The scorpion walking leg motor system: Muscle fine structure

1. 1. The fine structure of 7 leg muscles in the scorpion Paruroctonus mesaensis was studied. These muscles control the 3 leg joints most active during walking. 2. 2. The muscle fibers are small (15–30 μm diameter), and closely packed within each muscle. The fibers are tubular, consisting of a central region of nuclear material surrounded by radially arranged myofibrils, which occupy about 65% of the cross-sectional area. 3. 3. For all muscle fibers, the sarcomere lengths are relatively short, from 2 to 5 μm with a mean of about 3 μm (Fig. 5). Sarcomere lengths are relatively uniform, both within and between the 7 leg muscles studied (Table 1). 4. 4. The organization of the sarcoplasmic reticulum (SR) and transverse tubular system (TTS), the numbers and distribution of mitochondria, and the thin-thick filament ratios are similar for all 7 muscles. 5. 5. There is no structural evidence of specialized fast and slow muscle fibers in the scorpion walking leg muscles. This is in contrast to the better described insect and crustacean systems.