Increased motor neuron projection during development does not increase the number of neuromuscular synapses

We investigated in developing embryos whether the total number of neuromuscular synapses is determined by the muscle or by the number of innervating motor neurons. The superior oblique muscle of duck embryos was hyperinnervated by preventing the naturally occurring death of trochlear motor neurons using immunoglobulin G from patients with acquired myasthenia gravis. In spite of a significant increase in the number of motor neurons innervating the muscle, a corresponding increase in the number of neuromuscular synapses did not occur. These results suggest that the total number of synapses in a muscle is independent of the number of innervating motor neurons and that it is determined intrinsically by the muscle itself.