Fast and slow myosin in developing muscle fibres

Abstract

Slow and fast isoenzymes of chicken myosin coexist in all the fibers of a fast-twitch mammalian rat muscle during early development. They later become segregated into different populations of fibers. Slow myosin is most abundant when the speed of contraction of the muscle is slow and the fibers are multiply innervated; its synthesis in the majority of the fibers seems to be switched off when the speed of contraction increases and the fibers become innervated by single motoneurons.