Reflex activity of regenerating frog spinal motoneurons.

The reflex activity of frog spinal motoneurons whose axons had been sectioned 7 days to 10 months previously was examined. Before the severed axons reinnervated muscle, reflex latency was prolonged, and reflex amplitude was depressed. Examination of input-output relations revealed the presence of a subset of axotomized motoneurons having supranormal excitability. This subset was not present in normal preparations or in preparations tested after reinnervation took place. Following reinnervation of muscle, reflex latency returned to preoperative levels, while amplitude recovery was typically more variable. Decreased conduction velocity of the severed ventral root fibers accounted for the alterations in reflex latency. These and other data form the basis for concluding that motoneurons, even within a particular preparation, show a wide range of responses to section of their axons and will provide a framework for interpretation of more selective intracellular experiments.