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BMS 250 Lecture
Chapter 10
| Term | Definition |
|---|---|
| Functions of skeletal muscle | body movement, maintenance of posture, protection and support, regulating elimination of materials, and heat production |
| Characteristics of skeletal muscle | excitability, conductivity, contractility, extensibility, elasticity |
| Excitability | respond to a stimulus causing a local change in RMP |
| Conductivity | propagate an electrical signal along plasma membrane |
| Contractility | shortening of muscle as contractile proteins slide past one another decreasing degree of overlap |
| Elasticity | ability to return to original length following shortening or lengthening |
| Hierarchy of structures in a muscle | 1. whole muscle contains many fascicles 2. a fascicle contains many muscle fibers 3. a muscle fiber is a muscle cell |
| Epimysium | surrounds whole muscle |
| Is a muscle an organ | yes |
| Perimysium | surrounds each fascicle |
| Endomysium | surrounds each muscle fiber |
| What do epimysium, perimysium, and endomysium merge to form? | tendons and aponeurosis |
| Tendon | thick, cordlike structures composed of dense regular CT that attaches muscle to bone |
| Aponeurosis | thin, flattened sheet composed of dense regular CT that attaches muscle to bone |
| Groups of ____ fuse to form a single skeletal muscle cell during development | myoblasts |
| Sarcoplasm | cytoplasm in skeletal muscle that contains typical cell structures, contractile proteins, and other specialities |
| Sarcolemma | plasma membrane of skeletal muscle fiber that contains T-tubules; express VGNCs and VGKCs |
| T-tubules | invaginations of plasma membrane that extend into muscle fiber as a network of narrow tubules to meet the SR |
| Myofibrils | bundles of contractile proteins |
| Sarcoplasmic reticulum (SR) | endoplasmic reticulum of muscle fiber that fits around myofibrils like a sleeve of netting; express Ca2+ pumps and channels |
| Terminal cisternae | sacs at the end of sarcoplasmic reticulum sections that are reservoirs for Ca2+ ions |
| Triad | 2 terminal cisternae with central T-tubule |
| Fascicle | a bundle of structures, such as nerves or muscle fibers |
| How does the sarcoplasmic reticulum store Ca2+? | Ca2+ pumps move Ca2+ into SR, Ca2+ stored in SR bound to Ca2+ sequestering proteins (Calmodulin, Calsequestrin), Ca2+ channels open to release Ca2+ from SR and induce muscle contraction |
| Myofilaments | contractile proteins bundled within myofibrils; many successive units of myofilaments extend the entire length of a myofibril |
| Thick filaments | bundles of myosin proteins anchored at center of sarcomere called M line; has 2 heads and 2 intertwining tails with binding sites for actin and ATP on the head |
| How are thick filaments oriented? | tails point to center and heads point to ends |
| Thin filaments | primarily 2 strands of actin filaments twisted around each other; F-actin composed of G-actin proteins, 2 strand of F-actin twist together, each G-actin molecule has a myosin binding site |
| Tropomyosin | "stringlike" protein covers the myosin binding site in non-contracting muscle; blocks myosin binding sites |
| Troponin | "ball-like" protein has a Ca2+ binding site |
| Sarcomere | repeating units of myofilaments |
| What gives muscles a striated appearance? | when relaxed, thick and thin filaments partially overlap |
| What part of the sarcomere is lost when muscle contracts? | I band and H zone |
| When the muscle is contracted, thin filaments slide over... | thick filaments toward the M line |
| Z discs | ends of each sarcomere unit; anchor for thin filaments |
| I band | region containing only thin filaments |
| A band | central region containing entire thick filament and partially overlapping thin filaments |
| H zone | center of A band containing only thick filaments |
| M line | thin structure at center of H zone; attachment site for thick filaments |
| What type of neurons control skeletal muscle contraction? | somatic motor neurons |
| Motor unit | a single motor neuron and the muscle fibers it controls; size varies and determines degree of control; small=precise control, large=rough control |
| Neuromuscular junction (NMJ) | location on skeletal muscle fiber innervated by a motor neuron where the neuron synapses on the muscle fiber |
| 3 components of neuromuscular junction | synaptic knob, motor end plate, synaptic cleft |
| Synaptic knob | expanded tip of motor neuron, contains synaptic vesicles filled with acetylcholine (Ach) |
| Motor end plate | specialized region of sarcolemma of a muscle fiber that expresses Ach receptors (chemically-gated cation channels) |
| Synaptic cleft | narrow space between synaptic knob and motor end plate; contains acetylcholinesterase (enzyme that breaks down Ach) |
| Resting membrane potential | skeletal muscle fibers exhibit an electrical charge difference across the sarcolemma when the cell is at rest; -90mV in skeletal muscle fibers |
| Conditions of a skeletal muscle fiber at rest | RMP is -90mV (Na+ more concentrated in interstitial fluid, K+ more concentrated in cytosol), chemically-gated and voltage-gated channels are closed, Ca2+ stored in SR, contractile protein (myofilaments) within sarcomere are in relaxed position |
| 3 major phases of skeletal muscle contraction | excitation of the skeletal muscle fiber, excitation-contraction coupling, crossbridge-cycling |
| First phase of skeletal muscle contraction: excitation of the skeletal muscle fiber | Ca2+ enters synaptic knob, Ach released from vesicles, Ach binds Ach receptors (chemically-gated cation channels) |
| Second phase of skeletal muscle contraction: excitation-contraction coupling | end plate potential develops (-90mV to -65 mV), action potential propagates along sarcolemma and T-tubules, Ca2+ released from SR |
| What happens when end plate potential develops during the second phase of skeletal muscle contraction? | Na+ rapidly diffuses into motor end plate, membrane depolarizes to threshold potential (from -90mV to -65mV) |
| What happens when the action potential propagates along sarcolemma and T-tubules during the second phase of skeletal muscle contraction? | VGNCs open, Na+ diffuses in causing membrane to depolarize to +30mV, VGKCs open, K+ diffuses out causing membrane to repolarize to -90mV, AP propagates along length of sarcolemma w/ sequential opening of VGCs which continues along T-tubules |
| What happens when Ca2+ is released from sarcoplasmic reticulum during the second phase of skeletal muscle contraction? | AP reaches SR triggering opening of VGCCs expressed on terminal cisternae, Ca2+ diffuses out of the SR to mingle with myofilaments in the sarcoplasm |
| Third phase of skeletal muscle contraction: excitation-contraction coupling | crossbridge-cycling which causes muscle contraction by shortening sarcomeres |
| Crossbridge cycling steps | Ca2+ binds troponin, crossbridge forms (myosin heads bind to actin), myosin head power stroke , ATP binds causing myosin heads to release, ATP splits providing energy to reset myosin head |
| Power stroke | myosin heads swivel pulling thin filament past thick filament toward center of sarcomere. ADP and Pi are released exposing ATP binding site |
| How long does crossbridge cycling occur | if Ca2+ is still present, the process keeps repeating |
| Skeletal muscle relaxation steps | AP along motor neurons stops, AP along sarcolemma stops, crossbridge cycling stops |
| Skeletal muscle relaxation: AP along motor neuron stops | release of Ach from motor neuron stops, Ach receptors close, acetylcholinesterase removes Ach from synaptic cleft, motor end plate returns to RMP |
| Skeletal muscle relaxation: AP along sarcolemma stops | VGNCs and VGKCs close along sarcolemma and T-tubules, sarcolemma and T-tubules return to RMP, VGCCs in terminal cisternae close, Ca2+ in sarcoplasm pumped back into SR |
| Skeletal muscle relaxation: crossbridge cycling stops | troponin-tropomyosin complex moves to re-cover myosin binding site on actin, myosin heads can no longer bind actin, thin filaments slide away from thick filaments, sarcomeres return to resting length |
| Muscles have very high ____ demands | energy |
| How is ATP used by muscles? | myosin heads to reset during crossbridge cycling, calcium pumps to restore Ca2+ concentration gradient |
| 3 ways skeletal muscles generate ATP | phosphate transfer (immediate supply), glycolysis (short-term supply), and cellular respiration (long-term supply) |
| Criteria for classification of muscle fibers | type of concentration generated (differ in power, spped, and duration of contraction) and means for supplying ATP |
| Muscle fibers classified by type of contraction generated | fast-twitch fibers and slow-twitch fibers |
| Fast-twitch fibers | more powerful, rapid, brief contractions, contains a fast variant of myosin ATPase that splits ATP; less extensive vascular supply |
| Slow-twitch fibers | contains a slow variant of myosin ATPase that splits ATP; more extensive vascular supply |
| Muscle fibers classified by means for supplying ATP | oxidative fibers and glycolytic fibers |
| Oxidative fibers | "fatigue-resistant", provide ATP through aerobic cellular respiration; extensive capillary network, many mitochondria, large supply of myoglobin ("red fibers") |
| Glycolytic fibers | "fatigueable", provide ATP through glycolysis, large glycogen stores |
| Slow oxidative fibers (type 1) | slow and less powerful contractions, contract for long periods of time, appear dark red due to myoglobin and mitochondria (ex. posture and marathon running); fatigue-resistant |
| Fast oxidative fibers (intermediate type 11a) | fast and powerful contractions, contract for moderate periods of time, lighter dark red due to less myoglobin and mitochondria (ex. walking and biking); fatigue resistant |
| Fast glycolytic fibers (fast anaerobic) | most prevalent type of fibers that are fast and powerful, contract for short bursts, appear white due to lack of myoglobin and mitochondria (ex. sprinting and lifting weights); fatigueable |
| Do muscles contain a specific type of fiber? | No, they contain a mixture of fiber types which depend on the muscle |
| Muscle tension | force generated by the contraction of a muscle |
| Myogram | graphic recording of changes in muscle tension; muscles can be stimulated with electrodes to induce a contraction |
| Muscle twitch | a single, brief contraction period followed by a relaxation period of a skeletal muscle in response to a single stimulation |
| Threshold stimulus | minimum voltage needed to generate a twitch |
| Periods of a muscle twitch | latent, contraction, relaxation |
| Latent period | delay between stimulation and contraction; lag time is accounted for by events of excitation-contraction coupling and Ca2+ release from the SR |
| Contraction period | crossbridge cycling and sarcomere shortening |
| Relaxation period | release of crossbridges and return of Ca2+ to SR |
| Motor unit recruitment | the smooth, steady increase in tension produced by an increase in the number of activated motor units |
| All-or-none law | all: if a muscle contracts it contracts completely; none: if the stimulus is insufficient, the fiber will not contract |
| At what level is tension controlled? | motor unit |
| How do muscle exert varying degrees of force? | by controlling the number of motor units; smallest motor units activated first, the number gradually increases, tension peaks when all motor units in muscle activated |
| Factors influencing skeletal muscle tension | muscle tone (resting muscle tension), tension-resistance relationship, sarcomere length-tension relationship, muscle fatigue |
| Isometric contraction | muscle doesn't change length, tension generated is sufficient to maintain position; same length |
| Isotonic contraction | muscle changes length, tension is generated; concentric vs. eccentric; same force |
| Concentric | muscle shortens, tension is greater than resistance |
| Eccentric | muscle lengthens, opposing force is greater than the tension generated |
| Sarcomere length- tension relationship | the degree of thick and thin filament overlap influences the amount of tension that can be generated |
| Muscle fatigue | reduced ability of inability of skeletal muscle to produce muscle tension due to decreased glycogen stores or sustained exercise |
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kkade
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