The cytoskeleton of skeletal muscle: is it affected by exercise? A brief review.

The myofibrillar cytoskeleton of skeletal muscle is made up of two distinct sets of filaments, the exosarcomeric cytoskeleton and the endosarcomeric cytoskeleton. The exosarcomeric cytoskeleton consists of intermediate filaments (IF) composed of the proteins desmin, vimentin, and synemin. The IF are arranged both longitudinally and transversely around the fiber. The longitudinal filaments run from Z-disc to Z-disc, enveloping the myofibril in order to serve as attachment sites for mitochondria, nuclei, and the sarcolemma, as well as limiting the sarcomere's extensibility. The transverse filaments link adjacent myofibrils at the Z-disc and are responsible for the fibril's axial register, and thus the striated appearance of muscle. The endosarcomeric cytoskeleton acts as a third filament system that coexists with actin and myosin within the sarcomere. This system is believed to be extensible and is made up of the giant proteins, titin and nebulin. Titin is believed to be responsible for resting muscle elasticity, as well as the central position of myosin in the sarcomere. Nebulin's role is proposed to be the maintenance of actin's lattice array. Following various types of intense exercise, pathological changes in muscle morphology have been documented. These include Z-disc streaming, sarcomerogenesis, and decentralization of myosin filaments within the sarcomere. It is hypothesized that disruption of the transverse IF system may cause Z-disc streaming, whereas degradation of titin filaments may affect myosin's position in the sarcomere.