L6 - Eyes Flashcards
Why have people studied the eyes of Necturus?
The mudpuppy has particularly cells in its retina
In a vertebrate photoreceptor, where is the photopigment?
It is in the membrane of organelles called discs
What is the value of the membrane potential of a vertebrate photoreceptor in the dark?
-35
Which component of rhodopsin captures a light photon?
A rhodopsin molecule consists of:
- a protein called opsin
- an associated non-protein photon-absorbing group 1-cis-retinal
1-cis-retinal is found within opsin, covalently bonded to it. This entire molecule sits within the plasma membrane of the photoreceptor cell
How is cGMP involved in phototransduction?
Excited Rhodopsin combines and actives a protein called transducin
Tranducin activates PDE (phosphodiesterase)
PDE converts cGMP to GMP
cGMP in the dark binds to cation channels in the eye, letting Na+ to travel into the outer segment of the photoreceptor maintaining the resting potential. Therefore when cGMP changes to GMP, the membrane hyperpolarises
Can a photoreceptor respond to a single photon of light?
A photoreceptor in a fully dark-adapted eye is sufficiently sensitive to generate 1-5mV receptor potential for 1 light photon
In a photoreceptor, with a maintained light stimulus, the response rapidly becomes smaller.
What is this called?
Why does it matter?
Sensory Adaptation
When background light levels changes so does a photoreceptors sensitivity, allowing the eye to still be sensitive with changing levels of background light
What is the name given to the relative strengths of two stimuli?
Contrast
What kind of an eye does Limulus use to see other Limulus?
Compound eyes
What are the neurons called that receive signals from photoreceptors and carry spikes along their axons to the brain in Limulus?
Ommatidia
What is the effect on one of these cells of illuminating a wide area compared with a small area?
If neighbouring ommatidia lit, cell of steadily lit ommatidium is excited less.
Neighbouring eccentric cells inhibit (suppress) each other.
Immediate neighbours inhibit each other more strongly than more distant neighbours.
How does lateral inhibition help animals to see things?
Enhances contrast at edges.
Basis of several illusions.
As the rod cell hyperpolerizes in resonce to a flash of light, how does this effect the release of NT?
It decreases,
How does the absoption of light by rhodopsin hyperpolerise the cell?
Normally in the dark, Na+ and K+ both diffuse in and out of the outer segment freely - the channels are open. This keeps a level of around -35mV. When rhodopsin is excited by light, it initiates a cascade of events:
1) the photo excited rhodopsin combines with and activates a G-protein called transducin.
2) Activated transducin in turn activates a phosphodiesterase (PDE)
3) This converts cGMP to GMP. In the dark the cGMP in the outer segment binds to cation channels, keeping them open. But now that [cGMP] has dropped the channels close, NA+ cant get in and the cell hyperpolarises
Describe the organisation of the cells in the retina
1) Light travels through the layers of transparent neurons - ganglion, amacrine, bipolar and horizontal cells
2) it is the absorbed by rod and cone cells at the back of the retina. This alters the rate at which the photoreceptor cells release NT.
3) In between ganglion and photoreceptor cells are bipolar cells. This is whom the photoreceptor cells change the rate of NT release to. This change in NT level in turn causes the bipolar cells to release a different amount of NT.
4) the rate at which bipolar cells release NT, determines the rate of APs the ganglion cells fire.
3) Visual information so processing
