Fig. 2. Facilitation and summation affect the electrical response of the gm6 muscle to stimulation of LG. (A) Left: intracellular recording of gm6 muscle during stimulation of the LG motor neuron. Average of five sweeps with two-pulse stimulation. Interstimulus interval: 200 ms. Arrows show the times of stimulation. Right: average of five sweeps with two-pulse stimulation. The arrow points at the first EJP in each sweep. Interstimulus intervals: 100 ms, 200 ms, 400 ms, 800 ms, 1600 ms, 3200 ms. (B) Exponential decay function fit of the facilitation index. Average of nine animals. Arrow indicates maximum possible facilitation (as would occur at an interstimulus interval of 0; dotted lines, see Materials and methods section). ISI, interstimulus interval. F, facilitation index. (C) EJPs facilitate and summate with repetitive stimuli. 10 consecutive stimuli were used, followed by a test pulse. Left, 5 Hz stimuli; middle, 10 Hz stimuli; right, 20 Hz stimuli. (D) 5 Hz stimulation of LG. 10 stimuli plus a single test pulse with a delay of 500 ms were used. Left: original recording of 11 EJPs with different amplitudes. Middle: overlay of all EJPs. Right: Overlay of all EJPs after normalizing them the to peak depolarization of each EJP. (E) Left: original recording of a 20 Hz stimulus train. Circles indicate calculated baseline potential (white) and EJP amplitude (black). Gray circles mark the peak depolarization of the EJP. Right: the contribution of EJP amplitude (black circles) and baseline potential (white circles) to the peak amplitude (gray circles) of each EJP are given. (F–H) EJP amplitudes increased during 5 Hz (F), 10 Hz (G) and 20 Hz (H) train stimulation. Plot of 20 animals (white circles) plus average. Please note that averages were shifted slightly to the left for clarity. **Significantly different from first EJP, P<0.01. (I) Plot of averaged EJP amplitudes (N=20 for 5 Hz, 10 Hz and 20 Hz, N=10 for 30 Hz) over the time in the stimulus train. With higher stimulus frequencies, higher EJP amplitudes were reached in a shorter time. **Significantly different from stimulation with lower stimulus frequency, P<0.01. *Significantly different from stimulation with lower stimulus frequency, P<0.05.