Test 2 Gross Structure of Skeletal Muscle Fibers cylindrical cells that compose muscle o Fiber number largely fixed in second trimester of fetal development Fibers vary in length from a few mm to nearly 30 cm Levels of Organization Endomysium fine layer of connective tissue wraps each muscle fiber and separates it from neighboring fibers called a fasciculus Perimysium another layer of connective tissue surrounds a bundle of up to 150 fibers A fascia of fibrous connective tissue the epimysium surrounds the entire muscle Tendons dense strong connective tissue that connect both ends of the muscle to the periosteum the bone s outermost covering Tissues of tendon intermesh with collagenous fibers within bone forms a powerful link between muscle and bone that remains inseparable except during severe stress Force of muscle action transmits directly from CT harness to the tendons which then pull on bone at the point of attachment o F 20 to 50 N Origin the location where the tendon joins a relatively stable skeletal part generally the proximal or fixed end of the lever system or that nearest the body s midline Insertion the point of distal muscle attachment to the moving bone The protein collagen comprises about 70 of the tendon s dry mass Sarcolemma beneath the endomysium surrounds each muscle fiber a thin elastic membrane that encloses a fiber s cellular contents o Contains plasma membrane and basement membrane o Plasma membrane conducts the electrochemical wave of depolarization over the surface of muscle fiber Also insulates one fiber from another during depolarization o Basement membrane contains proteins and strands of collagen fibrils that fuse with the collagenous fibers in the outer covering of the tendon Satellite cells between the basement and plasma membranes myogenic stem cells o The normally quiescent myoblasts function in regenerative cellular growth possible adaptations to exercise training and recovery from injury o Incorporation of satellite cell nuclei into existing muscle fibers gives an explanation for exercise induced muscle fiber hypertrophy Sarcoplasm aqueous protoplasm contains enzymes fat glycogen particle nuclei that contain genes mitochondria and other organelles Sarcoplasmic reticulum an extensive longitudinal lacelike network of tubular channels and vesicles o Provides structural integrity to cell o Allows waves of depolarization to spread rapidly from the fiber s outer surface to its inner environment through T tubule system to initiate muscle action o Contains pumps that take up calcium from fiber s sarcoplasm Test 2 Chemical Composition Water makes up 75 of skeletal muscle mass Protein composes 20 Salts and other substances phosphates various enzymes sodium potassium and chloride ions amino acids fats and CHO make up 5 Myosin approx 60 of muscle protein actin and tropomyosin are most abundant muscle proteins Myoglobin oxygen binding conjugated protein Blood Supply Arteries and veins lie parallel to individual muscle fibers provide vascular supply o Divides into arterioles capillaries and venules Arteries provide oxygen while veins remove CO2 Blood flow fluctuates in rhythmic activities such as running swimming or cycling o Blood flow decreases during contraction and increases during relaxation o Between 200 and 500 capillaries deliver blood to each square millimeter of active muscle cross section with up to four capillaries directly contacting each fiber Endurance athletes 5 to 7 capillaries surround each fiber greater local Strenuous physical activity and blood flow present a different picture blood flow and adequate tissue oxygenation when needed o Elevated intramuscular pressure occludes local blood flow during contraction o With sustained high force contract intramuscular high energy P and glycolytic anaerobic reactions provide main energy for muscular effort Capillarization The trained muscles increased capillary to muscle fiber ratio helps explain improved exercise capacity with endurance training Enhanced capillary microcirculation expedites the removal of heat and metabolic byproducts from active tissues in additions to facilitating delivery of oxygen nutrients and hormones Enhanced vascularization on capillary level proves beneficial during exercise that requires a high level of steady rate aerobic metabolism Vascular stretch and shear stress on vessel walls from increased blood flow during exercise stimulate capillary development with intense aerobic training Skeletal Muscle Ultrastructure Fibrils or myofibrils approximately 1 micrometer in diameter contain even smaller subunits called filaments or myofilaments that lie parallel to the long axis of the myofibril Myofilaments contain actin and myosin that account for 85 of myofibrillar complex Tropomyosin located along the actin filament 5 Troponin located in actin filaments 3 Test 2 Alpha actinin distributed in Z band region 7 Beta actinin found in actin filaments 1 M protein in the region of M lines within sarcomere 1 C protein contributes to sarcomere s structural integrity 1 The Sarcomere Striated appearance light and dark bands along length of skeletal muscle o I band light area o A band darker area o Z line bisects the I band and adheres to the sarcolemma provides stability to entire structure Sarcomere consists of the basic repeating unit between two Z lines This structural entity compromises the functional unit of muscle fiber The actin and myosin contribute to muscle contraction The length of the sarcomere determines the muscle s functional properties Muscle Fiber Alignment Differences in sarcomere alignment and length strongly affect a muscle s force and power generating capacity Fusiform spindle shaped fibers run parallel to the muscle s long axis e g biceps brachii and taper at the tendinuous attachment Pennate fan shaped fibers fascicule bundles of fibers lie at an oblique pennation angle that varies up to 30 degrees o The degree of pennation directly impacts sarcomeres number per cross sectional muscle area no fibers run full muscle length o Allows individual muscle to remain short while overall muscle may attain considerable length Test 2 Physiologic cross sectional area PCSA total cross sectional area of all fibers within a particular muscle o An unusually large pennation angle of 30 degrees results on in 13 loss in an individual fiber s force capacity this makes for huge increase in total fiber packing pennation allows packing of a large number of fibers into a smaller cross
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