Skeletal muscle part 1

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University of Toronto Scarborough
Biological Sciences
Stephen Reid

Skeletal Muscle Lecture 8 Structure of Skeletal Muscle Bands of skeletal muscle are composed of bundles called fasciles. Each fascicle is surrounded by connective tissue and contains numerous muscle fibres (cells). Each muscle cell runs the entire length of the muscle and contains many nuclei. Muscle cells, in turn, are composed of bundles of myofibrils. Myofibrils are bundles of contractile filaments (actin and myosin) plus other components. The plasma membrane of a muscle fibre (cell) is called the sarcolemma. The sarcoplasmic reticulum is an endoplasmic reticulum-like compartment that contains stores of intracellular calcium. T-tubules penetrate the sarcoplasmic reticulum; connecting the sarcolemma to the sarcoplasmic reticulum. T-tubules are important in conducting waves of depolarisation from the sarcolemma to the sarcoplasmic reticulum. Longitudinal and cross sections of a myofibril illustrate specific patterns that have helped to elucidate the structure of skeletal muscle. In cross section, you see an orderly arrangement of large circular structures each surrounded by six smaller circular structures. The larger circular structures represent myosin molecules (thick filaments) while the smaller circular structures represent actin molecules (thin filaments). There are globular heads on the thick filaments (myosin) that form cross-bridges with the thin filaments (actin molecules). In a longitudinal section, you various lines, bands and zones. The lines, bands and zones illustrate areas of overlap (or lack of overlap) between the thick and thin filaments or (in the case of the Z-line) proteins. The sarcomere is the basic unit of muscle structure that repeats itself along the length of the muscle from Z-line to Z-line. The Thin Filament The thin filaments consist of actin molecules. Actin monomers (G-actin) form double-helical actin strands (i.e., two actin strands, composed of monomers, wound around each other). Each actin monomer also has a myosin-binding site. There are several regulatory proteins associated with the thin filaments. Tropomyosin is a long, thin fibrous protein that extends over many actin monomers and blocks the myosin binding site (at least under resting conditions). Troponin, the second regulatory protein associated with the thin filaments, binds actin, tropomyosin and calcium. Its role is to assist in the removal of tropomyosin from the myosin-binding sites on actin. The Thick Filament
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