Action potentials (nerve impulses) are changes in the membrane potential that, once started, will affect the entire excitable membrane. The first action potential is usually generated at the initial segment of the neuron's axon. This activity will test your understanding of the sequence of events that occur at the membrane of the initial segment of the axon during generation of an action potential.

Place the events involved in generation of an action potential in the correct order of occurrence from left to right.

1. K + efflux, Repolarization

2. Na + channels close, K + channels open

3. Hyperpolarization, K + channels close

4. Threshold stimulus, Na + channels open

5. Na + influx, Depolarization

The order of events would be:

4. Threshold stimulus, Na + channels open. This is the first event

5. Na + influx, Depolarization. Second event

2. Na + channels close, K + channels open. This is the third event

1. K + efflux, Repolarization. Fourth event

3. Hyperpolarization, K + channels close

Explanation:

In resting state, the membrane is negative inside, and K + concentration is higher than outside. Na + concentration is higher outside instead.

Sodium influx is responsible for membrane depolarization. It's the event to produce the action potential. When sodium channels are closing, potassium channels open and K + go outside until repolarization occurs. But now we have both ions in different sides of the membrane, and this can't be stimulated again to produce other action potential. The membrane will remain refractory to respond to the adequate stimulus until ions migrate to their original sites. This is Na + outside, and K + inside. This is responsibility of the Na+ / K + pump, that uses ATP to pump Na + out, and K + in. Once both ions had been relocated in the membrane, this is ready to receive a correct stimulus to produce a new action potential.