Abstract
In a previous paper 1 a range of latencies was postulated in the response of nerve fibers to short induction shocks. Using a 5-panel amplifier in conjunction with the von Ardenne cathode ray oscillograph, permitting the photographic recording of input potential changes significant to one microvolt and of time to 0.0lσ, we are now able to secure by an inertialess method records of the potential changes in nerve resulting from the activity of a single fiber.
The monophasic axon spike varies in amplitude from 10 to 300 microvolts, depending largely on the mass of inactive shunting tissue. In the a range the rising phase at 16°C. varies in different experiments from 0.3 to 0.7σ. The falling phase may be as short as l.0σ. The β spike seems to have comparable magnitudes, although statistically they may be a trifle longer. Spikes of B fibers in a gray ramus had a crest time, at 16°C, of 1.4σ Shapes and time functions of the axon spikes vary considerably in different experiments. The most important modifying factors seem to be the condition of the reacting fiber, the depth of tissue between it and the lead electrode, branches or adherent material between leads, potentials from remote active fibers, extraneous disturbances, the diphasic artifact, and the type of fiber most favorably placed in relation to the stimulating electrode.
The usual axon spike shows a short inception phase, most marked in nerves with sheaths, which we believe represents in part the potential change led through inactive tissue, ahead of the arrival of the excitatory process over the lead. Extremely small nerves may give axon spikes which show a vanishingly small period of inception; the rise then begins linearly, the crest is sharp, and the visible decline ends abruptly.
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