4/5/13Muscles1) 3rd Organ System composed of muscles, blood vessels, and nerves2) 3 types of muscles as organsa) Skeletal muscle tissuei) Elongated cells called skeletal muscle fiberii) Striationsiii) Multinucleateiv) Voluntary controlb) Cardiac muscle tissuei) Branched cells that form intercalated discsii) Striationsiii) Uninucleateiv) Involuntaryc) Smooth muscle tissuei) Spindle shaped cellsii) No obvious striationsiii) Uninucleateiv) Involuntary3) Skeletal Muscle Tissue organized with CT membranes to form skeletal musclea) Structural Organizationeach 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 convergentii) 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 neuronsb) Skeletal muscles attach to bones in the skeletal systemi) Each skeletal muscle has 2 attachment sites(1) Origin(2) Insertionc) When a skeletal muscle contracts it pulls the insertion towards the origini) The origin of a skeletal muscle is relatively stable when the skeletal muscle contractsd) Attachment of skeletal muscle may be:i) Direct(1) The epimysium of the skeletal muscle attaches directly to the surface of the bone (EX: skeletal muscles attached to the cranial bones)ii) Indirect(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 ofthe 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 fibersi) 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 ERii) Structure composed of terminal cisterna + T tubule + terminal cisterna = triadiii) 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 typesthick (16 nm in diameter) and thin filaments (8 nm in diameter)(a) Thick filaments (i) Composed of protein called myosin1. Myosin structure: rod-like tail that ends into globular headscalled the myosin globular heads(ii) Composed of 300 molecules of myosinthe tails form the core of these filaments with globular heads exposed(iii) Myosin globular head binding sites1. For Actin2. For ATP(iv) Globular heads contain the enzyme, ATPase1. Hydrolyzes ATP to ADP (b) Thin Filaments(i) Composed of 3 different proteins1. 1 structural protein called actina. Forms the structural frameworkb. Has a binding site for the myosin globular head2. 2 regulatory proteinsa. Troponini. Consists of 3 subunits: troponin T (TnT), inhibitory troponin (TnI), and troponin C (TnC)ii. TnT binds to tropomyosiniii. TnI binds to actiniv. TnC binds to ionic Calciumb. Tropomyosini. 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 discssarcomerea. 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 musclec. Sarcomeres are the smallest contractile units in a skeletal muscle hence, sarcomeres are the structural/functional units of skeletal musclei. Skeletal muscle construction can be explained at the level of sarcomeresd. Boundaries of each sarcomere is located between 2 Z discse. Thick filaments called A bands are located in the center of the sarcomerei. A bands alternated with thin filaments anchored by 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 appearanceof skeletal muscle (same striations appear in cardiac muscle)iv) Skeletal muscle organfasciclesskeletal muscle fibersmyofibrils 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 haltsmaximum shortening of the sarcomeres and maximum shortening of skeletal muscle occurs(3) What makes the thin
View Full Document
Unlocking...