An endogenous motor program for sand crab uropods

A stereotyped pattern of spontaneous, rhythmic bursting in motoneurons of three principal uropod muscles in the sand carb Emerita analoga has been recorded from a deafferented chain of the four most-posterior abdominal ganglia. This endogenous motor program resembles the electromyogram pattern recorded from return-stroke and power-stroke muscles in swimming crabs in that (1) latencies of power-stroke bursts and burst periods are positively correlated with each other and (2) durations of power-stroke bursts are brief and nearly invarient. The endogenous program differs from the electromyogram pattern in having longer periods and return-stroke bursts which are brief and sporadic. The neural oscillator underlying the endogenous motor program, therefore, appears to drive the power stroke. Circumstantial evidence suggests that it may also inhibit return stroke motoneurons concurrently with excitation of the power-stroke excitor.