Fig. 1.

Blocking chemical synaptic transmission also blocked expression of the normal swimmeret motor pattern.A, A diagram that illustrates the last thoracic ganglion (T5) and the six abdominal ganglia (A1–A6) of the isolated ventral nerve cord. Anterior is at the left. The squaresurrounding A4 shows the Vaseline well used to apply low Ca²⁺ saline just to that ganglion. Impulses in motor axons that projected to the pair of swimmerets innervated by A4 were recorded bilaterally with extracellular electrodes on the anterior (RS4L, RS4R) and posterior (PS4L, PS4R) branches of the swimmeret nerves. B, Three cycles of normal activity in A4 power-stroke (PS4L, PS4R) and return-stroke (RS4L, RS4R) motor axons recorded from branches of the left and right swimmeret nerves. Bathing the preparation in low Ca²⁺–high Mg²⁺ saline (Block A4) stopped all firing in these motor units. Restoring the normal extracellular concentrations of Ca²⁺ and Mg²⁺(Recovery) also restored coordinated firing of these units.