A scientist stimulates an axon two times within fifteen milliseconds. When measuring voltage changes on the other end of the axon, they only see one action potential. Why does this happen? 1. The membrane does not have enough time to return to its resting potential, and so no action potential can be initiated.

2. Voltage-gated Na+ channels are still inactivated after the first action potential, and so a second action potential can not be initiated.

3. Na+/K+ pumps are actively pumping Na+ out of the axon, which prevents the axon from reaching threshold.

4. Voltage-gated K+ channels are opposing the flow of current from the electrode, preventing the axon from reaching threshold.