Adaptability of ultrastructure in the mammalian muscle

All the skeletal muscle fibres taken from an adult mammal do not look alike. The structural differences are a result of adaptations which allow gradations in mechanical output to be achieved. The anatomy is described and the amounts of the subcellular components are measured by stereological techniques from electron micrographs. A population of normal, adult fibres is classified by the Z-line width, by the amounts of the mitochondria, T-system and terminal cisternae (TC), and by the isoforms of contractile proteins present. Classification of fibres by some of these ultrastructural components gives clusters named fast-twitch and slow-twitch types, but classification by other components gives a continuum of overlapping properties. Transformation from the fast- to the slow-twitch type or vice versa follows a specific alteration in the use of the fibre. The mechanical demand on the fibre is modified by changing the frequency of stimulation in the nerve with an implanted electrode. The time course of the changes in subcellular composition in the fibre during adaptation is followed for many weeks. Changes in the membrane systems begin within hours and are complete in days. Changes in the contractile proteins and metabolic systems begin in days and are complete in weeks. During these transitional phases of adaptation the fibres have an unusual complement of components never seen in a normal adult fibre. Extreme alterations, such as myofibril disassembly or supranormal amounts of mitochondria also result during some adaptive transitions. The aberrant appearance in the transitional fibres may be a result of doing the required mechanical work with a less than optimal set of proteins. At the end of the fibre type transformation, the fibre ultrastructure is indistinguishable from normal.