Lateral transmission of tension in frog myofibers: a myofibrillar network and transverse cytoskeletal connections are possible transmitters.

The extensibility of the sarcolemma of single myofibers can be reduced locally by leaving a segment covered by a sleeve of surrounding tissue composed of cut myofibers, blood vessels, and connective tissue, hereafter referred to as "the splint." Splinted fibers from frog semitendinosus muscle were used to study mechanical connections (transverse coupling) between myofibrillar components and sarcolemma. The transverse coupling is strong enough to insure a tight correlation between myofibril length and overlying sarcolemma length in both resting and activated fibers and to transmit nearly maximum isometric tension to the splint. Lateral transmission of active tension was demonstrated with a preparation which had the distal two-thirds of an intact fiber covered by a splint and the proximal third dissected clean. When the outer end of the splint was pinned down and only the distal tendon was held, tension generated in the splinted fiber was transmitted to, and recorded from, the splint. Parameters of isometric tension transmitted laterally were not significantly different from those of tension transmitted longitudinally. Myofibrils branch profusely and form a network that may act as a unitary force generator and transmitter. In splinted fibers its output is possibly picked up circumferentially and transmitted across the sarcolemma by a microfilament network. A cap of relatively inextensible sarcolemma "splints" myofiber ends. Resting tension is transmitted to and from the myofibrils by transverse coupling beyond the cap and the region of short sarcomere spacing it covers. Transverse cytoskeletal connections at Z and M regions are described. Immobilization of the sarcolemma allows study of myofibril-sarcolemma linkage in intact fibers. Both active and resting tension were transmitted laterally.