Sarcoplasmic reticulum of an unusually fast-acting crustacean muscle.
The fast-acting, synchronous "remotor" muscle of the lobster second antenna was examined by light and electron microscopy and was found to have a more profuse sarcoplasmic reticulum (SR) than any other muscle known. Myofibrils are widely separated from one another and occupy only about one-fourth of the volume of the muscle; most of the remaining volume is taken up by the SR, which resembles the smooth-surfaced reticulum of steroid-secreting cells. Dense granules (0.03-0.1 micro in diameter) are scattered through the reticulum. T-tubules penetrate into the fibers and form dyads along the A bands of myofibrils; however, ferritin-labeling experiments show that the volume of the T-system is very small compared with that of the SR. Myofibrils are approximately 0.5 micro x 1.0 micro in cross section and consist of thick filaments, which appear tubular except at the M region, and thin filaments, which are situated midway between neighboring thick filaments. The ratio of thin to thick filaments is 3:1. The extreme development of the SR in this muscle is discussed in relation to the exceedingly short duration of the contraction-relaxation cycle.
D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism; D003445:Crustacea; D066298:In Vitro Techniques; D008566:Membranes; D008854:Microscopy, Electron; D009119:Muscle Contraction; D009132:Muscles / Q000166:cytology* / Q000502:physiology; D009210:Myofibrils / Q000502:physiology; D013499:Surface Properties
D002118:Calcium