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Muscular System 5 characteristics: Responsiveness: ability to respond to chemical signals, stretch and electrical changes across the plasma membrane Conductivity: ability to pass an electric signal across the muscle fiber Contractility: the ability to shorten when stimulated Extensibility: the ability to be stretched Elasticity: returns to its resting length after being stretched Muscle Characterisitics Voluntary, striated (light at dark bands) muscle attached to bone Muscle fibers=myofibers Epimysium: surrounds the entire muscle; Perimysium: wraps around one fascicle (group of muscle fibers); Endomysium: surrounds one individual muscle fiber Sarcolemma- plasma membrane of sarcomere Sarcoplasm- cytoplasm of a muscle cell Single muscle fiber has hundreds of myofibrils that look spotty (because made of myofilaments, which make up sarcomeres) o A bands: dark (both thin and thick filaments) o I bands: light o H zone: thick filaments only o I band: thin filaments only o Sarcomere (smallest contractile unit of a muscle fiber, the functional unit) is from z disc to z disc o M line in middle just thick filaments Thick filaments: myosin, thin filaments: actin Tropomyosin: rod shaped protein that spiral around actin and stabilize it, block myosin binding sites when contraction is not needed Troponin o TnI- inhibitory subunit that binds to actin o TnT- binds to tropomyosin and helps position it on actin o TnC- binds calcium ions Sacrcolemma has tunnel like infoldings or transverse (T) tubules that carry electric current to cell interior Sarcoplasmic reticulum: network around each myofibril that stores calcium Triad- T tubule and 2 terminal cisternae Sliding of Filaments: Thick and thin filaments don’t change length during shortening, the overlap just changes as the sarcomeres shorten A band stays the same (makes sense, both thick and thin filaments) Actin filaments slide past myosin Motor Units: One motor neuron innervates many muscle fibers Recruitment: o Always recruit the smaller motor units first (because if you just need a fine touch/pressure, then that will do it) o Largest motor unit is the last one recruited (for extreme forces) o Smallest motor units are type 1/slow for endurance running; dark meat and lots of fat for long exercise o Largest are type 2/fast for explosive motion; white turkey because not much mitochondria because use lactic acid that makes your muscles hurt after sprinting o Exercise increases recruitment of fast and large motor units o Mismatch- using wrong size of motor unit for a certain force needed o Motor units die with age and you lose intricate control (loss of muscle as get older) Neuromuscular Junction: Connection between nerve fiber and muscle cell, ACh released from nerve fiber stimulates the muscle cell 1. Local depolarization (end plate potential) ACh opens chemically gated ion channels, causes Na/K and depolarizes 2. End plate potential spreads to adjacent membrane areas, voltage gated Na channels open, if threshold is reached, AP is generated 3. From there, AP is propagated along the sarcomere to T tubules, where voltage sensitive proteins stimulate Ca2+ release from SR which is needed for contraction 4. Calcium binds to troponin, which changes shape and moves tropomyosin away from the binding sites, allowing myosin to bind to actin 5. When nervous stimulation ends, Ca2+ pumped back into the SR and binds to calequestrin- which neutralizes the charge so calcium can keep flooding in without the gradient having any effect on the flow of calcium Cross Bridge Cycle: Continues as long as there is ATP and Calcium present 1. Cross bridge forms as an energized myosin head binds to actin 2. Power stroke occurs as ADP and P are released and the myosin head bends, causing the actin to slide toward the M line 3. Cross bridge detachment occurs when ATP attaches to myosin 4. Myosin head returns to high energy state, “cocking” and ATP is hydrolyzed to ADP and P RINSE AND REPEAT Relaxation of a Muscle: Nerve stimulation ceases and acetylchlolinesterase removes ACh from the receptors Active transport needed to pump calcium back into SR to bind to cal. Rigor Mortis: stiffening of the body beginning 3 to 4 hours after death -Happens because calcium will still be released, so myosin heads will attach to actin, but with no ATP, they will not detach from it so the muscles will stay contracted Length-Tension Relationship Amount of tension generated depends on length of muscle before it was stimulated (textbook lifting experiment) Overly contracted beforehand- weak contraction results because the thick filaments are too close to the Z disc and can’t slide Too stretched beforehand- no actin/myosin interaction because they are too far apart to reach each other Muscle Twitch (motor unit’s response to a single AP of its motor neuron) Latent period: first few milliseconds when EC coupling is occurring Period of contraction: cross bridges are active, if tension becomes great enough to overcome the resistance of the load, the muscle shortens Period of relaxation: final phase that is initiated by reentry of Ca2+ into the SR, muscle tension decreases to zero Muscle CONTRACTS faster than RELAXES Other stuff: Myostatin: negative regulator of skeletal muscle mass, limits growth, also limits cardiac growth Loss of myostatin (deficiency) causes uncontrolled muscle mass growth Good for animals that need a lot of muscles (dogs that race), but more muscle means less fat which means less flavor for food and such Fused tetanus: when all evidence of relaxation disappears and the contractions fuse into a smooth, sustained contraction plateau; happens infrequently- superhuman strength to lift a car for ex. Wave/temporal summation: two stimuli delivered in succession, second twitch will be stronger because second contraction occurred before it relaxed Incomplete/unfused tetanus: when wave summation becomes greater and greater and end up with a sustained but quivering contraction Isotonic Contraction: Concentric/eccentric- shortening/lengthening Isometric contraction: muscle tenses but doesn’t shorten/lengthen