Dissociated rat sympathetic neurons and skeletal myotubes were grown in mass cultures and microcultures as described in the accompanying paper (C. A. Nurse, 1981, Develop. Biol. 88, 55–70). Excitatory synaptic interactions developed between neuron and neuron and between neuron and myotube. Electrical coupling occurred rarely. More often, the interaction was chemical and as shown in the accompanying paper, cholinergic. At the chemical neuronmyotube junctions, spontaneous miniature potentials (mejp's), sensitive to d-tubocurarine, occurred infrequently (1–20/min) and their discharge appeared random; their amplitude distribution was skew at all ages (up to ca. 4 weeks) even when the myotube was innervated by a single neuron in microculture. The evoked postsynaptic potentials (ejp's) in the myotubes were sensitive in conventional ways to the extracellular calcium (Cao) and magnesium (Mgo) concentrations, and several tests suggested that transmission was quantal. In a few cases where a single neuron innervated a myotube in microculture, the estimated mean quantal unit size (assuming “Poisson” release) was similar to the mode of the spontaneous mejp amplitude histogram, suggesting that many of the spontaneous units were similar to the evoked units. At several junctions the quantal content mo, estimated by the “failure” method, varied nonlinearly with Cao over the range 0.2–1.2 mM; data could be fitted by a power relation where the power ranged from 2.6 to 5.2.
