ACTION POTENTIAL

A) What is an example of a possible depolarizing event.

An axon becoming more positive inside when sodium ions enter the cell during an action potential.

B) Explain the cause of the rapid change in membrane potential during the period in which it increases after the depolarizing event.

Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane. These channels are shut when the membrane potential is near the resting potential of the cell, but they rapidly begin to open if the membrane potential increases to a precisely defined threshold value. When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential. This then causes more channels to open, producing a greater electric current, and so on. The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential. The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate. As the sodium channels close, sodium ions can no longer enter the neuron, and they are actively transported out the plasma membrane. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state.

C) Explain how the membrane potential reestablishes the starting value at the resting membrane potential, in a period of rapid decreasing of membrane potential.

As the sodium channels close, sodium ions can no longer enter the neuron, and they are actively transported out the plasma membrane. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state. After an action potential has occurred, there is a transient negative shift, called the refractory period, due to additional potassium currents. This is the mechanism which prevents an action potential traveling back the way it just came.