all-trans retinol (Vitamin A)
Excitation of the Rod When Rhodopsin Is Activated by Light
Rhodopsin-retinal visual cycle in the rod, showing decomposition of rhodopsin during exposure to light and subsequent slow reformation of rhodopsin by the chemical processes.
The Rod Receptor Potential Is Hyperpolarizing, Not Depolarizing. When the rod is exposed to light, the resulting receptor potential is different from the receptor potentials in almost all other sensory receptors. That is, excitation of the rod causes increased negativity of the intrarod membrane potential, which is a state of hyper-polarization, meaning that there is more negativity than normal inside the rod membrane. This is exactly opposite to the decreased negativity (the process of "depolarization") that occurs in almost all other sensory receptors.
But how does activation of rhodopsin cause hyper-polarization? The answer is that when rhodopsin decomposes, it decreases the rod membrane conductance for sodium ions in the outer segment of the rod. This causes hyperpolarization of the entire rod membrane in the following way.
Figure 50-6 shows movement of sodium ions in a complete electrical circuit through the inner and outer segments of the rod. The inner segment continually pumps sodium from inside the rod to the outside, thereby creating a negative potential on the inside of the entire cell. However, the outer segment of the rod, where the photoreceptor discs are located, is entirely different; here, the rod membrane, in the dark state, is very leaky to sodium ions. Therefore, positively charged sodium ions continually leak back to the inside of the rod and thereby neutralize much of the negativity on the inside of the entire cell. Thus, under normal dark conditions, when the rod is not excited,
Theoretical basis for generation of a "hyperpolarization receptor potential" caused by rhodopsin decomposition, which decreases the flow of positively charged sodium ions into the outer segment of the rod.
there is reduced electronegativity inside the membrane of the rod, measuring about -40 millivolts rather than the usual -70 to -80 millivolts found in most sensory receptors.
Then, when the rhodopsin in the outer segment of the rod is exposed to light, the rhodopsin begins to decompose, and this decreases the outer segment membrane conductance of sodium to the interior of the rod, even though sodium ions continue to be pumped outward through the membrane of the inner segment. Thus, more sodium ions now leave the rod than leak back in. Because they are positive ions, their loss from inside the rod creates increased negativity inside the membrane, and the greater the amount of light energy striking the rod, the greater the electronegativity becomes—that is, the greater is the degree of hyperpolarization. At maximum light intensity, the membrane potential approaches -70 to -80 millivolts, which is near the equilibrium potential for potassium ions across the membrane.
Duration of the Receptor Potential, and Logarithmic Relation of the Receptor Potential to Light Intensity.
When a sudden pulse of light strikes the retina, the transient hyperpolarization that occurs in the rods— that is, the receptor potential that occurs—reaches a peak in about 0.3 second and lasts for more than a second. In cones, the change occurs four times as fast as in the rods. A visual image impinged on the rods of the retina for only one millionth of a second can sometimes cause the sensation of seeing the image for longer than a second.
Another characteristic of the receptor potential is that it is approximately proportional to the logarithm of the light intensity. This is exceedingly important, because it allows the eye to discriminate light intensities through a range many thousand times as great as would be possible otherwise.
Mechanism by Which Rhodopsin Decomposition Decreases Membrane Sodium Conductance—The Excitation "Cascade." Under optimal conditions, a single photon of light, the smallest possible quantal unit of light energy, can cause a measurable receptor potential in a rod of about 1 millivolt. Only 30 photons of light will cause half saturation of the rod. How can such a small amount of light cause such great excitation? The answer is that the photoreceptors have an extremely sensitive chemical cascade that amplifies the stimulatory effects about a millionfold, as follows:
1. The photon activates an electron in the 11-cis retinal portion of the rhodopsin; this leads to the formation of metarhodopsin II, which is the active form of rhodopsin, as already discussed and shown in Figure 50-5.
2. The activated rhodopsin functions as an enzyme to activate many molecules of transducin, a protein present in an inactive form in the membranes of the discs and cell membrane of the rod.
3. The activated transducin activates many more molecules of phosphodiesterase.
4. Activated phosphodiesterase is another enzyme; it immediately hydrolyzes many molecules of cyclic guanosine monophosphate (cGMP), thus destroying it. Before being destroyed, the cGMP had been bound with the sodium channel protein of the rod's outer membrane in a way that "splints" it in the open state. But in light, when phosphodiesterase hydrolyzes the cGMP, this removes the splinting and allows the sodium channels to close. Several hundred channels close for each originally activated molecule of rhodopsin. Because the sodium flux through each of these channels has been extremely rapid, flow of more than a million sodium ions is blocked by the channel closure before the channel opens again. This diminution of sodium ion flow is what excites the rod, as already discussed.
5. Within about a second, another enzyme, rhodopsin kinase, which is always present in the rod, inactivates the activated rhodopsin (the metarhodopsin II), and the entire cascade reverses back to the normal state with open sodium channels.
Thus, the rods have developed an important chemical cascade that amplifies the effect of a single photon of light to cause movement of millions of sodium ions. This explains the extreme sensitivity of the rods under dark conditions.
The cones are about 30 to 300 times less sensitive than the rods, but even this allows color vision at any intensity of light greater than extremely dim twilight.
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