1/4/2017. Introduction. Connective Tissue Coverings. 9.1: Structure of a Skeletal Muscle. Skeletal Muscle Fibers. Connective Tissue Coverings

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1 Introduction Chapter 09 Lecture Outline See separate PowerPoint slides for all figures and tables preinserted into PowerPoint without notes. Copyright McGraw-Hill Education. Permission required for reproduction or display. 1 There are 3 types of muscle tissue in the muscular system: Skeletal muscle: Attached to bones of skeleton Voluntary (consciously controlled) Cardiac muscle: Makes up most of the wall of the heart Involuntary (non-consciouslycontrolled) Responsible for pumping action of the heart Smooth muscle: Found in walls of internal organs, such as those of digestive tract Involuntary (non-consciouslycontrolled) 2 9.1: Structure of a Skeletal Muscle Skeletal Muscles: Attach to bones, and skin of face Under conscious control (voluntary) A skeletal muscle is an organ of the muscular system Skeletal muscles are composed of: Skeletal muscle tissue Nervous tissue Blood Connective tissues Connective Tissue Coverings Connective tissue coverings over skeletal muscles: Fascia Tendons Aponeuroses 3 4 Connective Tissue Coverings Muscle coverings: Epimysium: surrounds whole muscle Perimysium: surrounds fascicles within a muscle Endomysium: surrounds muscle fibers (cells) within a fascicle Skeletal Muscle Fibers Skeletal muscle fiber = muscle cell Multinucleated Sarcolemma Sarcoplasm Many myofibrils Myofibrils consist of: - thin actin filaments - thick myosin filaments Sarcomeres Sarcoplasmic reticulum (SR) Transverse ( T ) tubule Triad: 1 T tubule and 2 SR cisternae 5 6 1

2 Skeletal Muscle Fibers Skeletal Muscle Fibers Myofibrils consist of sarcomeres connected end-to-end Striation pattern is made by arrangement of myofilaments in myofibrils Sarcomeres contain these structures: I band (thin filaments) A band (thick & thin filaments) H zone (thick filaments) Z line (or Z disc) M line 7 Striation pattern has 2 main parts: I Band: Light band, composed of thin actin filaments A Band: Dark band, composed of thick myosin filaments overlapping with thin actin filaments H Zone: Center of A band; composed of thick myosin filaments Z Line: Anchors filaments in place; sarcomere boundary; center of I band M Line: Anchors thick filaments; center of A band 8 Skeletal Muscle Fibers Thick filaments: Composed of myosin protein; heads form crossbridges Thin filaments: Composed of actin protein; associated with troponin and tropomyosin, which prevent crossbridge formation when muscle is not contracting 9.2: Skeletal Muscle Contraction Contraction of a muscle fiber: Requires interaction from several chemical and cellular components Results from a movement within the myofibrils, in which the actin and myosin filaments slide past one another, shortening the sarcomeres Muscle fiber shortens and pulls on attachment points 9 10 Neuromuscular Junction Stimulus for Contraction Neuromuscular Junction: A type of synapse Also called a myoneural junction Site where an axon of motor neuron and skeletal muscle fiber interact Skeletal muscle fibers contract only when stimulated by a motor neuron Parts of a NMJ: Motor neuron Motor end plate Synaptic cleft Synaptic vesicles Neurotransmitters 11 Acetylcholine (ACh) is the neurotransmitter Nerve impulse causes release of ACh from synaptic vesicles ACh binds to ACh receptors on motor end plate ACh causes changes in membrane permeability to Na + and K + ions, which generates a muscle impulse (action potential) Impulse causes release of Ca +2 from SR, which leads to muscle contraction 12 2

3 Clinical Application 9.1 Myasthenia Gravis (MG) MG is an autoimmune disorder Antibodies attack Acetylcholine receptors on skeletal muscle fibers (motor end plates) in neuromuscular junctions Person may have only 1/3 normal number of ACh receptors Leads to widespread muscle weakness and muscle fatigue Treatments: - Drugs that inhibit acetylcholinesterase - Immunosuppressant drugs - Administering antibodies that inactivate harmful antibodies - Plasma exchange 13 Excitation-Contraction Coupling Excitation-Contraction Coupling: Connection between muscle fiber stimulation and muscle contraction During muscle relaxation: Ca +2 ions are stored in SR Troponin-tropomyosin complexes cover binding sites on actin filaments Upon muscle stimulation: Muscle impulses cause SR to release Ca +2 ions into cytosol Ca +2 ion binds to troponin to change its shape The position of tropomyosin is altered Binding sites on actin are now exposed Myosin heads bind to actin, forming cross-bridges 14 Excitation-Contraction Coupling The Sliding Filament Model Sliding Filament Model of Muscle Contraction: When sarcomeres shorten, thick and thin filaments slide past one another H zones and I bands narrow Z lines move closer together Thin and thick filaments do not change length Overlap between filaments increases Myosin head attaches to actin binding site, forming cross-bridge Myosin cross-bridge pulls thin filament toward center of sarcomere ADP and phosphate are released from myosin New ATP binds to myosin Linkage between actin and myosin cross-bridge break ATP splits Myosin cross-bridge goes back to original position Cross-Bridge Cycling 17 Relaxation When neural stimulation of muscle fiber stops: Acetylcholinesterase (enzyme) rapidly decomposes ACh remaining in the synapse Muscle impulse stops when ACh is decomposed Stimulus to sarcolemma and muscle fiber membrane ceases Calcium pump moves Ca +2 back into sarcoplasmic reticulum (SR) Troponin-tropomyosin complex again covers binding sites on actin Myosin and actin binding are now prevented Muscle fiber relaxes 18 3

4 Energy Sources for Contraction 1. ATP reserves: small amount 2. Creatine phosphate: initial source of energy to regenerate ATP from ADP and P 3. Cellular respiration Oxygen Supply & Cellular Respiration Cellular respiration: Anaerobic Phase: Glycolysis Occurs in cytoplasm Produces little ATP Aerobic Phase: Citric acid cycle and electron transport system Occurs in the mitochondria Produces the most ATP Myoglobin stores extra oxygen in muscles Oxygen Debt Oxygen Debt During rest or moderate exercise, respiratory & cardiovascular systems supply enough O 2 to support aerobic respiration Anaerobic (Lactic Acid) Threshold: Shift in metabolism from aerobic to anaerobic, during strenuous muscle activity, when the above systems cannot supply the necessary O 2. Lactic acid is produced. Oxygen debt: Amount of oxygen needed by liver cells to convert the accumulated lactic acid to glucose, and to restore muscle ATP and creatine phosphate concentrations Muscle Fatigue: Inability to contract muscle Muscle Fatigue Common causes of muscle fatigue: Decreased blood flow Ion imbalances across the sarcolemma Loss of desire to continue exercise Accumulation of lactic acid (controversial) Heat Production Heat is a by-product of cellular respiration in active cells Muscle cells are major source of body heat More than half the energy released in cellular respiration becomes heat; less than half is transferred to ATP Blood transports heat throughout body core Muscle Cramp: Sustained, involuntary muscle contraction May be caused by changes in electrolyte concentration in extracellular fluids in the area

5 9.3: Muscular Responses Muscle contraction can be observed by removing a single skeletal muscle fiber and connecting it to a device that senses and records changes in the overall length of the muscle fiber. Electrical stimulator promotes the contractions Threshold Stimulus Threshold Stimulus: Minimum strength of stimulation of a muscle fiber required to cause contraction When strength of stimulus reaches threshold, an action potential is generated Impulse spreads through muscle fiber, releasing Ca +2 from SR and activating crossbridge formation One action potential from a motor neuron releases enough ACh to produce threshold stimulus in muscle fiber, causing a muscle impulse Recording of a Muscle Contraction Twitch: Contractile response of a single muscle fiber to a single impulse Latent period Period of contraction Period of relaxation Length-Tension Relationship Length of muscle fiber before stimulation determines amount of force it can develop Optimum starting length is resting length of the muscle fiber; this allows the greatest force to develop Stretched muscle fibers develop less force, since some myosin heads cannot reach binding sites on actin Shortened muscle fibers also develop less force, since compressed sarcomeres cannot shorten further Summation Summation: Process by which the force of individual muscle fiber twitches combine Produces sustained contractions Can lead to partial or complete tetanic contractions Recruitment of Motor Units Motor Unit: A motor neuron + all of the muscle fibers it controls A whole muscle consists of many motor units Coarse movements are produced with large numbers of fibers in a motor unit Precise movements are produced with fewer muscle fibers in a motor unit

6 Recruitment of Motor Units Recruitment: Increase in the number of motor units activated, to produce more force Certain motor units are activated first, and others are activated only when the intensity of stimulus increases As intensity of stimulation increases, recruitment of motor units continues until all motor units are activated Sustained Contractions Smaller motor units (smaller diameter axons) - recruited first Larger motor units (larger diameter axons) - recruited later Summation and recruitment can produce sustained contractions of increasing strength Whole muscle contractions are smooth movements Muscle tone (tonus): Continuous state of partial contraction in resting muscles Types of Contractions Fast- and Slow-Twitch Muscle Fibers Isotonic: muscle contracts and changes length; equal force Concentric: shortening contraction Eccentric: lengthening contraction Isometric: muscle contracts but does not change length; change in force 33 Slow-twitch fibers (Type I): Always oxidative Resistant to fatigue Red fibers Abundant myoglobin Good blood supply Many mitochondria Slow ATPase activity; slow to contract Fast-twitch glycolytic fibers (Type IIb): Anaerobic respiration (glycolysis) White fibers (less myoglobin) Poorer blood supply Fewer mitochondria than fast-twitch More SR than fast-twitch Susceptible to fatigue Fast ATPase activity; contract rapidly Fast-twitch fatigue-resistant fibers (Type IIa): Intermediate twitch fibers Intermediate oxidative capacity Intermediate amount of myoglobin White fibers Resistant to fatigue Rapid ATPase activity 34 Clinical Application Use and Disuse of Skeletal Muscles Hypertrophy: Enlargement of skeletal muscle that is exercised Atrophy: Decrease in size and strength of skeletal muscle that is unused Aerobic exercise stimulates slow-twitch fibers. In response, fibers increase their capillaries and mitochondria. 9.4: Smooth Muscle Compared to skeletal muscle fibers, smooth muscle fibers are: Shorter Single, centrally located nucleus Elongated with tapering ends Myofilaments randomly organized Lack striations Lack transverse tubules Sarcoplasmic reticulum (SR) not well developed Forceful exercise stimulates mainly fast-twitch fibers. In response, fibers produce new actin & myosin filaments, and the muscle enlarges

7 Types of Smooth Muscle 2 types of smooth muscle: Multi-unit Smooth Muscle: Cells are less organized Function as separate units Fibers function independently Iris of eye, walls of blood vessels Stimulated by neurons, hormones Visceral Smooth Muscle: Single-unit smooth muscle; cells respond as a unit Sheets of spindle-shaped muscle fibers Fibers held together by gap junctions Exhibit rhythmicity Conduct peristalsis Walls of most hollow organs More common type of smooth muscle 37 Smooth Muscle Contraction Resembles skeletal muscle contraction in these ways: Interaction between actin and myosin Both use calcium and ATP Both are triggered by membrane impulses Different from skeletal muscle contraction in these ways: Smooth muscle lacks troponin; uses calmodulin instead Two neurotransmitters affect smooth muscle: Acetylcholine (Ach) and norepinephrine (NE) Hormones can stimulate or inhibit smooth muscle Stretching can trigger smooth muscle contraction Smooth muscle slower to contract and relax Smooth muscle more resistant to fatigue Smooth muscle can change length without changing tautness : Cardiac Muscle Characteristics of Muscle Tissues Cardiac Muscle: Located only in the heart Striated muscle cells Muscle fibers joined together by intercalated discs Fibers branch, contain a single nucleus Network of fibers contracts as a unit (syncytium) Self-exciting and rhythmic Longer refractory period than skeletal muscle No sustained or tetanic contractions : Skeletal Muscle Actions Skeletal muscles generate a great variety of body movements The action of each muscle mostly depends upon - the type of joint it is associated with - the way the muscle is attached on either side of the joint Body Movement When bones or body parts move, bones and muscles act as levers. 4 basic components of levers: Rigid bar or rod (bones) Fulcrum or pivot on which bar moves (joint) Object moved against resistance (weight) Force that supplies energy for movement (muscles)

8 Levers and Movement Origin and Insertion One end of a skeletal muscle is more fixed, and the other end is more movable: Origin: less movable end Insertion: more movable end When a muscle contracts, insertion is pulled toward origin Interaction of Skeletal Muscles Most skeletal muscle function in groups. Roles of muscles in performing certain actions: Agonist: muscle that causes an action 9.7: Major Skeletal Muscles Anterior View of the Superficial Skeletal Muscles: Prime mover: agonist primarily responsible for movement (In some cases, the terms agonist and prime mover are used interchangeably) Synergists: muscles that assist agonist / prime mover Antagonist: muscles whose contraction causes movement in the opposite direction of the prime mover Posterior View of the Superficial Skeletal Muscles: Major Skeletal Muscles 9.8: Lifespan Changes Myoglobin, ATP, and creatine phosphate decline, starting in the 40s Connective tissue and adipose cells replace some muscle tissue By age 80, almost half of muscle mass has atrophied Muscle strength decreases, and reflexes become slower Exercise helps to maintain muscle mass and function