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Anatomy and Physiology I Muscle Structure and Contraction Part II Instructor: Mary Holman Fig. 9.2 Basic Skeletal Muscle Structure Muscle Bone Fascicles Tendon Muscle fibers (cells) Fascia (covering muscle) Epimysium Perimysium Myofibrils Thick and thin filaments Endomysium Fascicle Axon of motor neuron Blood vessel Nucleus Sarcoplasmic reticulum Myofibril Filaments Muscle fiber Sarcolemma Actin Myosin Fig. 9.5a Sarcomere I band Z A band M I band Z H zone © H.E. Huxley Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 16,000x Three Types of Protein Associated with the Muscle Fiber • Contractile – Actin – Myosin • Regulatory – Troponin – Tropomyosin • Structural – Titin – Dystrophin – Myomesin – Nebulin Fig. 9.5b Sarcomere A band Titin I band I band Z line Z line Thin filaments Actin Thick filaments Myosin Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 9.6 Thick and Thin Filaments Myosin heads - Cross-bridges Thin filament Thick filament Thin filament Troponin Tropomyosin Myosin molecule Actin molecule Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 9.8c Neuromuscular Junction Synaptic vesicles Mitochondria Acetylcholine Synaptic cleft Folded sarcolemma Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Muscle Fibers innervated by Two Motor Neurons Motor neuron of motor unit 2 Motor neuron of motor unit 1 Branches of motor neuron axon Skeletal muscle fibers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Events Leading up to Muscle Contraction • Nerve impulse arrives at end of motor nerve axon causing • Acetylcholine (Ach) release into synapse via exocytosis • ACh floods across synaptic gap and attaches to receptors on the sarcolemma • Permeability of sarcolemma changes and Na+ enters cell • A muscle impulse is triggered • Muscle impulse travels via the transverse tubules throughout muscle cell • Ca++ diffuses from SR and binds to troponin on actin • Myosin cross bridges link with actin and muscle contracts Fig. 9.9a Relaxed muscle Tropomyosin Troponin Thin filament Actin monomers ADP + P ADP + P Thick filament 1 Muscle contraction begins and continues if ATP is available and Ca++ level in the sarcoplasm is high Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 9b Muscle Contraction Ca++ released from sarcoplasmic reticulum Ca+2 binds to troponin Tropomyosin pulled aside ATP Binding sites on actin exposed Ca+2 ADP + P Ca+2 ADP + P Ca+2 2 Exposed binding sites on actin molecules allow the muscle contraction cycle to occur Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 9.9c ADP + P 3 ADP + P Myosin heads bind to actin, forming cross-bridges ADP P ADP P ADP + P 4 Cross-bridges pull thin filament (power stroke), ADP and P released from myosin ATP ATP ATP ATP 5 New ATP binds to myosin, releasing linkages ADP + 6 P ADP + P ATP splits, which provides power to“cock” the myosin cross-bridges Fig. 9.10a Sarcomere A band Z line Z line 1 Relaxed Thin filaments Thick filaments 2 Contracting 3 Fully contracted Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 9.10b Sarcomere A band Z line Z line EM 23,000x © H.E. Huxley Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Muscle Fiber Excitation • Nerve impulse arrives at axon terminal • Triggers release of Ach by exocytosis • ACh diffuses across synaptic cleft • ACh binds to receptors on muscle motor end plate • Sarcolemma becomes more permeable to Na+ • Na+ triggers release of muscle action potential • Muscle action potential travels along outside of sarcolemma and into T tubules • Action potential triggers Ca++ release from SR • Ca++ binds to troponin on thin filament • Tropomyosin is pulled aside, revealing binding sites • Myosin links to & pulls actin to contract muscle Muscle Fiber Relaxation • Acetylcholinesterase decomposes ACh in synapse • Action potential (impulse) ends • SR actively pumps Ca++ back into SR • Tropomyosin moves back to cover binding sites • Myosin heads detach • Muscle fiber returns to its longer resting length Part II Text pgs 302 - 313 Muscle Metabolism Muscle Responses Smooth and Cardiac Muscle Fig. 9.11 Energy Sources for Muscular Contraction When cellular ATP is high Creatine Creatine P When cellular ADP Creatine ATP Creatine ATP is low P Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ADP ATP Immediate ATP from creatine phosphate Creatine ATP AATP ATP P Energy for muscle contraction Creatine phosphate ADP Relaxed muscle ADP Contracting muscle Short-term ATP from Anaerobic Respiration Muscle glycogen Or From blood Glucose Energy Net gain 2 ATP 2 Pyruvic acid 2 Lactic acid Into blood Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 9.12 Oxygen carried from the lungs by hemoglobin in red blood cells is stored in muscle cells by myoglobin and is available to support aerobic respiration. Lactic acid Mitochondria Pyruvic acid Cytosol Long-term ATP is provided by Aerobic Cellular Respiration Citric acid cycle Electron transport chain Synthesis of 34 CO2 + H2O + Energy Heat Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ATP Heat Production • 85% of heat generated in the body is from muscle contraction Muscle Fatigue • Defined as a loss of work out-put leading to reduced performance • Build-up of lactic acid • Depletion of muscle glycogen • Decrease in blood glucose • Increase in body temperature Oxygen Debt Recovery period - restores pre-exertion metabolic condition • convert lactic acid back into glycogen • resynthesize creatine phosphate • replenish oxygen storage in myoglobin Force of contraction Fig. 9.14 Myogram of a single muscle twitch Latent period Period of Period of contraction relaxation Time of stimulation Time Fig. 9.15 Force vs Muscle fiber length (a) Optimal length (c) Overly stretched Force (b) Overly shortened Muscle fiber length Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 9.16 Force of contraction Increasing Stimulation Frequency Force of contraction (a) Force of contraction (b) (c) Time Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fast Twitch and Slow Twitch Muscle Fibers Slow-twitch fibers (Type I) Slow to respond, slow to fatigue Fast-twitch glycolytic fibers (Type IIa) Fast to respond, fast to fatigue Fast-twitch fatigue-resistant fibers (Type IIb) Fast to respond, slow to fatigue Slow-twitch fibers (Type I) Slow to respond - slow to fatigue • Always oxidative • Resistant to fatigue • Red fibers • Most myoglobin • Good blood supply - more capillaries • Lots of mitochondria • Smallest fibers Fast-twitch glycolytic fibers (Type IIa) Fast to respond - fast to fatique • White fibers (less myoglobin) • Poorer blood supply • Susceptible to fatigue • Largest fibers • Lots of glycogen • Few mitochondria Fast-twitch fatigue-resistant fibers (Type IIb) Fast to respond - slow to fatique • Intermediate fibers • Oxidative • Intermediate amount of myoglobin • Intermediate amount of mitochondria • Pink to red in color • Resistant to fatigue Fig 9.17 Muscle Fibers innervated by Two Motor Neurons Motor neuron of motor unit 2 Motor neuron of motor unit 1 Branches of motor neuron axon Skeletal muscle fibers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 9.18 Isotonic Concentric Eccentric Isometric No movement Movement (a) Muscle contracts with force greater than resistance and shortens (concentric contraction) Movement (b) Muscle contracts with force less than resistance and lengthens (eccentric contraction) (c) Muscle contracts but does not change length (isometric contraction) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Types of Muscle Tissue • Skeletal muscle • General characteristics: • Muscle cells also called muscle fibers • Contractile • Three (3) types: • Skeletal muscle • Smooth muscle • Cardiac muscle • Attached to bones • Striated • Voluntary • Smooth muscle • Walls of organs • Skin • Walls of blood vessels • Involuntary • Non-striated • Cardiac muscle • Heart wall • Involuntary • Striated • Intercalated discs Fig. 5.28 Skeletal Muscle Tissue Striations Nuclei Portion of a muscle fiber (a) (b) b: © The McGraw-Hill Companies, Inc./Al Telser, photographer Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Fig. 5.29 Smooth Muscle Tissue Cytoplasm Nucleus (a) (b) b: © The McGraw-Hill Companies, Inc./Dennis Strete, photographer Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Smooth Muscle Contraction From: Principles of Anatomy & Physiology Tortora & Grabowsky Fig. 5.30 Cardiac Muscle Cells Striations Nucleus Intercalated disc (a) desmosome b: © The McGraw-Hill Companies, Inc./Al Telser, photographer gap junction Copyright © The McGraw-Hill Companies, Inc . Permission required for reproduction or display Fig. 9.20a First-Class Lever Resistance Force (Effort) EFR Force (Effort) Resistance Fulcrum Fulcrum Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 9.20b Second-class Lever Resistance Fulcrum FRE Resistance Force Effort Fulcrum Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Force Effort Fig. 9.20c Third-class lever Resistance FER Force Effort Resistance Fulcrum Force - Effort Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fulcrum Fig. 9.22 Coracoid process Origins of biceps brachii Tendon of long head Tendon of short head Biceps brachii Origin = Stable bone Insertion = Moveable bone Radius Insertion of biceps brachii Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
