Vision L6 Flashcards
which light retina lcells are used in the mesophic range?
cones and rods
Scotopic sensitivity is highest iwhere?
Scotopic sensitivity is highest in the parafoveal region, which has the highest rod density.
describe the sliding slace of brightness
2 subtypes of adaptation
Field adaptation, also known as light adaptation, is the rapid and reversible change in sensitivity which takes place when the steady intensity is altered.
Bleaching adaptation is the profound decrease in sensitivity induced by very bright light, which recovers only slowly thereafter upon dark adaptation.
Increment-threshold experiment - describe how it tests light adaptation
describe webers law
ithin the Weber range of intensities, I/I is constant; this ratio is the threshold contrast.
The constant threshold contrast corresponds to the sliding scale of intensity.When the background becomes very bright, the rod system saturates, resulting in a steep increase in threshold with background intensity. Under normal conditions, when the rod system is not artificially isolated by this stimulus, the less sensitive cone system takes over well before rod saturation, and exhibits its own Weber law adaptation.
do cones take over before rods are competely saturated?
do rod photoreceptors adapt to light?
During background light the response to a given flash is smaller than in darkness: the rod has adapted to the background according to Weber’s law.
As well as becoming less sensitive during steady light, the responses of rods also become ……
As well as becoming less sensitive during steady light, the responses of rods also become faster, as shown in the right hand panel.
Thus photoreceptors are able to respond to more rapid changes in bright light than in dim light. Similar light adaptation also takes place in cone photoreceptors.
How does photoreceptor light adaptation arise?
throguh modulation of the transduction mechanism which controls the concentration of cyclic GMP.
Essentially its a fall in cytoplasmic Ca2+;
The absorption of light triggers an enzymatic cascade which culminates in the destruction of cyclic GMP and the resultant closing of channels in the outer segment membrane. These channels normally let both Ca2+ and Na+ enter the outer segment. The Ca2+ which enters is pumped out again via the sodium-calcium exchanger. When Ca2+ influx decreases in the light, this efflux continues for a while, so the cytoplasmic Ca2+ concentration falls. This fall in Ca2+ concentration plays a crucial role in photoreceptor light adaptation; the most telling evidence is that when it is prevented, light adaptation is abolished also, and the receptor saturates at a relatively low intensity.
how does a fall in CA2+ make the photoreceptors less sensitive to light?
Ca2+ acts on the transduction cascade in several ways.
First, Ca2+ inhibits guanylyl cyclase, which produces cyclic GMP. In darkness, when the Ca2+ concentration is relatively high, guanylyl cyclase is partially inhibited.
But in the light, when the Ca2+ concentration falls, this inhibition is reduced, allowing cyclic GMP production to increase.
Second Ca2+ is believed to prolong the activation of photoisomerized rhodopsin, so that its activation switches off more rapidly when the Ca2+ concentration falls during illumination.
Ca2+ also affects the affinity of the cyclic GMP activated channel.
The increased rate of destruction of cyclic GMP by the phosphodiesterase plays an important role in light adaptation, by allowing changes in cyclic GMP concentration to follow changes in phosphodiesterase activity more closely.
This process plays a crucial role in the changes in response kinetics and sensitivity upon adaptation.
what are rod adaptation pools?
Remember that the outputs of individual rods are summed together within the retina through convergence. It is this summed output which provides not only the visual signal, but also the signal for adaptation. Rods are connected together to form adaptation pools, which adaptation of the rod system to take place within the retina, somewhere between the receptors and the ganglion cells.
describe how the dark likght limits the absolute threshold for vision
Even in darkness the rod system is not free of fluctuations, as rhodopsin can spontaneously isomerise due to thermal agitation.
This results in a spontaneous isomerisation every couple of minutes in any given rod.
These spontaneous quantal events are believed to cause the dark light in the retina, which sets the absolute threshold for scotopic rod vision.
label the dark adaptation curve
what is purkinje shift?
During dark adaptation the spectral sensitivity of vision progressively changes in the Purkinje shift. Before the rod-cone break, vision relies on middle and long wavelength cones and the threshold is lowest at 550 nm. But later when the rods take over, the wavelength of peak sensitivity changes to the peak wavelength of rhodopsin at 500 nm.
describe what this picture shows
Following exposure to intense light bleached rhodopsin is gradually regenerated. By the time of the rod-cone break, even though the regeneration of rhodopsin is more than 90% complete, the rod system is still desensitised by several hundred fold: threshold rises approximately exponentially with the fraction of pigment bleached.
Bleaching also directly desensitises photoreceptors.
how?
This bleaching desensitisation is believed to involve persistent excitation of the phototransduction mechanism by rhodopsin photoproducts, leading to a reduced cytoplasmic Ca2+ concentration as in light adaptation.
does Bleaching also results in an increased rate of spontaneous quantal events in each rod?
yes
rods after bleaching a few percent of the rhodopsin, and will also to some extent limit the detection within the retina of the responses to dim flashes as described above. This is believed to result from back reactions by quenched forms of photoisomerised rhodopsin, some of which can also weakly excite phototransduction directly.Such post-bleach noise has been seen in amphibian
how does visual acuity change with intensity?
Changing illumination has profound consequences for the spatial acuity of the visual system.
At low intensities, acuity is low over the entire retina, as the high- convergence rod pathway is used.
But as the light intensity increases, foveal acuity improves dramatically as its tightly-packed densely sampled cones are brought into play.
So high spatial resolution requires light intensities sufficient to adequately stimulate the cone system.
how does the critical fusion frequency change with intensity?
Changing illumination also profoundly affects the temporal properties of vision, which can be assessed using the critical fusion frequency: the frequency above which a flickering light is perceived as steady. At low intensities vision depends on the rods, which are best stimulated by blue-green light. Rod responses are, however, quite slow, giving rise to a low flicker fusion frequency which never exceeds 15 Hz. At higher intensities the cones take over, and exhibit far better temporal resolution. At high intensities at which the cone system responds most rapidly, the flicker fusion frequency approaches 60 Hz. If at low intensities a long wavelength stimulus is used which barely stimulates rods, only the contribution of the cone system is seen. These observations show that as the mean intensity increases, the visual system becomes progressively better at following fast changes.
CONES ARE FASTER THAN RODS = MORE SENSITIVE
