Muscles are made up of both contractile and non-contractile elements. Each muscle fiber is a long multinucleated cell that can reach a length of up to 50cm, as occurs for example in the Sartorius muscle. The muscle fibers are surrounded by a thin membrane called the endomysium or basement membrane, and are grouped into bundles covered by the perimysium. Finally, the entire muscular structure is enclosed by the epimysium, the most 'stiff' and strongest of the three membranes. The membranes constitute the non-contractile component of the muscle and cover the entire muscle length from tendon to tendon. In this way the entire contractile/non-contractile structure forms the muscle-tendon unit. The non-contractile structures therefore act as a skeleton for the muscle fibers, a skeleton that transmits the forces created by the contraction of a single point to the muscle-tendon junction (MTJ). An additional membrane, the sarcolemma, surrounds the individual muscle cells. The sarcolemma is important and is electrically conductive; it has within it the sarcoplasm containing reserves of 'fuel', i.e. glycogen and enzymes important for muscle contraction. Inside the sarcoplasm there is a very intricate membrane, the sarcoplasmic reticulum which contains transverse tubules, each of which ends on the muscle cell surface with a lateral sac for the transmission of calcium into the muscle to trigger the contraction process. The sarcolemma contains two important molecules, integrin at the end of the MTJ, dystrophin, or rather a dystrophin-glycoprotein complex, concentrated mainly in the MTJ but also present in the sarcolemma. These molecules give the sarcomere its tensile strength, the MTJ is able to tolerate forces of 1000kg during muscular effort.
Looking more closely at the muscle fibers, you may notice lighter stripes alternating with darker stripes. These stripes correspond to different muscle proteins. The light areas are composed of actin filaments that wrap around the dystrophin at the MTJ, while the dark areas correspond to more rigid myosin filaments. Myosin has projections called crossbridges that come out of it like anchors, very similar to those of ships and the movement of one muscle protein on another forms the muscular sliding that defines muscle contraction
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