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Lecture 7

Week/Lecture 7: Muscles.pdf

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Humber College
BIOL 171

Muscles Skeletal Muscle attached to bone, skin or fascia (fibrous tissue layer) alternating light and dark bands (striations (ridges)) on cells under voluntary control Muscle Functions give shape produce movement maintain posture support other structure generate heat regulate organ volumes (sphincters: circular muscles that maintain constriction of opening) Properties of Muscle Tissue excitability conductivity contractility extensibility elasticity Composition skeletal muscles are composed of bundles of muscle fibre called fascicles fascicles consist of bundles of elongated, striated muscle cells (muscle fibres) Connective Tissue Components epimysium surrounds entire muscle perimysium surrounds individual fascicles endomysium surrounds individual muscle fibres all connective tissue layers extend beyond the muscle belly to form the tendon tendon attaches muscle to bone Nerve and Blood Supply each muscle is supplied by a nerve, artery, vein each muscle fibre is supplied by a motor neuron and 1-2 capillaries (located in the endomysium) each motor neuron innervates (supply with nerves) several muscle cells (motor unit) Formation of Muscle Fibres during embryonic development, many myoblasts fuse to create a mature muscle cell (multinucleate (many nuclei) but cannot divide) become more specialized- function as muscle cell normally, increased muscle size is due to hypertrophy of muscle cells (cell size is growing) some myoblasts remain unfused to create satellite cells (can divide) satellite cells left over that can divide, but rare muscle fibres made of myofibrils made of sarcomeres Sarcoplasm (sarco = muscle) most of volume occupied by myofibrils also contains glycogen and myoglobin (red-coloured protein that binds oxygen) Transverse Tubules invaginations of sarcolemma (cell membrane) carry muscle action potentials deep into sarcoplasm ensures that action potential is transmitted across entire cell and into centre Sarcoplasmic Reticulum muscle fibre equivalent to endoplasmic reticulum 2+ storehouse for Ca Myofibrils contractile elements of skeletal muscle extend entire length of muscle fibre long chains of sarcomeres Sarcomere region between Z-discs basic functional unit of striated muscle composed of overlapping thick and thin myofilaments components Z-discs: on ends M-line: median line create striation Myofilaments thick and thin filaments overlap each other in a pattern that creates striations (dark A bands and light I bands) striation from overlapping layers of proteins/filaments thick filaments composed of myosin myosin molecule resembles two golf clubs twisted together myosin heads (cross bridges) extend toward the thin filaments held in place by the M line proteins thin filaments consist of actin, troponin, tropomyosin in relaxed muscle, myosin-binding site on each actin molecule is covered by tropomyosin supported by troponin don't want myosin binding all the time, or muscle would never relax to get myosin to bind, must get rid of tropomyosin and troponin held in place by Z disc proteins Muscle proteins myofibrils contain 3 kinds of protein contractile proteins (myosin and actin) regulatory proteins that turn contraction off and on (troponin and tropomyosin) structural proteins that provide proper alignment, elasticity and extensibility (*titin, myosin, nebulin, and dystrophin) *only important one titin third most plentiful protein in skeletal muscle huge molecule anchors thick filament to the Z disc and M line Sliding Filament Mechanism of Contraction (theory) myosin cross bridges pull on thin filaments thin filaments slide inward (across thick filament) Z discs come toward each other (distance shortens) sarcomeres shorten --> muscle fibre shortens --> muscle shortens the thick and thin filaments do not shorten Mechanism of Contraction neuromuscular junction synapse between a motor neuron (from brain/spinal cord) and skeletal muscle fibre 1. nerve impulse reaches an axon terminal (synaptic end bulb), synaptic vesicles release ACh (acetylcholine) 2. ACh opens Na+ channels/receptors and Na+ rushes into the muscle cell 3. a muscle action potential spreads across the sarcolemma, down into the transverse tubules (all is stimulated at the same time -- T tubules) 4. SR (sarcoplasmic reticulum) opens and releases stored Ca2+ into the sarcoplasm (muscle cell "cytoplasm") when Na+ influx occurs 5. Ca2+ binds to troponin, causes troponin-tropomyosin complex to move, exposing myosin binding sites on actin 6. contraction cycle begins Contraction Cycle repeating sequence of events that cause the thick and thin filaments to move past each other Steps: 1. Myosin heads activated (cocked) by ATP (ADP+P) 2. Myosin heads bind to actin (releases P-- binds to actin) 1."cross-bridge" 3. Power stroke: myosin heads pull actin towards centre of sarcomere ("springs" back into relaxed position, bringing thin filament a small bit along with it) 4. Myosin heads bind ATP (relaxing myosin), cross bridges detach cycle repeats continuously as long as: ATP is available Ca2+ level near the troponin-tropomyosin complex is high relaxation AChE breaks down ACh within the synaptic cleft muscle action potential ceases (no more trigger for Na+) Ca2+ channels close Ca2+ pumped back into SR by active transport tropomyosin-troponin complex covers binding site on the actin Muscle Metabolism Production of ATP in Muscle Fibres active muscles use AT
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