Heterogeneity of spider leg muscle: Histochemistry and electrophysiology of identified fibers in the claw levator

Summary

Our combined histochemical/electrophysiological examination of single, identified muscle fibers demonstrates surprising functional heterogeneity in the claw levator muscle (CL) of spider legs:

• 1. The muscle raises and retracts the claws and is also involved in tarsal depression (Fig. 1a). InCupiennius salei it is organized in 2 bilaterally asymmetrical bundles: a small ‘K-bundle’ with 37 short fibers and a relatively massive ‘L-bundle’ with 43 long fibers (Table 1).

• 2. Histochemical staining for relative succinic dehydrogenase activity, for relative myofibrillar (actomyosin) ATPase activity (including acid preincubation), and for glycogen content in muscle cells reveals 4 distinct fiber populations A, B, C, and D, each occurring in clusters (Figs. 2, 3). A-type fibers comprise the bulk of the claw levator and occur only in the L-bundle. B-fibers appear in both muscle bundles, while the occurrence of type C and D is restricted to the K-bundle (Table 1).

• 3. Two excitatory axons innervate the claw levator (Fig. 1 c): fast (FLE) and slow levator excitor (SLE). A-fibers are innervated by FLE alone; B and C-fibers receive innervation from both excitors; D-fibers receive excitatory innervation only by the SLE and are addtionally innervated by an inhibitory axon (Figs. 4, 5). Each fiber group differs with respect to the membrane time constantT and facilitation properties (Table 2).

• 4. Although detailed force measurements and biochemical analyses of isolated muscle fibers are not available, we discuss how each of the 4 distinct fiber types is possibly involved with the different functions required for various locomotory behaviors ofCupiennius salei.