Figure 2. Intrinsic variability in the leech nervous system. A. A record of continuous electrical activity from one peripheral nerve of an isolated leech nervous system. The observed activity varies with time despite the absence of variations in input, and includes periods of activity known to be associated with well defined output behaviors such as swimming (the bursting activity beginning at the arrow). B. Variability in response to a fixed stimulus. Shown are five trials on each of which the same electrical stimulus (line below bottom trace) was applied to a nerve other than the one being recorded from. Notice that the same stimulus sometimes, but not always (B4) causes swimming. Even on trials when swimming does result, there is substantial variability in the pattern and duration of activity prior to the initiation of swimming , as well as in the swimming activity itself (B1,2,3,5). C. A stimulus response curve for the isolated leech nervous system. For each of a series of stimulus voltages (abscissa) the figure shows the percentage of trials (ordinate) using that voltage on which a swimming episode resulted. Notice that, over a wide voltage range, the same stimulus sometimes does and sometimes does not trigger a swim. D. A stimulus response curve for the isolated leech nervous system in the absence of the most head ganglia. Conventions as in C. Notice that there is now a sharp transition between stimulus voltages which reliably do and do not trigger swimming episodes. Unpublished studies by Peter Brodfuehrer and Paul Grobstein.
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