Anatomy 2 Study Guide Test 1 Muscular System and Review 1 There are 660 skeletal muscles in the body made up of water proteins and substrates 45 of body weight and is the largest organ system important to bioenergetics and major site for energy transduction and storage End organ for primary support system involved in exercise like pulmonary and cardiovascular Epimysium Perimyseum outermost layer of dense irregular connective tissue encircling the entire muscle dense irregular connective tissue surrounds groups of 10 to 100 or more muscle fibers separating them into bundles called fascicles Endomysium muscle cell penetrates the interior of each fascicle and surrounds each separate muscle fiber Skeletal Myofiber Composed of myofibrils which have myofilaments composed of sarcomeres which account for roughly 80 of cellular volume and contain the contractile elements of the muscle cell Striated due to repeating series of dark A bands and light I bands H Zone central region of the A band where there is no thick and thin filament overlap H zone is bisected by the M line composed of proteins that keep the sarcomere in proper spatial orientation as it lengthens and shortens Z line separates each sarcomere Multi nucleated highly plastic Load driven size alterations Increase hypertrophy Decrease atrophy Ultrastructure and Molecular Composition of the Myofilaments Sarcomere smallest contractile unit of the skeletal myofiber composed of the following 2 Types Thick filaments composed of bundles of myosin 50 of total myofibrillar proteins Key importance for dev of muscular force and velocity of contraction and thin filaments composed of actin G Actin globular actin polymerize into F Actin F Actin filamentous actin arranged in a double helix These are contractile proteins Tropomyosin and Torponin are regulatory proteins present in thin filaments Sarcolemma Plasma membrane basement membrane that fuses with tendon bone Sarcoplasm cytoplasm contains proteins minerals fats organelles glycogen myoglobin Transverse Tubules transport system deep into fiber allows rapid conduction of impulse because no innervation exists here They are infoldings of the sarcolemma that conduct electrical impulses from the surfaces of the cell to the terminal cisternae Covers each myofibril parallel to it and stores the Ca2 Sliding Filament Theory At the onset of contraction which means that muscle action potential propagates across the sarcolemma and into the t tubules SR releases Ca2 into the sarcoplasm Ca2 binds to Troponin which moves Tropomyosin away from the binding sites on acting for myosin Once binding sites are free the contraction cycle beings a Myosin head includes an ATP binding site and an ATPase When the ATP is hydrolyzed the myosin head is energized ADP and Pi still att to head b The energized myosin head attaches to the actin and releases the previously att phosphate group this is referred to as a cross bridge c after cross bridges from the power stroke occurs In this time the site on the cross bridge where ADP is opens and the cross bridge rotates to release the ADP as it rotates towards the center of the sarcomere the cross bridge generates forces sliding the thin filament towards the thick filament towards the M line d At the end of the power stroke the cross bridge remains firmly att to the actin until it binds another ATP As the new ATP binds to that myosin the myosin detaches from the actin As long as ATP is available and Ca2 level near the thin filament is sufficiently high the contraction cycle will continue Neuromuscular Junction between a neuron and muscle cell motor neuron with the motor end plate the highly excitable region of muscle fiber plasma membrane responsible for initiation of APs across the muscle s surface causing muscle to contract Signal passes through NMJ via ACh synapse of the axon terminal of a a Arrival of nerve impulse to synaptic end bulb opens voltage gated channels which allow CA2 to enter Synaptic vesicles att to motor neurons plasma membrane liberating ACh which diffuses along the synaptic cleft b Activation of ACh receptors opens ion channels allowing Na to flow through c The inflow of Na causes a change in membrane potential which produces an action potential which then propagates along the sarcolema into the T tubules which causes the release of Ca2 into sarcoplasm d ACh is broken down by AChE Excitation Contraction Coupling induces the release of Ca2 form the sarcoplasm and the mechanical response is muscle contraction the electrical stimulus is usually an action potential that Coordinated Coupling Gated Mechanism discontinuous such that each junction consists discrete RyR1 channel subunits this mechanism may allow the concerted activation of RyR1 Ca2 release channels during EC coupling by triggering on RyR1 channel that may activate all associated RyR1 channels in a junction junctional regions between t tubules and SR are Length Tension Rel a muscle fiber develops it s greatest tension when there is optimal zone overlap between thick and thin filament Not over stretch and not over contracted because myosin and actin heads cannot make contact 2 Muscle fibers are organized into functional units called motor units defines as one motor neuron and all the fibers it innervates yet one fiber cannot be innervated by multiple motor neurons There are 3 types of muscle fibers Each types provides different energy transduction kinetics and cross bridge turnover rates during contraction Strength comes from size of myofibers and proteins As you grow older you lose more type IIx Type I Slow aerobic exercise uses triglycerides ex distance running Type IIa Fast fatigue resistant long term anaerobic Type IIx Fast and fatiguable short term anaerobic Type llb Fast not in humans short term anaerobic Stem Cells Satellite Cells mechanism for replacement and repair of muscle Help produce cell type according to needs defined by type of training Located along the peripheral edge of the fibers and look like fiber nuclei 3 Functions and Characteristics Functions Characteristics Produce movement maintain posture stabilize joints generate heat Excitability Contractility Extensibility Elasticity stretching Types of muscle contractions Isotonic Eccentric Isometric tension remains constant a muscle length decreases or increases Ex Concentric and tension increases greatly without change in muscle length Ex isometric 4 Fascicle Arrangements Circular fascicles arranged in rings Ex around eyes
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