Anatomy & Physiology Chapter 10 Muscle Tissue PDF

Chapter 10 Muscle Tissue 1  Functions of Skeletal Muscle 1. Move the body Move bones, make facial expressions, speak, breathe, swallow 2. Maintain posture Stabilize joints, maintain body position 3. Protect and support Package internal organs and stabilize...

Chapter 10 Muscle Tissue 1  Functions of Skeletal Muscle 1. Move the body Move bones, make facial expressions, speak, breathe, swallow 2. Maintain posture Stabilize joints, maintain body position 3. Protect and support Package internal organs and stabilize them 4. Regulate elimination of materials Sphincters control passage of materials at orifices 5. Produce heat Help maintain body temperature 9-2  General Properties of Muscle All muscles share four main characteristics: – Excitability: capacity of muscle to respond to a stimulus (from our nerves) – Conductivity: ability to send an electrical charge down the length of the cell membrane – Contractility: ability of a muscle to shorten with force – Extensibility: muscle can be stretched to its normal resting length and beyond to a limited degree – Elasticity: ability of muscle to recoil to original resting length after stretched 9-3  Gross Anatomy of Skeletal Muscle Each skeletal muscle is an organ – Multiple types of tissues working together: skeletal muscle fibers, connective tissue, blood vessels, and nerves Muscle fibers bundled within a fascicle – A whole muscle contains many fascicles – A fascicle consists of many muscle fibers © Ed Reschke Myo, mys, and sarco are prefixes for muscle 9-4 Gross Anatomy of Skeletal Muscle Connective Tissue Muscle coverings: – Endomysium – surrounds muscle fibers (cells) within fascicle – Perimysium – surrounds fascicles within muscle – Epimysium – surrounds whole muscle  Gross Anatomy of Skeletal Muscle Muscles span joints and attach to bones Attachments can be direct or indirect Aponeuroses – Direct (fleshy): endomysium fused to periosteum of bone or perichondrium of cartilage – Indirect: connective tissue wrappings extend beyond muscle as ropelike Skeletal muscles tendon or sheetlike aponeurosis Deep fascia is dense irregular CT that Tendons separates individual muscles and binds muscles with similar functions – Continuous with epimysium 6 Gross Anatomy of Skeletal Muscle Muscular fascia Each muscle receives a nerve, (surrounds individual muscles and groups of muscles) Epimysium Artery Vein (surrounds muscles) Perimysium artery, and veins (surrounds fasciculi) Nerve Endomysium – Skeletal muscle is innervated by (surrounds muscle fibers) Muscle somatic motor neurons fiber Artery Nerve Contracting muscle fibers Vein require huge amounts of Fasciculus Capillary oxygen and nutrients – Highly vascularized to provide nutrients and remove wastes Axon of motor neuron Synapse or neuromuscular junction Microscopic Anatomy of Skeletal Muscle Sarcoplasm (cytoplasm) Multinucleate from the fusion of myoblasts Sarcolemma (plasma membrane) Myofibrils Cisternae of Myofibril Nucleus sarcoplasmic reticulum Transverse tubule Triad – Actin myofilaments – Myosin myofilaments Sarcoplasmic reticulum Openings into transverse tubules Mitochondria Nucleus Thick and thin filaments Sarcolemma Sarcoplasm 8  Microscopic Anatomy of Skeletal Muscle T tubules – Formed by protrusion of sarcolemma deep into cell interior Increase muscle fiber’s surface area greatly – Important in impulse transmissions deep into interior of cell – Tubules run between terminal cisterns of SR to form triads Notice their orientation around each myofibril… 9  Microscopic Anatomy of Skeletal Muscle Sarcoplasmic reticulum: network of smooth endoplasmic reticulum tubules surrounding each myofibril – SR functions in regulation of intracellular Ca2+ levels Triad Myofibrils Cisternae of – Transverse (‘T’) tubule sarcoplasmic reticulum Triad Nucleus Transverse tubule – Cisternae of SR Sarcoplasmic reticulum Openings into transverse tubules Mitochondria Nucleus Thick and thin filaments Sarcolemma 10  Microscopic Anatomy of Skeletal Muscle Triad – Transverse (‘T’) tubule – These membranes have integral proteins important in producing action during an impulse – Cisternae of SR – Associated with T-tubules – Contains calcium release channels triggered by electrical signal traveling down T-tubule 11  Microscopic Anatomy of Skeletal Muscle Myofibrils are composed of neatly arranged myofilaments that interact with each other to cause contraction – Thick filaments consist of many myosin protein molecules with heads that have an affinity to actin – Thin filaments are twisted strands of actin proteins with tropomyosin and troponin as regulatory proteins 12  Microscopic Anatomy of Skeletal Muscle Organization of a sarcomere – Myofilaments arranged in repeating units called sarcomeres Smallest contractile unit (functional unit) of muscle fiber – Composed of overlapping thick and thin filaments 9-13 Microscopic Anatomy of Skeletal Muscle Striated appearance due to numerous sarcomeres – Delineated at both ends by Z discs Anchors for thin filaments – The positions of thin and thick filaments give rise to alternating I- bands and A-bands – A bands: dark regions where the filaments overlap – I bands: lighter regions at the ends of the sarcomeres where actin is tethered 14 Structure of a Sarcomere H zone: lighter region in middle of dark band where myosin is bound – M line: center of the sarcomeres Z disc (line): sheet of proteins on midline I band Connectin includes an elastic filament of the protein titin Extends from Z disc to M line Maintain alignment of sarcomeres Dystrophin links thin filaments to proteins of sarcolemma 9-15  Microscopic Anatomy of Skeletal Muscle Muscle fibers have abundant mitochondria for aerobic ATP production – Myoglobin within cells allows storage of oxygen used for aerobic ATP production – Glycogen is stored for when fuel is needed quickly – Creatinine phosphate can quickly give up its phosphate group to help replenish ATP supply 16  Innervation of Skeletal Muscle Motor unit: a single motor neuron and all muscle fibers innervated by it Motor neuron – Axons of motor neurons from spinal cord (or brain) of motor unit 2 innervate numerous muscle fibers Motor neuron Small motor units have less than of motor unit 1 five muscle fibers » Allow for precise control of force output Large motor units have thousands of muscle fibers » Allow for production of large amount of force Muscle fibers of a motor unit are Branches of motor neuron spread throughout the whole muscle Skeletal muscle axon fibers Neuromuscular Junction Site where an axon and muscle fiber meet Parts – Synaptic knob – Motor end plate – Synaptic cleft  Neuromuscular Junction Synaptic knob – Expanded tip of the motor neuron axon – Houses synaptic vesicles Small sacs filled with neurotransmitter acetylcholine (ACh) Motor end plate – Specialized region of sarcolemma with numerous folds – Has many ACh receptors on ligand gated channels Synaptic cleft – Narrow fluid-filled space – Acetylcholinesterase resides here 19  Skeletal Muscle Fibers at Rest Muscle fibers function by “potentials” Muscle fibers resting membrane potential (RMP) – Membrane voltage difference across membranes (polarized) – Must exist for action potential to occur Membrane proteins required to establish and maintain RMP – Ligand-gated – Voltage-gated – Leak channels are always open – Each protein is specific for certain ions! 20  Overview of Events in Skeletal Muscle Contraction Three steps must occur for skeletal muscle to contract: 1. Muscle fiber excitation 2. Excitation-contraction coupling 3. Cross bridge cycling 21  Events at the Neuromuscular Junction 1. AP arrives at axon terminal 2. Voltage-gated calcium channels open, calcium enters motor neuron 3. Calcium entry causes release of ACh neurotransmitter into synaptic cleft 4. ACh diffuses across to ACh receptors (Na+ chemical gates) on sarcolemma 5. ACh binding to receptors, opens gates, allowing Na+ to enter resulting in end plate potential 6. Acetylcholinesterase degrades ACh 22  Muscle Fiber Excitation Action potential is caused by changes in electrical charges Occurs in three steps 1. Generation of end plate potential ACh binds receptors and opens Na+ gated channels 2. Depolarization Voltage gated channels open for more Na+ 3. Repolarization Voltage gated Na+ channels close and Voltage gated K+ channels open  Excitation-Contraction Coupling Stimulation of the fiber is coupled with the sliding of filaments Action Potential across the membrane and down the T-tubules causes the release of Ca2+ from the sarcoplasmic reticulum 24  Excitation-Contraction Coupling AP is propagated along sarcolemma and down into T tubules, where voltage-sensitive proteins in tubules stimulate Ca2+ release from SR Ca2+ release leads to contraction AP is brief and ends before contraction is seen 25 Action Potentials Action potentials result when the motor end plate depolarizes enough to open voltage gated channels Depolarization results from an increase in the permeability of the plasma membrane to Na+ – Such as AcH binding to a receptor that holds the gate open for it!!! An all-or-none action potential is produced if depolarization reaches threshold 9-26 Action Potentials Threshold is when voltage gated Na+ channels open Repolarization phase occurs when those channels close, K+ channels open, and the resting potential is established. 9-27  Excitation-Contraction Coupling AP is propagated along sarcolemma and down into T tubules, where voltage-sensitive proteins in tubules stimulate Ca2+ release from SR Ca2+ release leads to contraction by interacting with the myofilaments 28  Actin and Myosin Myofilaments Sarcomere Cross-bridge M line Actin myofilament Myosin myofilament Titin Z disk Z disk (a) Myosin molecule F actin molecules Troponin Tropomyosin Active sites Actin (thin) myofilament Myosin (thick) myofilament (b) Myosin Rod portion Troponin Tropomyosin Coiled portion of the two α helices Myosin light chains G actin molecules Hinge region Binds to Binds to Two myosin heavy chains of myosin Active G actin tropomyosin Binds sites to Ca 2+ (c) 9-29  Myosin Myofilaments Each thick filament consists of many myosin molecules whose heads protrude at opposite ends of the filament 9-30 Actin Myofilaments A thin filament consists of two strands of actin subunits twisted into a helix plus two types of regulatory proteins troponin tropomyosin 9-31  Excitation-Contraction Coupling Action Potentials cause SR to release calcium ions into cytosol – Calcium binds to troponin to change its shape – The position of tropomyosin is altered – Binding sites on actin are now exposed 32 32  Cross Bridge Cycling – Myosin head attaches to actin binding site, forming cross-bridge – Phosphate released from myosin 33  Cross Bridge Cycling – Myosin cross-bridge pulls thin filament – ADP released from myosin 34  Cross Bridge Cycling – New ATP binds to myosin – Linkage between actin and myosin cross-bridge break 35  Cross Bridge Cycling – ATP splits – ADP and Phosphate remain attached to myosin head 36 Cross Bridge Cycling – Myosin cross-bridge goes back to original position – If Ca+ is still present, the heads bind to actin again and the process repeats 37 Cross-Bridge Movement 9-38  Sarcomere Shortening Sarcomeres are the functional units of muscle cells When sarcomeres shorten, thick and thin filaments slide past one another – H zones and I bands narrow – Z lines move closer together Thick and thin filaments remain the same length but slide past each other – Known as the sliding filament theory 39  Relaxation Acetylcholinesterase – rapidly decomposes Ach remaining in the synapse – Muscle impulse stops – Calcium moves back into sarcoplasmic reticulum (SR) – Myosin and actin binding prevented – Muscle fiber relaxes 40  Rigor ATP is also needed for cross bridge detachment When tissues die, membranes can no longer maintain their RMP – Ca+ leaks out into the sarcoplasm and myosin and actin bind – With no ATP available, myosin head stays bound to actin, causing constant state of contraction Muscles stay contracted until muscle proteins break down, causing myosin to release 41  Energy for Contraction ATP supplies the energy needed for the muscle fiber to: – Move and detach cross bridges – Pump calcium back into SR – Pump Na+ out of and K+ back into cell after excitation- contraction coupling Available stores of ATP depleted in 4–6 seconds ATP is the only source of energy for contractile activities; therefore it must be regenerated quickly  Energy for Contraction ATP is regenerated quickly by three mechanisms: – Creatine phosphate – Glycolysis – Aerobic cellular respiration  Energy for Contraction Creatine phosphate – Contains a high-energy bond between creatine and phosphate – Phosphate can be transferred to ADP to form ATP – Catalyzed by creatine kinase When cellular ATP is high When cellular ATP is low Creatine P ADP Creatine P ADP Creatine ATP Creatine ATP 44  Energy for Contraction Glycolysis does not require oxygen Glucose broken down into two pyruvate molecules Net production of 2 ATP and 2 NADH molecules Glycolysis can be repeated with lactic acid fermentation Fermentation oxidizes NADH so that it can repeat glycolysis Pyruvate is reduced to lactate Lactate may be utilized by muscle fibers if oxygen is adequate Lactic acid cycle - cycling of lactate to liver where it’s converted to glucose, and transport of glucose back to muscle  Energy for Contraction Cellular respiration: Glucose 2 In the absence of Energy 2 ATP Aerobic Phase sufficient oxygen, glycolysis leads to Cytosol lactic acid accumulation. Pyruvic acid Lactic acid – Citric acid cycle – Electron transport system Mitochondria – Occurs in the mitochondria 1 Oxygen carried from the lungs by Citric acid cycle – Produces most ATP hemoglobin in red blood cells is stored in muscle cells by – Myoglobin stores extra oxygen myoglobin and is available to support aerobic respiration. – Glycogen stores extra glucose Electron transport chain Synthesis of 34 ATP CO2 + H2O + Energy Heat 46  Energy Supply & Varying Intensity of Exercise For a 50-meter sprint (less than 10 seconds): ATP supplied primarily by phosphate transfer system For a 400-meter sprint (less than a minute): ATP supplied primarily by glycolysis after first few seconds For a 1500-meter run (more than a minute): ATP supplied primarily by aerobic processes after first minute 47  Oxygen Debt For a muscle to return to its pre-exercise state: – Oxygen reserves are replenished on hemoglobin and myoglobin – Glycogen stores are replaced – ATP and creatine phosphate reserves are resynthesized – Lactic acid is reconverted to pyruvic acid or glucose All replenishing steps require extra oxygen, so this is referred to as excess postexercise oxygen consumption (EPOC) Classification of Skeletal Muscle Fibers Skeletal muscle fibers classified based on: 1. Type of contraction generated 2. Means for supplying ATP Type of contraction generated – Differences in power, speed, and duration Power related to diameter of muscle fiber Speed and duration related to type of myosin ATPase, quickness of action potential propagation, and quickness of Ca2+ release and reuptake by sarcoplasmic reticulum – Fast-twitch fibers are more powerful and have quicker and briefer contractions than slow twitch fibers 49 Classification of Skeletal Muscle Fibers Skeletal muscle fibers classified based on: 1. Type of contraction generated 2. Means for supplying ATP Means for supplying ATP – Oxidative versus glycolytic fibers Oxidative fibers (fatigue-resistant) use aerobic cellular respiration – Extensive capillaries, many mitochondria, large supply of myoglobin – Red fibers Glycolytic fibers (fatigable) use anaerobic cellular respiration – Fewer capillaries, fewer mitochondria, small supply of myoglobin, large glycogen reserves – White fibers 50  Classification of Skeletal Muscle Fibers Based on these two criteria, skeletal muscle fibers can be classified into three types: 1. Slow oxidative (SO) fibers (type I) Contractions are slower and less powerful High endurance since ATP supplied aerobically About half the diameter of other fibers, red in color due to myoglobin 2. Fast oxidative (FO) fibers (type IIa, intermediate) Contractions are fast and powerful Primarily aerobic respiration, but delivery of oxygen lower Intermediate size, light red in color 3. Fast glycolytic (FG) fibers (type IIx, fast anaerobic) Contractions are fast and powerful Contractions are brief, as ATP production is primarily anaerobic Largest size, white in color due to lack of myoglobin 51 Distribution of Skeletal Muscle Fiber Types A single muscle contains a mixture of fiber types There are variations in proportions – Hand muscles have a high percentage of fast glycolytic fibers for quickness – Back muscles have a high percentage of slow oxidative fibers to continually maintain postural support – Long-distance runners Higher proportion of slow-oxidative fibers in legs – Sprinters Higher percentage of fast glycolytic fibers Determined primarily by genes, and partially by training 52  Muscle Tension in Skeletal Muscle Muscle tension – Force generated when a muscle is stimulated to contract – Lab experiments measure tension and graph it (myogram) Access the text alternative for slide images. 53  Muscle Twitch A muscle fiber’s response to a single action potential from motor neuron – Muscle fiber contracts quickly, then relaxes Twitch recorded as a myogram shows three periods – Latent period No change in tension – Contraction period Time when tension is increasing – Relaxation period Time when tension is decreasing Generally lasts a little longer than contraction period  Graded Muscle Responses Normal muscle contraction is relatively smooth, and strength varies with needs – A muscle twitch is seen only in lab setting or with neuromuscular problems, but not in normal muscle Graded muscle responses vary strength of contraction for different demands Responses are graded by: – Wave summation – Recruitment  Summation Process by which individual twitches combine Occurs when stimuli are received in rapid succession – Fibers have little to no time to relax between potentials – Additional Ca2+ stimulates more shortening Produces sustained contractions Can lead to tetanic contractions 56 Myogram  Recruitment Recruitment - increase in the number of motor units As intensity of stimulation increases, more motor neurons are stimulated – explains how muscles exhibit varying degrees of force Motor units in muscle usually contract asynchronously – Some fibers contract while others rest – Helps prevent fatigue Small motor units are recruited first with the largest last, until all motor units within the muscle are activated  Muscle Tone Constant, slightly contracted state of all muscles – Generated by involuntary nervous stimulation of muscle – Some motor units stimulated randomly at any time – Change continuously so units not fatigued – Do not generate enough tension for movement Keeps muscles firm, healthy, and ready to respond  Muscular Responses Same principles apply to contraction of both single fibers and whole muscles Contraction produces muscle tension, the force exerted on load or object to be moved Contraction may/may not shorten muscle – Isometric contraction: no shortening; muscle tension increases but does not exceed load – Isotonic contraction: muscle shortens because muscle tension exceeds load 59 Types of Contractions Isometric – muscle contracts but does not change length Isotonic – muscle contracts and changes length – Concentric – shortening contraction – Eccentric – lengthening contraction 60  Force of Contraction Depends on number of cross bridges attached, which is affected by four factors: 1. Frequency of stimulation 2. Number of muscle fibers stimulated 3. Relative size of fibers 4. Degree of muscle stretch  Length-Tension Relationship Optimal length is where crossbridges can easily form Overly shortened muscles have no more crossbridges to form Overly stretched muscles can’t form as many cross bridges 62  Muscle Fatigue The reduced ability to produce muscle tension – Primarily caused by a decrease in glycogen stores with prolonged exercise Other possible causes of fatigue – Excessive excitation at neuromuscular junction Insufficient Ca2+ to enter synaptic knob Decreased number of synaptic vesicles – Excitation-contraction coupling Altered ion concentrations impair action potential conduction and Ca 2+ release from sarcoplasmic reticulum – Crossbridge cycling Excessive Pi slows release of Pi from myosin head Less Ca2+ available for troponin (some Ca2+ is bound to Pi) 63  Effects of Exercise Changes in muscle from a sustained exercise program – Endurance exercise leads to better ATP production – More mitochondria, myoglobin and vascularization – Resistance exercise leads to hypertrophy – Increases in size due to addition of contractile proteins – Muscle also increases glycogen reserves and mitochondria – Limited amount of hyperplasia (increase in number of fibers) Changes in muscle from lack of exercise – Atrophy: decrease in size due to lack of use – For example, someone wearing a cast – Initially reversible, but becomes permanent if extreme 64  Effects of Aging Loss of muscle mass with age – Slow loss begins in person’s mid-30s due to decrease in activity – Decreased size, power, and endurance of skeletal muscle – Loss in fiber number and diameter (decrease in myofibrils) – Decreased oxygen storage capacity – Decreased circulatory supply to muscles with exercise Reduced capacity to recover from injury – Decreased number of satellite cells – Fibrosis: muscle mass often replaced by dense regular connective tissue – Decreased flexibility 65  Cardiac Muscle Short, branching fibers One or two nuclei Striated (contain sarcomeres) Aerobic with many mitochondria Intercalated discs join ends of neighboring fibers Contractions started by heart’s autorhythmic pacemaker cells Heart rate and contraction force influenced by autonomic nervous system 66  Cardiac Muscle Fibers Intercalated disks: specialized cell-cell contacts. – Cell membranes interdigitate with numerous gap junctions – Gap junctions allow action potentials to move efficiently Fewer but bigger T-tubules Well developed SR but unlike the SR of skeletal muscle is not closely associated with T-tubules 67  Cardiac Muscle Fibers Electrically, cardiac muscle of the atria and cardiac muscle of the ventricles behave as individual units Action potentials spread from pacemaker cells to other cells by gap junctions Action potentials are of longer duration and longer refractory period because: – SR is not closely associated with T-tubules so there is delay in Ca2+ release from it – Ca2+ partially diffuses into the cell from the sarcolemma adding delay in contraction – These Ca2+ gated channels delay repolarization 68  Smooth Muscles Smooth muscle is found in a variety of organ systems with a variety of roles – Examples: – In blood vessels of cardiovascular system » Helps regulate blood pressure and flow – In bronchioles of respiratory system » Controls airflow to alveoli – In intestines of digestive system » Mixes and propels materials – In ureters of urinary system » Propels urine from kidneys to bladder – In uterus of female reproductive system 69  Smooth Muscle Anatomy Smooth muscle cells have fusiform shape – Wide in the middle with tapered ends Smaller than skeletal muscle fibers Sarcolemma has varied types of Ca2+ channels (gated by chemicals, voltage, etc.) Transverse tubules absent – Surface area increased by caveolae (flasklike invaginations) Sarcoplasmic reticulum sparse – Outside of cell is important source of Ca2+ 70  Smooth Muscle Different from skeletal muscle in the arrangement of anchoring proteins and contractile proteins of smooth muscle – Cytoskeleton composed of extensive intermediate filaments Dense bodies: points where intermediate filaments interact within sarcoplasm Dense plaques: points where intermediate filaments attach on inner sarcolemma – Contractile proteins Attached to dense bodies and dense plaques Oriented at oblique angles Contraction causes a twisting motion – Lack sarcomeres and Z discs (smooth = no striations) 71  Smooth Muscle Like skeletal muscle, smooth muscle filaments have actin and myosin Unlike skeletal muscle, smooth muscle – Filaments have myosin heads along their entire length; can form additional cross bridges – Has calmodulin: protein that binds Ca2+ to trigger contraction – Filaments can perform the latchbridge mechanism: myosin attaches to actin for extended time without using extra ATP – Has myosin light-chain kinase (MLCK): enzyme that phosphorylates myosin heads when activated by calmodulin – Has myosin light-chain phosphatase: enzyme that dephosphorylates myosin head (required for relaxation) 72  Smooth Muscle Contraction Different from skeletal muscle in stimulation: – Smooth muscle lacks NMJ’s (use varicosities) – Numerous smooth muscle cells stimulated simultaneously – Stimulation spread from cell to cell via gap junctions so contraction is synchronous – Hormones affect smooth muscle Acetlycholine (Ach) and norepinephrine (NE) – Stretching can trigger smooth muscle contraction 73  Functional Categories of Smooth Muscle Visceral Smooth Muscle Multi-unit Smooth Muscle – Single-unit smooth muscle – Less organized – Sheets of muscle fibers – Function as separate units – Fibers held together by gap junctions – Fibers function separately – Exhibit rhythmicity – Iris of eye – Exhibit peristalsis – Walls of blood vessels – Walls of most hollow organs – Arrector pili Smooth muscle does not follow all-or-none law Contraction regulated by nervous system and by hormones May have pacemaker cells 74  Smooth Muscle Contraction Long latent period – Takes time to phosphorylate myosin head – Slow ATPase activity Long duration – Slow calcium pumps – Need for dephosphorylation of myosin head Slowness fits its functional requirements – Extended contractions maintain continuous tone Fatigue-resistant – Energy requirements low compared to skeletal muscle Broad length-tension curve – Lacks limitations because no Z discs 75  Study Questions 1. What are the five major functions of skeletal muscle? 2. Compare and contrast the skeletal muscle characteristics of contractility, extensibility, and elasticity. 3. Sketch a diagram of a cross section of a muscle, and label the fascicle, muscle fiber, myofibrils, and their associated connective tissue coverings. 4. Draw and label a diagram of a sarcomere. 5. Place the following gross anatomic and microscopic anatomic structures in order from largest to smallest: fascicle, myofibril, myofilament, muscle, muscle fiber, and sarcomere. Describe their anatomic relationship. 6. Describe a motor unit, and explain why motor units vary in size. 7. Diagram and label the anatomic structures of a neuromuscular junction. 8. What triggers the binding of synaptic vesicles to the synaptic knob membrane to cause exocytosis of ACh? 9. What two events are linked in the physiologic process called excitation-contraction coupling? 10. Describe the events of excitation-contraction coupling. 11. What is the function of Ca2+ in skeletal muscle contraction? 12. What triggers the binding of synaptic vesicles to the synaptic knob membrane to cause exocytosis of ACh? 76  Study Questions 13. What two events are linked in the physiologic process called excitation-contraction coupling? 14. Describe the events of excitation-contraction coupling. 15. What is the function of Ca2+ in skeletal muscle contraction? 16. Additional ATP is made immediately available in skeletal muscle through which phosphate-containing molecules? 17. What are the various means for making ATP available in a 1500-meter race? 18. What is oxygen debt, and how is the additional oxygen used following intense exercise? 19. Explain how a fast-twitch fiber differs from a slow-twitch fiber and how an oxidative fiber differs from a glycolytic fiber. 20. Which skeletal muscle fiber type is slow and fatigue-resistant? What is the advantage of this skeletal muscle fiber type? 21. Muscles that maintain posture are composed primarily of what type of skeletal muscle fibers? 22. What events are occurring in a muscle that produce the different components of a muscle twitch (latent period, contraction, and relaxation)? 23. What is recruitment? Explain its importance in the body. 24. What happens to skeletal muscle during wave summation? Explain its importance in the body. 25. Explain the function of skeletal muscle tone. 26. When you flex your biceps brachii while doing “biceps curls,” what is the type of muscle contraction – is it an isometric contraction or an isotonic contraction? 77  Study Questions 27. Describe the relative force of contraction that can be developed in your back muscles when you bend at the knees to lift an object and when you bend at the waist to lift an object, based on the length-tension relationship. Explain the significance. 28. How can muscle fatigue result from changes in each of the three primary events of skeletal muscle contraction? 29. What anatomic changes occur in a skeletal muscle fiber when it undergoes hypertrophy? 30. What are three anatomic or physiologic differences between skeletal muscle and cardiac muscle? 31. Where is smooth muscle located in the human body? 32. How are anchoring proteins and contractile proteins in smooth muscle cells arranged 33. What is the specific role of the following structures within smooth muscle cells: calmodulin, myosin light-chain kinase, and myosin light-chain phosphatase? 34. What are the steps of smooth muscle contraction? 35. What unique characteristics of smooth muscle allow it to fulfill its functions? Explain. 36. What are the various forms of stimulation for controlling smooth muscle? 37. Explain the stress-relaxation response of smooth muscle. 38. Explain why smooth muscle of the eye is multiunit smooth muscle and the smooth muscle in the wall of digestive organs is single-unit smooth muscle. 78