Non-transmitting neuromuscular junctions during an early stage of end-plate reinnervation.

1. Electrophysiological studies were made on regenerating motor end-plates in frog cutaneous pectoris muscle after crushing the motor nerve.2. The pattern of degenerative and regenerative changes is similar to that already described for the frog sartorius, although it occurs more quickly in the cutaneous pectoris. Resumption of synaptic transmission first occurs after 9 days, in muscle fibres closest to the nerve crush. During the subsequent 12 days synaptic contact is progressively re-established across the muscle.3. During the period of regeneration muscle fibres can be found which have been reinnervated, but which do not produce an end-plate potential in response to stimulation of the nerve trunk. This is referred to as the ;non-transmitting' stage of regeneration.4. Increases in extracellular potassium, addition of lanthanum to the bath and focal extracellular stimulation all indicate that the axon terminals at non-transmitting junctions do have a releasable supply of neurotransmitter. Focal stimulation occasionally sets up active responses in these nerve terminals.5. It is concluded that this ;non-transmitting' stage results from failure of action potential propagation in the regenerating neurone proximal to the end-plate region.