The characteristics of the medial and lateral superficial extensor muscles (sem and sel) in the crayfish Orconectes limosus abdomen and their developmental and activity-dependent plasticity were studied. It was shown that both muscles are innervated by at least five excitatory and one inhibitory motor neuron in a nonuniform pattern. The muscles are composed of at least three different mATPase histochemistry-based fiber types that are all different from a fourth type in the uniform deep extensor muscles. sem and sel are composed of different ratios of these fiber types but do not show a constant fiber type pattern between segments and even between hemisegments. The three histochemically defined superficial extensor-fiber types have characteristic electrophysiological properties. The fiber types were shown to develop successively during the first postembryonic stages of development without a change in the number of muscle fibers. Based on histochemical ATPase staining after 21 days of chronic stimulation by means of an implantable, double-hook electrode, we show preliminary evidence that the fiber composition in the sem can switch from the presumably fast fiber type III to an intermediate type II. Repeated axotomy up to 53 days had no effect on the fiber type composition of the muscles.
Copyright 2002 Wiley-Liss, Inc.