In a single-ion system, reversal potential is synonymous with equilibrium potential; their numerical values are identical, and it:

1. Is a function of the concentration of that ion on the extracellular side of the membrane

2. Is a function of the concentration of that ion on the intracellular side of the membrane

3. Is the potential at which there is no movement of that ion across the membrane due to electrochemical forces

4. Is the potential at which there is equal movement of that ion into and out of the cell across the membrane due to electrochemical forces

The period of time during which a second action potential cannot be initiated, no matter how large the applied stimulus is the absolute refractory period. Which statement is true for this period?

1. Potassium channels are open and the membrane is hyperpolarized
2. It immediately follows the relative refractory period
3. It allows an action potential to proceed in two directions along the length of an axon
4. It occurs when the sodium channels are open