A SEPARATE LOCAL PATTERN-GENERATING CIRCUIT CONTROLS THE MOVEMENTS OF EACH SWIMMERET IN CRAYFISH

1. Within an abdominal segment, the motor output from the segmental ganglion to the swimmerets consists of coordinated bursts of impulses in the separate pools of motor neurons innervating the left and right limbs. This coordinated motor pattern features alternating (out-of-phase) bursts of impulses in the power-stroke (PS) and return-stroke (RS) motor axons that innervate each swimmeret. PS bursts on both sides of each segment occur simultaneously (in-phase), and so RS bursts on both sides are also in-phase. 2. With all intersegmental connections interrupted, isolated abdominal ganglia were able to sustain the normal swimmeret motor pattern of alternating PS/RS activity that was bilaterally in-phase. 3. After an isolated ganglion was surgically bisected down the midline, the isolated hemiganglia that resulted could produce stable, coordinated alternation of PS and RS bursts. 4. The neuropeptide proctolin could induce rhythmic oscillations of membrane potential in swimmeret neurons when spiking was blocked by tetrodotoxin (TTX). For neurons within the same hemiganglion, these oscillations retained the same phase relations they displayed in controls, but the oscillations of neurons in different hemiganglia became uncoordinated. 5. Synaptic transmission between swimmeret neurons in the same hemiganglion persisted in the presence of TTX. Swimmeret interneurons that could activate the pattern-generating circuitry under control conditions could induce membrane-potential oscillations in swimmeret neurons of the same hemiganglion when TTX was present. 6. We conclude that a separate hemisegmental pattern-generating circuit controls the rhythmic PS and RS movements of each swimmeret. Each circuit is located in the same hemiganglion as the population of motor neurons that innervates the local swimmeret. Graded transmission is sufficient to coordinate the timing of oscillatory activity within the hemisegmental circuitry. These hemisegmental circuits are coupled by intersegmental and bilateral coordinating pathways that are dependent on sodium action potentials for their operation.