Question

How are thick and thin filaments arranged

Answer 1

The fine structure and arrangement of thick and thin filaments in the main closer muscle were investigated for pincer, snapper and transforming claws of Alpheus armillatus. The snapper main closer muscle is composed of a uniform population of tonic-type fibers with long sarcomeres and a thin-to-thick filament ratio of 7 to 1. In contrast, the pincer main closer muscle is made up of two populations of fibers. Phasic type fibers have large diameters, short sarcomeres and a thin-to-thick filament ratio of 3 to 1. Intermediate-type fibers have smaller diameters, intermediate sarcomere lengths and a thin-to-thick filament ratio of between 5 and 6 to 1. In the pincer the thick filaments have a hollow appearance. During the transformation of a pincer to a snapper claw, the main closer muscle fibers develop tonic-type characteristics. The sarcomeres increase in length, and the ratio of thin-to-thick filaments increases to 7 to 1. Furthermore, the thick filaments increase in both length and diameter, and develop a more uniform structure. These findings suggest that changes in myosin packing accompany transformation, with predictable consequences in both thick filament diameter and thin filament/thick filament ratio.