  The system of modular neural circuits that controls crustacean swimmerets
drives a metachronal sequence of power-stroke and return-stroke movements that
propels the animal forward efficiently. These neural modules are synchronized
by an intersegmental coordinating circuit that imposes characteristic phase
differences between these modules. Using a semi-intact preparation that left
one swimmeret attached to an otherwise isolated crayfish central nervous system
of the crayfish, we investigated how the rhythmic activity of this system
responded to imposed movements. We recorded extracellularly from the Ps and Rs
nerves that innervated the attached limb and from coordinating axons that
encode efference copies of the periodic bursts in Ps and Rs axons.
Simultaneously we recorded from homologous nerves in more anterior and
posterior segments. Maintained retractions did not affect cycle period, but
promptly weakened Ps bursts, strengthened Rs bursts, and caused corresponding
changes in the strength and timing of efference copies in the coordinating
axons. These changes in strength and timing of these efference copies then
caused changes in the phase and duration, but not the strength, of Ps bursts in
modules controlling neighboring swimmerets. These changes were promptly
reversed when the limb was released. Each swimmeret is innervated by two
nonspiking stretch receptors, Nssrs, that depolarize when the limb is
retracted. Voltage-clamp of an Nssr changed the durations and strengths of
bursts in Ps and Rs axons innervating the same limb, and caused corresponding
changes in the efference copies of this motor output.
