4 5 13 Muscles 1 3rd Organ System composed of muscles blood vessels and nerves 2 3 types of muscles as organs a Skeletal muscle tissue i Elongated cells called skeletal muscle fiber ii Striations iii Multinucleate iv Voluntary control b Cardiac muscle tissue i Branched cells that form intercalated discs ii Striations iii Uninucleate iv Involuntary c Smooth muscle tissue i Spindle shaped cells ii No obvious striations iii Uninucleate iv Involuntary 3 Skeletal Muscle Tissue organized with CT membranes to form skeletal muscle a Structural Organization each skeletal muscle fiber is covered by the endomysium i A bundle of endomysium covered skeletal muscle fibers wrapped in the perimysium forms a fascicle 1 The arrangement of the fascicles in a skeletal muscle may be parallel pennate or convergent ii A bundle of perimysium covered fascicles wrapped in the epimysium forms the organ called skeletal muscle which is infiltrated by blood vessels and axon terminals from motor neurons b Skeletal muscles attach to bones in the skeletal system i Each skeletal muscle has 2 attachment sites 1 Origin 2 Insertion i Direct ii Indirect c When a skeletal muscle contracts it pulls the insertion towards the origin i The origin of a skeletal muscle is relatively stable when the skeletal muscle contracts d Attachment of skeletal muscle may be 1 The epimysium of the skeletal muscle attaches directly to the surface of the bone EX skeletal muscles attached to the cranial bones 1 Skeletal muscle attaches indirectly to the bone by tendon which extends from the epimysium extending from the skeletal muscle 2 Most skeletal muscles use this attachment a Advantages i Attachments by tendons occupy smaller surface area of the bones allowing for other skeletal muscles to attach to the same bone for efficient attachment of skeletal muscles ii Attachments by skeletal muscles act as levers when the skeletal muscles contract to produce movements by the body iii Tendons attaching skeletal muscles to surface of bones protect skeletal muscles from direct contact with the surface of the bone rough surface that can cause tearing damage to skeletal muscles e Physiology of skeletal muscle contraction is explained at the cellular level structure function of skeletal muscle cells skeletal muscle fibers i Each skeletal muscle fiber has 1 Plasma membrane called sarcolemma 2 Infoldings invaginations of the sarcolemma are called transverse tubules T tubules 3 Cytoplasm called sarcoplasm which contains several nuclei 4 All the cytoplasmic organelles such as mitochondria smooth ER called sarcoplasmic reticular SR which is a specialized smooth ER that stores releases calcium ions 5 Inclusions called glycosomes which is stored glycogen 6 Red pigment called myoglobin in sarcoplasm that binds releases O2 to support aerobic respiration occurring in the mitochondria 7 T Tubules are located between the 2 terminal cisternae which are the expanded ends of the smooth ER ii Structure composed of terminal cisterna T tubule terminal cisterna triad iii Skeletal muscle fibers have rod like structures called myofibrils 1 80 of the volume of skeletal muscles are myofibrils 2 Myofibrils run the entire length of the skeletal muscle fibers a Skeletal muscle fibers run the entire length of a skeletal muscle 3 Each myofibril is composed of thinner rod like structures called myofilaments 2 types thick 16 nm in diameter and thin filaments 8 nm in diameter a Thick filaments i Composed of protein called myosin 1 Myosin structure rod like tail that ends into globular heads called the myosin globular heads ii Composed of 300 molecules of myosin the tails form the core of these filaments with globular heads exposed Myosin globular head binding sites iii 1 For Actin 2 For ATP iv Globular heads contain the enzyme ATPase 1 Hydrolyzes ATP to ADP b Thin Filaments i Composed of 3 different proteins 1 1 structural protein called actin a Forms the structural framework b Has a binding site for the myosin globular head 2 2 regulatory proteins a Troponin i Consists of 3 subunits troponin T TnT inhibitory troponin TnI and troponin C TnC ii TnT binds to tropomyosin iii TnI binds to actin iv TnC binds to ionic Calcium b Tropomyosin i Rod like flexible protein that spirals around the actin and blocks myosin binding sites on actin when the skeletal muscle is relaxed ii Anchored by disc like proteins called Z discs or Z lines 1 The distance between 2 successive Z discs sarcomere a Sarcomeres are arranged end to end through the entire length of a myofibril b Myofibrils run the entire length of the skeletal muscle fibers while skeletal muscle fibers run the entire length of the skeletal muscle c Sarcomeres are the smallest contractile units in a skeletal muscle hence sarcomeres are the structural functional units of skeletal muscle i Skeletal muscle construction can be explained at the level of sarcomeres d Boundaries of each sarcomere is located between 2 Z e Thick filaments called A bands are located in the center of the sarcomere i A bands alternated with thin filaments anchored by discs Z discs 4 In a relaxed skeletal muscle the thick and thin filaments do not overlap hence the region of thin filaments are not overlapping with the A bands are referred to as I bands abut the Z discs 5 The region of the A bands not overlapping with thin filaments is referred to as the H zone which is the middle of the sarcomere a Running through the H zone is the M line which anchors the A bands in a sarcomere 6 In myofibrils the thick and thin filaments are arranged in an alternating pattern which explains the banded or striated appearance of skeletal muscle same striations appear in cardiac muscle iv Skeletal muscle organ fascicles skeletal muscle fibers myofibrils myofilaments either thick or thin v Sliding Filament Mechanism explains how skeletal muscles shortens when sarcomeres shorten 1 States that when the thin filaments slide inward into the H zone toward the M line the Z discs are pulled inward and hence the distance between 2 successive discs is shortened a Sarcomere shortens 2 With complete overlap between the thin filaments and the A bands the H zone and the I bands are obliterated a Once H zone is obliterated sliding of thin filaments halts maximum shortening of the sarcomeres and maximum shortening of skeletal muscle occurs 3 What makes the thin filaments slide inward into the H zone toward the M line a Explained by Excitiation contraction coupling E C coupling excitation of a
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