes isomerisation - where it straightens out to become trans-retinal
What happens after isomerisation of trans-retinal?
Several unstable chemical intermediates form and disappear - leading to the production of a receptor potential
What happens to the retinal after it has been straightened.
It detaches from the opsin in a method known as bleaching (because the final products look colourless)
What is the function of retinal isomerase?
It converts trans-retinal back into cis-retinal, so it can bind to opsin (reforming a functional photopigment) in a process called regeneration
How does the pigmented area contribute to regeneration?
It stores a large quantity of vitamin A for the retinal in the rods
Describe the release of neurotransmitters by the photoreceptors in darkness.
1) cGMP gated sodium channels open
2) sodium influx
3) membrane potential increases (partial depolarisation)
4) causes constant glutamate release at the synaptic terminals and inhibition of the bipolar cells
Describe neurotransmitter release during light transduction.
1) isomerisation of retinal activates an enzyme that breaks down cGMP
2) cGMP gated sodium channels close
3) sodium inflow slows down
4) hyperpolarisation induces a receptor potential
5) this turns off neurotransmitter release, exciting the bipolar cell
What happens to glutamate release in dim light?
Small and brief receptor potentials are activated that only partially turn off glutamate release
How many rods synapses with a single bipolar cell?
6-600 depending on the area
- this increases the light sensitivity of rod vision, but slightly blurs the perceived image
