Figure 8.

Diagram of potential consequences of PD axon peripheral spike initiation for muscle contraction. A, In a muscle with slow contraction dynamics the tension summates at normal pyloric cycle frequencies. At steady state, phasic contractions ride on top of a tonic baseline tension. The relative amplitudes of the tonic and phasic responses are dependent on the cycle frequency of the bursting input. Modified from Morris and Hooper (1998) and Hooper and Weaver (2000). B, As the rhythmic input decreases in frequency the phasic contractions increase and the tonic contraction decreases. C, If peripheral spike initiation increases with lowered cycle frequencies, the tonic spiking during the interburst intervals provides a means to keep tonic muscle tension at a comparable level, whereas the timing of bursting input is changed.