Action Potential Propagation
Fred Rieke
Learning Objectives and Quick Synopses
1. Describe how changes in resting membrane potential affect the action potential.
Changes in resting potential impact action potential generation in two ways: (1) manipulations that make the resting potential farther from threshold can mean that a larger stimulus is required to reach threshold; (2) manipulations that make the resting potential considerably more positive can cause failure of Na+ channels to recover from inactivation.
2. Explain the propagation of an action potential down an axon and describe how the refractory period contributes to this.
The propagating action potential leaves in its wake an area of membrane that is still refractory, and this prevents the action potential from propagating in both directions along the axon.
3. Describe the importance of myelination for action potential propagation.
Myelin decreases the membrane capacitance and increases its resistance. Both of these mean that less current is lost as the action potential propagates. This enables “saltatory conduction” wherein the action potential is regenerated at specific sites along the axon that lack myelin; transmission between these sites is passive (without regeneration).
Overview
In the Action Potential, Threshold, Refractory Period Chapter we studied how the gating of Na+ and K+ channels produces the action potential. Here we study two additional aspects of action potentials: (1) how changes in resting potential alter excitability – i.e. the number of action potentials that result from a given input to a cell, or alternatively how likely a small input is to generate an action potential; and, (2) how action potentials propagate down axons. Both of these are implicated in disease.
A key fact about the threshold for action potential generation is that it is not determined as a fixed voltage relative to the resting potential, but is instead determined by the voltage at which enough Na+ channels open to enter the positive feedback Hodgkin cycle. Thus threshold is determined by the number of available Na+ channels and their voltage dependence. Manipulations that change the resting potential can also change the voltage difference between it and action potential threshold; this in turn changes excitability by either bringing the voltage closer to threshold or moving it farther from threshold. Manipulations that cause a cell’s resting potential to be farther from the threshold (i.e. that make the resting potential more negative) decrease excitability – i.e. the cell is less likely to generate an action potential for a given stimulus. Manipulations that produce modest depolarization cause a cell’s resting potential to be closer to threshold for action potential generation and hence increase excitability — i.e. the cell is more likely to generate an action potential for a given stimulus.
More extreme depolarization can completely suppress action potential generation as a cell goes into “de