Electromechanical coupling in tubular muscle fibers. I. The organization of tubular muscle fibers in the scorpion Leiurus quinquestriatus.

The tubular fibers of the claw-closer muscle of the scorpion have a central core containing nuclei and mitochondria. The myofibrils have the shape of thin lamellae (1 micro) extending radially from the core to the surface membrane (20 micro). The thick myofilaments are organized in a hexagonal array with orbits of 10-13 thin myofilaments. The ratio of thick-to-thin filaments is 1:5. Transverse tubular system (TS) openings are located between lamellated myofibrils. In each sarcomere two TS's are found, one on each side of the H band. The TS is composed of a transverse tubule and tubular pockets (TP). The TP's form diadic contact with the terminal cisternae of the sarcoplasmic reticulum. The TS can be traced from the cell membrane down to the cell core. The surface area of the TS was calculated to be six times that of the outer surface membrane.