Structure of Limulus striated muscle. The contractile apparatus at various sarcomere lengths.
The musculature of the telson of Limulus polyphemus L. consists of three dorsal muscles: the medial and lateral telson levators and the telson abductor, and one large ventral muscle; the telson depressor, which has three major divisions: the dorsal, medioventral, and lateroventral heads. The telson muscles are composed of one type of striated muscle fiber, which has irregularly shaped myofibrils. The sarcomeres are long, with discrete A and I and discontinuous Z bands. M lines are not present. H zones can be identified easily, only in thick (1.0 microm) longitudinal sections or thin cross sections. In lengthened fibers, the Z bands are irregular and the A bands appear very long due to misalignment of constituent thick filaments. As the sarcomeres shorten, the Z lines straighten somewhat and the thick filaments become more aligned within the A band, leading to apparent decrease in A band length. Further A band shortening, seen at sarcomere lengths below 7.4 microm may be a function of conformational changes of the thick filaments, possibly brought about by alterations in the ordering of their paramyosin cores.
D000818:Animals; D003386:Brachyura / Q000033:anatomy & histology*; D008854:Microscopy, Electron; D009124:Muscle Proteins; D009132:Muscles / Q000033:anatomy & histology / Q000166:cytology*; D009210:Myofibrils; D011487:Protein Conformation
D009124:Muscle Proteins