Fig. 4.

Repetition priming in the feeding circuit. Plotted is the firing frequency of a radula closer motor neuron during radula protraction (x-axis) and radula retraction (y-axis). Dashed lines mark clusters of ingestive and egestive activity. Cycles of activity were evoked with an intercycle interval of 30 s by using either CBI-2 to induce ingestive repetition priming (A) or the EN to induce egestive repetition priming (B). Insets are typical intracellular recordings from a radula closer motor neuron (RC; top trace) and extracellular recordings from the I2 nerve (bottom trace). Activity in the I2 nerve marks the protraction phase of the motor program. Note that in both A and B, the first cycle that was triggered was poorly articulated. The radula closer motor neuron fired at a relatively low frequency (cycle 1 in both A and B). With repeated input activation, however, program definition occurred. For example, with repeated stimulation of CBI-2, motor activity became clearly ingestive (e.g., the firing frequency of the radula closer motor neuron increased during retraction; A). With repeated stimulation of the EN, motor programs became egestive (e.g., the firing frequency of the radula closer motor neuron increased during protraction; B). [Data are replotted from Friedman et al. (2009) and Friedman and Weiss (2010).]