Plasticity in the cockroach neuromuscular system

The retrograde transport of wheat germ agglutinin-conjugated horseradish peroxidase extracellularly injected into a leg muscle was used to identify the regenerating cockroach motor neurons that have grown an axonal branch into that muscle. At least 66% of the animals with crushed nerve roots eventually reform the original innervation pattern of this muscle with no mistakes. In spite of this apparent specificity the cockroach neuromuscular system can express plasticity as evidenced by the correction of mistakes made at early stages of regeneration. These mistakes are corrected through elimination during the time interval between 40 and 60 days after nerve crush. In addition, when the distal segments of the leg are removed, thus depriving some motor neurons of their normal target muscles, many of them form stable inappropriate axonal branches in denervated as well as fully innervated muscles. These observations are discussed in terms of possible mechanisms responsible for the specificity of the cellular interactions and in terms of their relevance to understanding the development of vertebrate neuromuscular systems.