1. B. They are all inactivated. Upon depolarization they opened and inactivated, and since the transmembrane voltage was never returned to the resting value, the channels have not recovered from inactivation. Regarding answer D, note that the cell cannot generate repetitive action potentials because it is voltage clamped: The value of the transmembrane voltage is set by the experimenter, not by the properties of the membrane. For this reason the density of potassium channels (answer E) is irrelevant—the density of potassium channels will affect the current through those channels, but since the cell is voltage clamped, the current through potassium channels will have no effect on transmembrane voltage and therefore no effect on the behavior of the voltage-gated calcium channels.
2. E. Answers A through D are true. The influx of calcium acts, through the voltage change it causes, to increase the influx of calcium—this is why the generation of an action potential is an example of positive feedback.
3. D. Statements A, B, C, and E are all true in themselves but are not reasons why action potentials in nerve cell axons are shorter than those in sea urchin eggs. Concerning answer C, the sodium ions do not have to be removed from the cytosol in order for the action potential to terminate. The very small amounts of sodium that do enter (see page 331) are removed over a timescale of seconds by the Na+/K+ ATPase. If the Na+/K+ ATPase is blocked by the toxin digitalis (page 271), a nerve cell can continue to generate action potentials as normal. Only after many thousands of action potentials have occurred in a digitalis-poisoned nerve cell will the cytosolic sodium concentration rise to a concentration sufficient to significantly affect the threshold for action potential generation.
4. C. There are no voltage-gated calcium channels at the node. All the other statements are true: although some charge is being carried out of the axon at the node by potassium, the current carried by the inward movement of sodium is larger, so there is a net inward current. The excess charge is carried out of the node axially.
6. B. Statements A, D, and E are untrue. Statement C is true in itself in that, like most cells, the sperm plasma membrane will contain some potassium channels that will pass an outward current, but as the answer states, the action of these will be to repolarize the egg membrane, not depolarize it. The depolarizing effect of the calcium influx through the sperm calcium channels, which are open even at the resting voltage, is greater.
7. A. Potassium ions are usually attracted to the negative interior of the cell, and this force opposes the tendency of potassium ions to leave down their concentration gradient. Depolarization reduces the inward electrical force, so potassium ions leave at a greater rate than at rest. At the same time, voltage-gated sodium channels are carrying an inward current, but that movement is not relevant to the question.
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