Fig. 4. (A–C) Synaptic activity of DBNi1-2 during motor activity. (A) Intracellular recording of DBNi1-2 (lower trace, hyperpolarised with -2nA) and extracellular recording of antennal motoneurons from N4B (upper trace) as a measure of antennal motor activity. Movements of the antenna were not registered. Large excitatory synaptic potentials (arrows) and action potentials (open arrows) are absent during a spontaneous motor neuron burst (arrowhead) and during respiratory-related bursts (asterisks). (B) During forced antennal movements, excitatory postsynaptic potential (EPSP) frequency was also reduced in the presence of high levels of antennal motor activity. Solid bars represent episodes of low synaptic activity in DBNi1-2 and the dashed bar represents episodes of high activity. (C) Box-and-whisker plot showing the median, the second and third quartiles and the fifth and ninety-fifth percentiles of normalised integrals of 176 spontaneous EPSPs plotted against normalised motor spike count in motor nerve N4B from four crickets (see Materials and methods). Large synaptic potentials did not occur in the presence of high motor spike counts. The integral values were binned into classes with a width of 0.1 relative motor spike count units. The maximum integral values decrease with increasing spike count (Spearman rank correlation coefficient r=-0.85, P<0.001). Numbers indicate numbers of samples in each class.