Muscles and Muscle Tissue

1 Muscles and Muscle Tissue Chapter 9 2 Overview of Muscle Tissues Compare and Contrast the three basic types of muscle tissue List four important functions of muscle tissue 3 Muscle Terminology Muscle Fibers (skeletal and smooth muscle cells) Myo and sarco = muscle Sacroplasm, sarcolemma 1

3/15/15 4 Types of Muscle Tissue Skeletal Muscle Longest muscle cells Striated Voluntary muscle Very powerful, easily fatigued Highly adaptable 5 Types of Muscle Tissue Cardiac Muscle Striated Involuntarily controlled Connected by intercalated discs Can contract without any nervous system input 6 Types of Muscle Tissue Smooth Muscle Found in walls of hollow organs Elongated cells No striations Involuntary Slow sustained contractions 2

Special Characteristics of Muscle Tissue 1. Excitability 2. Contractility 3. Extensibility 4. Elasticity 7 8 Muscle Functions Movement Production Maintain Posture and Body Position Joint Stabilization Heat Generation Additional Functions Organ Protection Valve formation Pupil constriction 9 Check Your Understanding When describing muscle, what does striated mean? Andrew is pondering an exam question that asks, Which muscle type has elongated cells and is found in the walls of the urinary bladder? How should he respond Reed? 3

10 Skeletal Muscle Describe the Gross Structure of a Skeletal Muscle Describe the microscopic structure and functional roles of the myofibrils, sarcoplasmic reticulum, and T tubules of skeletal muscle fibers Describe the sliding filament model of muscle contraction Gross Anatomy of a Skeletal Muscle Each muscle is a discrete organ Nerve and Blood supply Connective Tissue Sheaths Epimysium Perimysium and fascicles Endomysium Attachments Direct/Fleshy Attachments Indirect Attachments 11 12 Microscopic Anatomy of a Skeletal Muscle Fiber (Cell) Sarcolemma Multinucleate Sarcoplasm Glycosomes Myoglobin 4

13 Microscopic Anatomy of a Skeletal Muscle Fiber Myofibrils Striations, Sarcomeres, and Myofilaments. Dark A Bands H Zone M Line Light I Bands Z Disc Sarcomeres Myofilaments Thick Filaments (myosin) Thin Filaments (actin) 14 Molecular Composition of Myofilaments Thick Filaments Myosin Elastic Filaments (Titin) Thin Filaments Actin Tropomyosin Troponin Sarcoplasmic Reticulum and T- Tubules Sarcoplasmic Reticulum (SR) Most tubules run longitudinully Terminal Cistern Pairs T-Tubules Continuous with the extracellular fluid Form Triads with the terminal cistern pairs Extension of the sarcolemma 15 5

Sliding Filament Model of Contraction In a relaxed muscle fiber, thick and thin filaments overlap only at the ends of the A band. 16 The sliding filament model states that during contraction, the thin filaments slide past the thick filaments so that the actin and myosin filaments overlap to a greater degree. Pg. 285 http://www.youtube.com/ watch?v=ct8abzn_a8a 17 Check Your Understanding How does the Term Epimysium relate to the role and position of this connective tissue sheath? Which Myofilaments have binding sites for calcium? What specific molecule binds calcium? Which region or organelle -cytosol, mitochondrion, or SRcontains the highest concentration of calcium ions in a resting muscle fiber? Which structure provides the ATP needed for muscle activity? 18 Physiology of Skeletal Muscle Fibers Explain how muscle fibers are stimulated to contract by describing events that occur at the neuromuscular junction. Describe how an Action Potential is Generated Follow the events of excitation-contraction coupling that lead to cross bridge activity. 6

19 Activation and Excitation-Contraction Coupling Activation Step 1: The fiber must be activated, that is, stimulated by a nerve ending so that a change in membrane potential occurs. Step 2: Next, it must generate an electrical current, called an action potential, in its sarcolemma. Excitation-Contraction Coupling Step 3: The action potential is automatically propagated along the sarcolemma. Step 4: Then, intracellular calcium ion levels must rise briefly, providing the final trigger for contraction. The Nerve Stimulus and Events at the NMJ NMJatatheNMJ Somatic Motor Neurons Neuromuscular Junction Synaptic Cleft Synaptic Vesicles (ACh) 20 How does a motor neuron stimulate a skeletal muscle fiber? Step 1: When a nerve impulse reaches the end of an axon, the axon terminal releases ACH into the synaptic cleft Step 2: ACh diffuses across the cleft and attaches to ACh receptors on the sarcolemma of the muscle fiber. Step 3: ACh binding triggers electrical events that ultimately generate an action potential 21 Generation of an Active Potential Across the Sarcolemma Action Potential: The predictable sequence of electrical changes across a membrane. Step 1: Generation of an end plate potential Step 2: Depolarization: Generation and Propagation of an action potential Step 3: Repolarization: Restoring the Sarcolemma to its original state 7

22 Excitation-Contraction Coupling Step 1: Action Potential Propagation Step 2: Calcium ion release Step 3: Calcium binds to Troponin and removes the blocking action of tropomyosin Step 4: Contraction Begins 23 Cross Bridge Cycling http://www.youtube.com/watch?v=ct8abzn_a8a 24 Check Your Understanding What are the three structural components of a neuromuscular junction? What is the final trigger for contraction? What is the initial trigger? What prevents the filaments from sliding back to their original position each time a myosin cross bridge detaches from actin? What would happen if a muscle fiber suddenly ran out of ATP when sarcomeres had only partially contracted? 8

25 Contraction of Skeletal Muscle Define motor unit and muscle twitch, and describe the events occurring during the three phases of muscle twitch. Explain how smooth, graded contractions of a skeletal muscle are produced. Differentiate between isometric and isotonic contractions. Types of Muscle Contraction Muscle tension versus load Isometric versus isotonic 26 The Motor Unit 27 One motor neuron and all of its innervated fibers Innervated fibers are spread throughout entire muscle 9

28 The Muscle Twitch The motor units response to a single action potential from its motor neuron 3 Phases of a twitch myogram Phase 1: Latent Period Phase 2: Period of Contraction Phase 3: Period of Relaxation Graded Muscle Responses Can be Graded in two ways 1.) By changing the frequency of stimulation Temporal summation unfused (incomplete) tetanus fused (complete) tetanus 2.) By changing the strength of stimulation Recruitment (multiple motor unit summation) Sub threshold stimuli threshold stimulus maximal stimulus 29 Size Principle 30 The motor units with the smallest muscle fibers are activated first As motor units with larger and larger muscle fibers begin to be excited, contractile strength increases. The largest motor units are only activated when maximal contraction is required. Prevents fatigue due to asynchronous contraction 10

31 Isotonic and Isometric Contractions Isotonic: Muscle length changes Concentric: Muscle shortens Eccentric: Muscle Lengthens Isometric: Muscle length does not change 32 Check your understanding What is a motor unit What is happening in a muscle during the latent period of a twitch contraction? Matt is competing in a chin up competition, What type of muscle contractions are occurring in his biceps muscles? 33 Muscle Metabolism Describe three ways in which ATP is regenerated during skeletal muscle contraction. Define EPOC and muscle fatigue. List possible causes of muscle fatigue. 11

34 Providing Energy for Muscle Contraction ATP is the only energy source used directly for contractile activities Muscles store only 4-6 seconds worth Therefore ADP must be converted to ATP as quickly as ATP is used as energy 3 Pathways Pathway #1 Direct Phosphorylation of ADP by Creatine Phosphate Creatine Phosphate + ADP -----------> Creatine + ATP Pathway is viable for roughly 15 seconds 35 36 Pathway #2 Anaerobic Pathway: Glycolysis and Lactic Acid Formation Glucose is broken down in to two Pyruvic acid molecules releasing 2 ATP molecules Glycolysis occurs both in the presence and absence of oxygen Viable as a primary energy source for 30-40 seconds Ordinarily the pyruvic acid byproducts enter the mitochondria for further metabolism However At 70% maximal contractile activity blood vessels are compressed preventing aerobic mitochondrial metabolism. Under these circumstances (anaerobic glycolysis) most of the pyruvic acid is converted to lactic acid 12

37 Pathway #3 Aerobic Respiration During rest, light, and moderate exercise, this pathway provides 95% of ATP supply. Occurs in the mitochondria Requires oxygen Glucose + Oxygen --------> Carbon Dioxide + water + 32 ATP Slowest of three systems Fuel source progression: 1. Muscle Glycogen 2. Bloodborne glucose, pyruvic acid, free fatty acids 3. After 30 minutes, free fatty acids are the primary source of fuel 38 Energy Systems During Sport Aerobic Endurance Anaerobic Threshold Weightlifting: Direct Phosphorylation On off activities such as tennis, soccer, 100m swim: Anaerobic Prolonged jogging: Aerobic 39 Muscle Fatigue Physiological inability to contract in the presence of stimuli Caused by ionic disturbances that alter E-C coupling 13

Excess Post-exercise Oxygen Consumption (EPOC) Post exercise, muscle tissue must replenish its myoglobin bound oxygen reserves convert excess lactic acid into pyruvic acid replace glycogen stores Resynthesize ATP and creatine phosphate reserves The increased oxygen demand during this recovery period is referred to as the EPOC or oxygen debt 40 41 Heat Production Only 40% of energy used during muscle contraction is converted into useful work 60% is converted into heat 42 Check Your Understanding Clayton has just finished jogging and is breathing heavily. Why is Clayton breathing heavily? What metabolic product might account for his sore muscles and muscle weakness? 14

43 Forces of Muscle Contraction Describe factors that influence the force, velocity, and duration of skeletal muscle contraction Describe three types of skeletal muscle fibers and explain the relative value of each type 44 Muscle Contraction Force Influencing Factors Number of fibers recruited Size of muscle fibers Frequency of stimulation Degree of muscle stretch 45 Velocity and Duration of Contraction Influencing factors Muscle Fiber Type Load Recruitment 15

46 Muscle Fiber Type Classified based on two criteria Speed of contraction Slow fibers Fast Fibers Major pathways for forming ATP Glycolytic Oxidative 47 3 Fiber Types Slow Oxidative Fast Oxidative Fast Glycolytic 48 Load Greater load results in a longer latent period a slower contraction a shorter duration of muscle contraction 16

49 Recruitment The greater number of motor units recruited The faster the contraction The more prolonged the contraction 50 Check Your Understanding List two factors that influence contractile force and two that influence velocity of contraction Adaptations to Exercise 51 Compare and Contrast the effects of aerobic and resistance exercise on skeletal muscles and on other body systems 17

52 Aerobic (endurance) Exercise Number of capillaries surrounding the muscle fibers increases Number of mitochondria within the muscle fibers increases Concentration of myoglobin increases Affects all fiber types, conversion is possible 53 Resistance Exercise Causes muscle hypertrophy Muscle fibers increase in diameter, not number 54 Study guide 18