Anatomy 2 Study Guide: Test 1Muscular System and Review1. 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 : outermost layer of dense irregular connective tissue, encircling the entire muscle.- Perimyseum : dense irregular, connective tissue, surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called fascicles.- Endomysium : penetrates the interior of each fascicle and surrounds each separate muscle fiber (muscle cell).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 contractionand 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 TheoryAt 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 : synapse of the axon terminal of a 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. 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 : the electrical stimulus is usually an action potential that induces the release of Ca2+ form the sarcoplasm and the mechanical response is muscle contraction.- Coordinated-Coupling Gated Mechanism: junctional regions between t-tubules and SR are 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.- 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 anaerobicType IIx: Fast and fatiguable - short-term anaerobicType 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 : Produce movement, maintain posture, stabilize joints, generate heat.- Characteristics: Excitability, Contractility, Extensibility, Elasticity (stretching).Types of muscle contractions...- Isotonic : tension remains
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