Collateral sprouting of insect motorneurons.

The metathoracic extensor tibiae muscle of the cricket, Teleogryllus oceanicus, is innervated by two excitatory axons: a fast axon, which initiates large twitches to single stimuli, and a slow axon, which evokes minute twitches to single stimuli, but which, through facilitation and summation, evokes readily measurable tension to repetitive stimulation. The fast axon and the slow axon leave the metathoracic ganglia in different nerve roots, the fast axon through nerve 5 and the slow axon through nerve 3. The fast axon innervates muscle fibers in the middle of the extensor tibiae, and the slow axon innervates muscle fibers at the proximal and distal ends of the muscle. A central region of muscle fibers is innervated by only the fast axon. This region is flanked on either side by dually innervated fibers, fibers that receive both the fast and the slow axons. Fibers with only slow axon innervation are restricted to a wedge-shaped patch in the proximal extensor tibiae and a larger region in the most distal portion of the muscle. Sectioning nerve 5 containing the fast axon, or nerve 3 containing the slow axon, partially denervates the extensor tibiae. Functional transmission by the fast axon fails 7-10 days after nerve section. The innervation field of the intact motorneuron expands in a partially denervated muscle. The linear expansion rate of the slow axon field is about 20-40 micrometer per day. The enlarged slow field does not regress when axons regenerate to the muscle through nerve 5. The progressive expansion of the slow innervation field suggests that the expansion is due to collateral sprouting of slow axon terminals.