Diversity of Striated Muscle

A broad comparative survey has been made correlating ultrastructureof cross-striated fibers with contractile properties in bothinvertebrates and vertebrates. Most of the muscles were foundto be heterogeneous in fiber-composition as indicated by: lengthof sarcomere, extent of SR, number of invaginating tubules,numbers of mitochondria, etc. Z discs and M bands have markedlydifferent structures in different fibers. The general conceptof the "fibrillar" nature of striated muscle is challenged.It is suggested that following excitation the responses of individualsarcomeres, or parts of sarcomeres, are relatively independent.The possibility that all striated muscles contain a very thinelastic filament in parallel with actin and myosin, which mayalso be contractile, is raised.