Skeletal Muscle • Human body contains over 400 skeletal muscles • 40-50% of total body weight • Functions of skeletal muscle • Force production for locomotion and breathing • Force production for postural support • Heat production during cold stress
Structure: Connective Tissue • Epimysium • Outermost; surrounds entire muscle • Perimysium • Surrounds bundles of muscle fibers • Fascicles • Endomysium • Surrounds individual muscle fibers • Sarcolemma • Muscle cell membrane
Muscle Cells • Multi-nucleic • Allow for greater protein synthesis • Striated appearance, alternating light and dark bands • Dark bands contain contractile proteins • Satellite cells: undifferentiated cells that play role in muscle growth and repair • Increase number of nuclei during/after strength training
Microstructure of Muscle Fibers • Sarcomere: basic unit of muscle (“Z-line to Z-line”) • Z line (divide sarcomeres) • M line • H zone (myosin without actin overlap) • A band (myosin) • I band (actin) With contraction, what do you think occurs with the Z-lines?
Microstructure of Muscle Fibers • Myofibrils • Threadlike structures that contain contractile proteins • Actin (thin filament) • Myosin (thick filament) • Tropomyosin • Part of actin that contains attachment site for myosin • Troponin • Attached to tropomyosin • Covers attachment site for myosin
Microstructure of Muscle Fibers • Sarcoplasmic reticulum:pumps Ca+ ions • Surrounds each myofibril and runs parallel to it • Terminal cisternae: Storage sites for calcium • Transverse tubules • Invaginations in sarcolemma (membrane) • Linked closely to sarcoplasmic reticulum and terminal cristae • Allows quick depolarization of the inside of muscle cell
Neuromuscular Junction (NMJ) • Junction between motor neuron and muscle fiber • Motor unit • Motor neuron and all fibers it innervates • Motor end plate • Pocket formed around motor neuron by sarcolemma • Neuromuscular or Synaptic cleft • Short gap between neuron and muscle fiber • Acetylcholine is released from the motor neuron • Causes an end-plate potential (EPP) • Depolarization of muscle fiber
The Sliding Filament Theory • Contraction is considered excitation-coupling • Muscle shortening occurs due to the movement of the actin filament over the myosin filament • Formation of cross-bridges between actin and myosin filaments • Muscle shortening: reduction in distance from Z-line to Z-line • “Power stroke” • Generation of force
Sarcomere Shortening during Muscle Contraction How can we use this graphic to better understand active and passive insufficiency?
Power Stroke • Think of a crew team with synchronized oar strokes • Shorten muscle fiber by approximately 1% • Many muscles can shorten up to 60% of its resting length • Clear that contraction cycle can be repeated over and over
Energy for Contraction • ATP is required for muscle contraction • Myosin ATPase breaks down ATP as fiber contracts • Enzyme located on head of myosin cross bridge • ATP ADP + Pi • Sources of ATP • Phosphocreatine (PC) • Glycolysis • Oxidative phosphorylation So…is ATP needed in order for the actin/myosin cross bridge to form or separate?
Excitation-Contraction Coupling • Depolarization of motor end plate (excitation) is coupled to muscular contraction • Acetylcholine binds to motor end plate first • Action potential travels down transverse tubules and causes release of Ca+2 from SR • Ca+2 binds to troponin and causes positional change in tropomyosin • Exposing active sites on actin • Strong binding state formed between actin and myosin cross bridges • Contraction occurs
Thoughts… • What is needed for a contraction cycle to be continued indefinitely? • What is the signal to stop a contraction? • How are troponin and tropomyosin regulators of muscle contraction?
Step-by-Step Summary of Excitation-Contraction Coupling • Excitation • Action potential in motor neuron causes release of acetylcholine from nerve fiber branches into synaptic cleftof neuromuscular junction. • Acetylcholine binds to receptors on motor end plate of sarcolemma • This binding leads to depolarization that is conducted down transverse tubules. • This causes release of Ca+2 from sarcoplasmic reticulum (SR). SR runs parallel to the myofibrils.
Step-by-Step Summary of Excitation-Contraction Coupling • Contraction • At rest, myosin cross-bridges in weak binding state (allows muscles to stretch quite easily). • Ca+2 binds to troponin, causes shift in tropomyosin to uncover active sites on actin • As myosin heads move to bind to actin, Pi releases; the resultant connection is called a cross bridge. • Cross-bridge movement or “power stroke” occursand ADP is released from myosin. • ATP attaches to myosin, breaking the cross-bridge and forming weak binding state, also called recovery stroke. • Then ATP binds to myosin, broken down to ADP+Pi, which energizes myosin.
Video • Sliding Filament Theory • Myosin and Actin
Muscle Fatigue • Decline in muscle power output due to: • Decrease in force generation • Decrease in shortening velocity • High-intensity exercise (~60 seconds) • Accumulation of lactate, H+, ADP, Pi, and free radicals • Diminishes cross bridges bound to actin and thus force production • Long-duration exercise (2–4 hours) • Muscle factors • Accumulation of free radicals • Electrolyte imbalance • Glycogen depletion What factors effect the fatigue process?
Exercise-Induced Muscle Cramps • Spasmodic, involuntary muscle contractions • Multiple theories • Electrolyte depletion and dehydration theory • Water and sodium loss via sweating causes spontaneous muscle contractions (motor nerve terminals spontaneously discharge) • What population could this apply to most? • Altered neuromuscular control theory • Muscle fatigue causes abnormal activity in muscle spindle and Golgi tendon organ • Increase in spindle, decrease in GTO • Leads to increased firing of motor neurons in SC > involuntary muscle contractions
Characteristics of Muscle Fiber Types • Biochemical properties • Oxidative capacity • Number of capillaries, mitochondria, and amount of myoglobin • Type of myosin ATPase • Speed of ATP degradation
Characteristics of Muscle Fiber Types • Contractile properties • Maximal force production • Fiber specific tension: Force per unit of cross-sectional area • Speed of contraction (Vmax) • Highest speed at which a fiber can shorten • Determined by the rate of cross bridge cycling, myosin ATPase activity, and Ca+ release • Maximal poweroutput • Force generation X shortening velocity • Muscle fiber efficiency • Less E to perform a certain amount of work
Characteristics of Individual Fiber Types • Type I fibers • Slow-twitch fibers • Slow-oxidative fibers • Type IIa fibers • Intermediate fibers • Fast-oxidative glycolytic fibers • Type IIxfibers (Type IIb) • Fast-twitch fibers • Fast-glycolytic fibers
Comparison of Maximal Shortening Velocities Between Fiber Types What does Type II fibers having larger shortening velocities tell us about the activities they are utilized with?
Fiber Types & Performance • Non-athletes • Have approximately 50% slow and 50% fast fibers • Power athletes • Sprinters, O-line, D-Line, pole vaulters • Higher percentage of fast fibers • Endurance athletes • Distance runners • Higher percentage of slow fibers
Speed of Muscle Action and Relaxation • Muscle twitch • Contraction as the result of a single stimulus • Latent period • Lasting ~5 ms • Contraction • Tension is developed • 40 ms • Relaxation • 50 ms (longest)
Muscle Twitch SINGLE STIMULUS
Types of Muscle Action • Isometric • Muscle exerts force without changing length (static exercise) • Pulling against immovable object • Postural and core muscles • Dynamic (isotonic) • Concentric • Muscle shortens during force production • Eccentric • Muscle produces force but length increases • Associated with muscle fiber injury and soreness • High stress within sarcomere
Force Regulation in Muscle • Types and number of motor units recruited • More motor units = greater force • Do fast or slow twitch fibers have a high number of motor units recruited? • Initial muscle length • “Ideal” length for force generation • Resting? Stretched? Contracted? • Increased cross-bridge formation
Force-Velocity Relationship • At any absolute force the speed of movement is greater in muscle with higher percent of fast-twitch fibers • The maximum velocity of shortening is greatest at the lowest force • True for both slow and fast fibers
Power-Velocity Relationship • “At any given velocity of movement, the peak power generated is greater in muscle that contains a high percentage of fast fibers than in muscle with a high percentage of slow fibers.” • “The peak power generated by any muscle increases with increasing velocities of movement up to movement speed of 200-300 degrees/second. • Power decreases beyond this velocity because force decreases with increasing movement speed