Selective regeneration of the neuromuscular connections in the pteropod mollusc Clione limacina

In the pteropod mollusc Clione limacina, two different groups of motoneuron s innervate two physiologically identical wing muscles (dorsal and ventral) . When motoneuron axons are crushed in the nerve of whole animals regenerat ion starts. In its course motoneurons initially project to both the correct and incorrect muscles. Then incorrect connections and neurites are elimina ted and the original innervation is restored. Here we investigated neuromus cular regeneration when one of the muscles was removed or the muscle size w as reduced. Motoneurons formed both correct and incorrect connections not o nly in vitro when the pedal ganglion was attached to only one of two wing m uscles, but also in whole-animal 'no choice' experiments when only one musc le was available for reinnervation. In these experiments incorrect connecti ons were stable and were not eliminated at the later stages, as happened in experiments in which both muscles were accessible. In whole-animal experim ents with reduced size of the muscles, a normal pattern of regeneration was conserved although not all incorrect connections were eliminated. Thus, in the course of regeneration: (i) locomotor motoneurons make connections wit h both correct and incorrect muscles; (ii) if for some group of motoneurons the correct targets are unavailable, the incorrect connections survive and become stable; (iii) if both groups of motoneurons have a choice between t he correct and incorrect targets, initial mixed innervation is replaced by purely correct innervation; (iv) elimination of incorrect synapses could be a result of the competition between correct and incorrect synapses of the same neuron.