The graph represents the changes in membrane potential across an axon membrane during the passage of a single action potential.
Which of the following statements correctly describes the events occurring at the marked points on the graph?
At II, voltage-gated Na+\text{Na}^+Na+ channels are closed and Na+\text{Na}^+Na+ ions are diffusing out of the axon; at V, voltage-gated K+\text{K}^+K+ channels are closed.
At II, voltage-gated Na+\text{Na}^+Na+ channels are open and Na+\text{Na}^+Na+ ions are diffusing into the axon; at IV, voltage-gated K+\text{K}^+K+ channels are open and K+\text{K}^+K+ ions are diffusing out of the axon.
At III, voltage-gated Na+\text{Na}^+Na+ channels are fully open; at V, voltage-gated K+\text{K}^+K+ channels are closed and the sodium-potassium pump has stopped working.
At IV, voltage-gated Na+\text{Na}^+Na+ channels are open and Na+\text{Na}^+Na+ ions are diffusing out of the axon; at V, voltage-gated K+\text{K}^+K+ channels are closed.