Projection pattern and target selection of Clione limacina motoneurons sprouting within an intact environment

In the pteropod mollusc Clione limacina, two groups of locomotor motoneuron s, located in the pedal ganglion, innervate the dorsal and ventral muscle l ayers of the ipsilateral wing through the wing nerve. Separate branches of this nerve go either only to the dorsal muscle layer or only to the ventral one. In the present study, growth of novel neurites of the wing motoneuron s was induced by cutting the wing nerve. In addition, all other peripheral nerves and connectives of the pedal ganglion were cut, except for the pedal commissure to the contralateral pedal ganglion. Thus, the neurites were al lowed to grow only towards the contralateral pedal ganglion. We have found that the novel neurites, entering the contralateral pedal ganglion, were ca pable of growing everywhere inside the central nervous system (CNS) and int o any peripheral nerve. However, they preferred the wing nerve. This findin g suggests that the preference is caused by the guiding cues in the wing ne rve or the at;tractive influence of the wing muscles. Because the contralat eral pedal ganglion and nerves were left intact. the growth direction of th e new neurites could be determined only by factors permanently existing in the CNS, rather than induced by nerve injury or muscle denervation. Within the wing nerve, the neurites could not discriminate between the nerve branc hes going to the dorsal and ventral muscle layers. They formed synapses on muscles of both layers, despite the fact that the muscles were innervated b y their own motoneurons. J. Comp. Neurol. 423.220-226, 2000. (C) 2000 Wiley -Liss. Inc.