Action Potential, Threshold, Refractory Period
Fred Rieke
Learning Objectives and Quick Synopses
1. Explain the ionic basis of the action potential.
The action potential is produced by the opening of voltage-activated Na+ and K+ channels. Na+ channel opening creates the positive change in voltage of the initial upswing of the action potential. K+ channel opening helps the voltage repolarize and return to its resting value.
2. Explain the concept of threshold in terms of the underlying ion channel activity.
As the cell membrane is depolarized from the resting potential, threshold is the voltage at which Na+ channels begin to dominate the membrane potential. Their opening makes the voltage more positive, which then causes more channels to open, which makes the voltage yet more positive … . This positive feedback cycle creates the threshold.
3. Describe the refractory period and how it is produced mechanistically.
The refractory period refers to the time following an action potential that a cell is unable to generate another action potential. This is produced by the time required for Na+ and K+ channels to return to their normal resting states following activation during the action potential.
Overview
This chapter: (1) introduces the action potential and some of its key properties; (2) reviews the key aspects of electrical signaling that provide the mechanistic basis of the action potential; and (3) explains the action potential by stepping through the learning objectives. The Action Potential, Propagation chapter builds on these ideas to understand how action potentials propagate down axons to carry signals long distances.
Key properties of action potentials
A nerve cell has a negative membrane potential at rest – meaning the inside of the cell has an excess of negative charge. If we inject a weak, brief depolarizing current (i.e. an inward movement of positive charge, making the membrane potential less negative) into such a cell, we don’t see much interesting; specifically, the voltage changes smoothly in response and generally follows the shape of the injected current (Figure 1, far left trace). As we increase the magnitude of the injected current, at some point something very different happens (Figure 1, second trace from left): the membrane voltage makes a large and rapid excursion in response to injected current. This is an action potential – a large, rapid and discrete (i.e. stereotyped) change in membrane voltage. Action potentials are the basic unit of signaling in the central nervous system. All the information our brain receives about the sensory world, all the thoughts and dreams generated in the brain, and all the signals sent from the brain to muscles to generate motor outputs are conveyed via action potentials.
A key role of action potentials is transmitting signals over long distances with minimal delay. The sensory signals that initiate a spinal reflex, for example, can travel the ~1 m distance from your finger tips to the spinal co