neurobio 915 ch 9 of Bear's book: the eye part 2 (cornea sclera retina fovea lens bipolar cell
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what does bleaching of rhodopsin mean?
the absorption of light causes a change in the conformation of retinal so that it activates the opsin. This process is called bleaching because it changes the wavelengths absorbed by the rhodopsin.
Explain what happens after rhodopsin is bleached
the bleaching of rhodopsin stimulates a G protein called transducin in the disk membrane, which in turn activates the effector enzyme phosphodiesterase, which breaks down the cGMP that is normally present in the cytoplasm of the rod (in the dark). The reduction of cGMP causes the Na channels to close and the membrane to hyperpolarize
our visual system is capable of detecting as little as a single photon because of what?
What happens to rods in bright sunlight? what is the consequence?
they become saturated; additional light causes no more hyperpolarization. thus, vision during the day depends entirely on the cones, whose photopigments require mor eenergy to become bleached
what does the trichromacy theory state?
that at each point in the retina there exists a cluster of three receptor types, each type being maximally sensitive to either blue, green, or red.
explain dark adaptation
dark adaptation involves the regeneration of unbleached rhodopsin and an adjustment of the functional circuitry of the retina so that info. from more rods is available to each ganglion cell.
explain light adaptation
light adaptation reverses the changes in the retina that result from dark adaptation.
When you step out into bright light from a movie theater, initially the cones are hyperpolarized as much as possible (toward the equilibrium potential for K). If the cones stayed in this state, we would be unable to see changes in light level. as we discussed above, the constriction of the pupil helps a bit in reducing the light entering the eye. However, the most important change is a gradual depolarization of the membrane back to about -35 mV.
The reason this happens stems from the fact that the cGMP-gated sodium channels we discussed previously also admit calcium, In the dark, Ca enters the cones and has an inhibitory effect on guanylyl cyclase. When the cGMP-gated channels close, the flow of Ca into the photoreceptor is curtailed; as a result, more cGMP is synthesized (because the enzyme is less inhibited), thereby allowing the cGMP-gated channels to open agian. Stated more simply, when the channels close, a process is initiated that gradually reopens them even if the light level doesn't change. Calcium also appears to affect photopigments and phosphodiesterase in ways that decrease their response to light.
in the outer plexiform layer, photoreceptors synapse with what two types of cells?
bipolar and horizontal cells
what do ON bipolar cells do in response to glutamate?
what do OFF bipolar cells do in response to glutamate?
the glutamate opens glutamate-gated cation channels and causes Na to diffuse in, causing a depolarizing EPSP
bipolar cells are connected via ________ cells
what is the receptive field of a bipolar cell?
the area of retina that, when stimulated with light, changes the cell's membrane potential
true or false. The response of a bipolar cell's membrane potential to light in the receptive field center is the same to that of the light in the surround receptive field
false, it is opposite.
how are ganglion cells distinguished on the basis of size?
m type ganglion cells are larger than P type ganglion cells
cells that don't fit the category of M type of P type are called what? what percentage of ganglion cells do these cells account for?
nonM-nonP ganglion cells. account for 5% of ganglion cells
color sensitive ganglion cells are called what?
explain why color opponent cells get their name
the response to one wavelength in the receptive field center is canceled by showing another wavelength in the receptive field surround.
what are the types of color opponency?
red versus green and blue versus yellow
consider if you have a cell with a R+G- receptive
field. What would happen if one was to shine red light upon the entire
receptive field? What parts of the receptive field would become activated? Is
this an effective stimulus?
Red light bathing the entire receptive field is an effective
stimulus. However, the response is reduced because red light has some effect on
green cones (recall the overlap of the red and green sensitivity curves) that
feed into the green OFF surround. The response to red is only canceled by green
light on the surround.
consider if you have a cell with a R+G- receptive
field. What would happen if one was to shine white light on the entire
Because white light contains all visible wavelengths, both
center and surround would be equally activated, thereby canceling the response
of the cell.
what is the most abundant type of ganglion cell? M-type, P-type, or nonM-nonP type?
P-type. It constitutes 90% of all ganglion cells in human retina
how do M and P cells differ from one another in terms of visual response properties?
M cells have larger receptive fields, conduct action potentials more rapidly in the optic nerve, and they are more sensitive to low-contrast stimuli. In addition, M cells respond to stimulation of their receptive field centers with a transient burst of action potentials, while P cells respond with a sustained discharge as long as the stimulus is on.