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Lecture 12: Molecular Basis of Skeletal Muscle Contraction Structure of the Skeletal Muscle: a)A whole muscle is composed of hundreds to thousands of muscle cells, which run longitudinally along the entire length of the muscle. Imagine holding a hundred strands of spaghetti in your hand. Each spaghetti strand represents a muscle cell running along the entire in a longitudinal direction. These muscle cells, which are also referred to as muscle fibers, are wrapped by connective tissue known as the endomycium. Multiple muscle fibers are packed together as a muscle fascicle. There are multiple muscle fascicles in a whole muscle. Each muscle fascicle is wrapped by a sheath of connective tissue known as a perimysium. An entire skeletal muscle, from tendon to tendon, is wrapped by a fibrous sheath known as the epimysium. This connective tissue protects the muscle during contraction. Muscle tissues are supplied with nerve fibers that carry messages to and from the central nervous system (brain and spinal cord) and blood vessels that supply the tissue with oxygen and nutrients and carry away waste products. b)As mentioned above, skeletal muscle appears striated when viewed under a microscope. These striations are due to the molecular organization of thick and thin myofilamentswithin structural units known as sarcomeres. c)The thick myofilamentsare made up of multiple myosin molecules organized together. Each myosin molecule is a consists of two identical subunits wrapped around each other at the alpha-helical ends of the protein and with the globular portion of each subunit protruding outwards. The butt ends of a pair of myosin molecules are attached together such that the globular ends of these pair of myosin molecules are positioned far apart from one another. We will talk about how these globular heads are able to swing back and forth,