The Intracellular Action Potential
You should already be familiar with the characteristics of an intracellular action potential (from
BGYB30). It is important to understand the intracellular AP in order to understand the compound
action potential (CAP) that you will be recording in the lab.
The important characteristics of the intracellular AP are:
1) They are triggered by an increase in membrane potential (i.e., a depolarisation).
2) A certain threshold potential (level) must be reached during the depolarisation before an AP
can be fired. If membrane potential rises but does not reach threshold then an AP will not occur.
There is also a threshold voltage (electrical stimulus) that is required to generate a CAP. The
basis of this threshold is similar (but not exactly the same) as the basis for the intracellular AP.
A sub-threshold stimulus (depolarisation) will never generate an intracellular AP. A supra-
threshold (greater than threshold) stimulus (depolarisation) will generate an intracellular AP but
the amplitude of the intracellular AP will not be larger than the AP that was generated with the
threshold level of depolarisation.
A supra-threshold electrical stimulus will generate a CAP that is larger than the CAP generated
by the threshold stimulus. Ultimately there will be a maximum electrical stimulus above which
further increases in the stimulus voltage will not cause an increase in the amplitude of the CAP.
3) Intracellular AP are all or none events under normal conditions. When the membrane potential
reaches threshold an AP is generated that is always the same amplitude.
4) As an intracellular AP travels down an axon it retains the same shape. This is not necessarily
true of a CAP traveling down a nerve in the experiments you are doing in the lab. The signal may
deteriorate if the recording electrodes are placed far away from stimulus electrode.
5) During an action potential, the membrane potential rises, becomes positive and then reverses
direction and decreases again until it falls below the resting potential (the undershoot).
6) During the period of repolarisation (when the membrane potential is returning from the peak
level to baseline) and the period of hyperpolarisation (when membrane potential has fallen below
resting potential), there is a refractory period in which it is either not possible to generate a
second action potential (absolute refractory period) or it is possible to generate a second AP but
with a greater than normal stimulus (relative refractory period).
Myelination and Saltatory Conduction