SKELETAL MUSCLE 1 Structure and function Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton Th.
SKELETAL MUSCLE Structure and function Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton The muscle cells of skeletal muscles are much longer than in the other types of muscle tissue, and are often known as muscle fibers The muscle tissue of a skeletal muscle is striated – having a striped appearance due to the arrangement of the sarcomeres Skeletal muscles are voluntary muscles, meaning you control how and when they move and work Nerves in your somatic nervous system send signals to make them function If you reach for a book on a shelf, you’re using skeletal muscles in your neck, arm and shoulder Function The skeletal muscles are a vital part of your musculoskeletal system They serve a variety of functions, including: - Chewing and swallowing, which are the first parts of digestion - Expanding and contracting your chest cavity so you can inhale and exhale at will - Maintaining body posture - Moving the bones in different parts of your body - Protecting joints and holding them in place Skeletal Muscle Structure A skeletal muscle contains multiple fascicles – bundles of muscle fibers Each individual fiber, and each muscle is surrounded by a type of connective tissue layer of fascia Muscle fibers are formed from the fusion of developmental myoblasts in a process known as myogenesis resulting in long multinucleated cells In these cells the nuclei termed myonuclei are located along the inside of the cell membrane Muscle fibers also have multiple mitochondria to meet energy needs Muscle fibers are in turn composed of myofibrils The myofibrils are composed of actin and myosin filaments called myofilaments, repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fiber necessary for muscle contraction Muscles are predominantly powered by the oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used, particularly by fast twitch fibers These chemical reactions produce adenosine triphosphate (ATP) molecules that are used to power the movement of the myosin heads Types of muscle contraction There are two primary types of muscle contractions They are isotonic contraction and isometric contraction The types of contractions depend on the changes in the length and tension of the muscle fibres at the time of its contraction Isotonic contraction (iso – same, tonweight/resistance) In isotonic contraction the length of the muscle changes but the tension remains constant Here, the force produced is unchanged Example: lifting dumb bells and weightlifting Isometric contraction (iso – same, metric – distance) In isometric contraction the length of the muscle does not change but the tension of the muscle changes Here, the force produced is changed Example: pushing against a wall, holding a heavy bag Types of Skeletal Muscle Fibres The muscle fibres can be classified on the basis of their rate of shortening, either fast or slow and the way in which they produce the ATP needed for contraction, either oxidative or glycolytic Fibres containing myosin with high ATPase activity are classified as fast fibres and with lower ATPase activity are classified as slow fibres Fibres that contain numerous mitochondria and have a high capacity for oxidative phosphorylation are classified as oxidative fibres Such fibres depend on blood flow to deliver oxygen and nutrients to the muscles The oxidative fibres are termed as red muscle fibres Fibres that contain few mitochondria but possess a high concentration of glycolytic enzymes and large Skeletal muscle fibres are further classified into three types based on the above classification They are slow – oxidative fibres, fast – oxidative fibres and fast – glycolytic fibres Slow – Oxidative Fibres Have low rates of myosin ATP hydrolysis but have the ability to make large amounts of ATP These fibres are used for prolonged, regular activity such as long distance swimming Long – distance runners have a high proportion of these fibres in their leg muscles Fast – Oxidative Fibres Have high myosin ATPase activity and can make large amounts of ATP They are particularly suited for rapid actions Fast – Glycolytic Fibres Have myosin ATPase activity but cannot make as much ATP as oxidative fires, because their source of ATP is glycolysis These fibres are best suited for rapid, intense actions, such as short sprint at maximum speed Isometric: A muscular contraction in which the length of the muscle does not change Isotonic: A muscular contraction in which the length of the muscle changes Eccentric: An isotonic contraction where the muscle lengthens Concentric: An isotonic contraction where the muscle shortens Types of Contractions There are three types of muscle contraction: concentric, isometric, and eccentric Isometric: Of or involving muscular contraction against resistance in which the length of the muscle remains the same Isotonic: Of or involving muscular contraction against resistance in which the length of the muscle changes The process of muscular contraction occurs over a number of key steps, including: Depolarisation and calcium ion release Actin and myosin cross-bridge formation Sliding mechanism of actin and myosin filaments Sarcomere shortening (muscle contraction) Isotonic contractions – These occur when a muscle contracts and changes length and there are two types: Isotonic concentric contraction – This involves the muscle shortening Isotonic eccentric contraction – This involves the muscle lengthening whilst it is under tension The first step in the process of contraction is for Ca ++ to bind to troponin so that tropomyosin can slide away from the binding sites on the actin strands This allows the myosin heads to bind to these exposed binding sites and form cross-bridges Abstract Skeletal muscle is one of the most dynamic and plastic tissues of the human body In humans, skeletal muscle comprises approximately 40% of total body weight and contains 50-75% of all body proteins Skeletal muscles only pull in one direction For this reason they always come in pairs When one muscle in a pair contracts, to bend a joint for example, its counterpart then contracts and pulls in the opposite direction to straighten the joint out again This is a table of skeletal muscles of the human anatomy There are around 650 skeletal muscles within the typical human body Almost every muscle constitutes one part of a pair of identical bilateral muscles, found on both sides, resulting in approximately 320 pairs of muscles, as presented in this article