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Muscle Contraction We have 400+ muscles in the body which comprise 40-50% of the total body weight There are three types of muscle: Smooth Cardiac Skeletal Skeletal muscles functions are: Force generation for movement Force generation for postural support Heat production during periods of cold stress Skeletal Muscle Structure Skeletal muscles are wrapped in a fibrous layer called epimysium Each Fascile (bundle of fibres) is wrapped in perimysium Each muscle fibre is wrapped in endomysium • These fibrous layers provide support for nerves and vessels and maintain elasticity in the muscle • They also merge to form the tendons which connect the muscle to the bone Skeletal muscles fibres (cells) are long, striated and multinucleated. They are formed of myofibrils, mitochondria and other organelles in the sarcoplasm Each cell is enveloped in a cell membrane called the sarcolemma Inside a Muscle Fiber/Myofibril (a single cell) Cell membrane = Sarcolemma Cytoplasm = sarcoplasm Myofibrils= many myofilaments made up of proteins Actin and Myosin From outside in: Muscle belly (bundle of fascicles) -> Fascicle (bundle of fibers) -> Muscle Fiber (also called myofibril - a single cell)-> Myofilament (made of actin and myosin) are divided into Sarcomeres *see handout pg.44 and “The Structure of the Skeletal Muscle” Sliding Filament Theory Muscle contraction occurs because of the interaction between actin and myosin in the myofibrils. Simultaneous shortening of adjacent sarcomeres results in a contraction of the entire myofibril. Myosin cross-bridges connect the actin to the myosin These cross-bridges draw the actin together, toward the centre of the sarcomere. They are like ‘extending’ arms from myosin which attach on the actin filaments Z Band are plates that hold the actin filaments During contraction, the distance between the Z bands gets smaller (actin and myosin filaments DO NOT shorten) *See handout p14 What happens when the muscle gets stimulated? An simplified overview: Nerve impulse -> sarcolemma -> sarcoplasmic reticulum which discharges Calcium ions -> triggers myosin cross-bridges to pull actin -> causes Z bands to move = muscle shortening (contraction!!! :) ATP (adenosine triphosphate) is required for this process (provides energy)
