Serotonin-induced spike narrowing in a locomotor pattern generator permitsincreases in cycle frequency during accelerations

During serotonin-induced swim acceleration in the pteropod mollusk Clione l imacina, interneurons of the central pattern generator (CPG) exhibit signif icant action potential narrowing. Spike narrowing is apparently necessary f or increases in cycle frequency during swim acceleration because, in the ab sence of narrowing, the combined duration of the spike and the inhibitory p ostsynaptic potential (IPSP) of a single cycle is greater than the availabl e cycle duration. Spike narrowing could negatively influence synaptic effic acy in all interneuron connections inhibitory connections between the two g roups of antagonistic CPG interneurons as well as the interneuron-to-motone uron connections. Thus compensatory mechanisms must exist to produce the ov erall excitatory behavioral change of swim acceleration. Such mechanisms in clude 1) a baseline depolarization of interneurons, which brings them close r to spike threshold, 2) enhancement of their postinhibitory rebound, and 3 ) direct modulation of swim motoneurons and muscles, all through inputs fro m serotonergic modulatory neurons.