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The AP so far considered is the sum of contributions of many fibers of similar kind. The impulses in these fibers travel at velocities including a rather narrow range, so the AP is a relatively uniform peak. This is the A elevation, due to the Type A fibers. When the stimulus is increased beyond a certain point, the record becomes distorted.

The initial peak amplitude increases as more fibers are activated. The trailing currents seem to diminish but become prolonged. To understand this phenomenon, note that a cross-section of the nerve shows a variety of diameters of nerve fibers. The large fibers have a low internal longitudinal resistance compared to the smaller fibers. Therefore sufficient charge is easily deposited to produce the critical level of depolarization. The larger fibers will be the first excited as the stimulus is increased from a subthreshold level. For the same reason, propagation is easier (velocity is greater) for the larger than for the smaller fibers.

The large, relatively uniform initial peak of the AP comes from the population of the largest fibers. Stronger shocks bring in the slower axons, which make their contribution of leading current later than the faster ones.

Thus the upward deflections from the slower fibers may come during the downward phase of the faster APs from the Type A fibers. We can isolate the time course of a slower component, due to Type B fibers, by subtracting records evoked by low- and by high-intensity stimulation.

 
 
Figure 21. AP and fiber groups.

 
 
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