stimulus. In Fig. 6 the action potential amplitude increases as a function of the stimulus intensity. The key to understanding this apparent paradox is that we are now dealing with a nerve bundle that contains many different axons. The individual axons in the nerve have different diameters and therefore different thresholds to extracellularly applied stimulating currents. As a result, when the stimulus intensity is increased, the first axon to initiate an action potential is the nerve axon that has the lowest threshold. That action potential is then sensed by the recording electrodes and displayed on the recording device. As the stimulus intensity is increased further, more axons are brought to threshold so that there are multiple action potentials propagating along the nerve bundle, and these individual action potentials will each make a contribution to the signal sensed by the recording electrodes. Eventually, a stimulus intensity is reached that brings all the axons above threshold so that further increases in the stimulus intensity initiate no additional action potentials, and the size of the extra-cellularly recorded action potential remains at its peak value. The action potential that is recorded with extracellular electrodes from a nerve bundle is known as a compound action potential (compound because it is a summation of contributions from the individual action potentials in the axons that compose the nerve bundle).
One additional variant on this theme is illustrated in Fig. 7. This is a case in which the stimulating and recording electrodes are separated by a considerable distance. An interesting observation is that the potential change recorded can be quite different from the shape of the potential changes seen in Figs. 5 and 6. Previously, there was a relatively smooth rise, fall, and then recovery—a diphasic action potential. In this new case, the action potential has multiple peaks (in some cases, there can be three or more different peaks). What is the origin of these different peaks? The important point is that a nerve bundle may contain many different axons, many of which have different diameters. In addition, some of these axons may be myelinated and others unmyelinated. As a result, action potentials that are initiated in these axons will have different propagation velocities. The action potentials will reach the recording electrodes at different times so the different peaks reflect differences in the time of arrival of action potentials. The situation is somewhat analogous to a fast runner
FIGURE 7 Recording the compound action potential at a point distant from the stimulus site. Multiple peaks can be observed due to the contributions of fibers with different conduction velocities.
and a slow runner in a race. Initially, they are both at the starting line, but over a period of time the faster runner outdistances the slower runner and reaches the finish line sooner. The stimulating electrodes are analogous to the starting line, whereas the recording electrodes are analogous to the finish line.
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