Kin 290, Week 9
Kin 290, Week 9 Kin 290
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This 19 page Class Notes was uploaded by Leonard Carey on Monday April 11, 2016. The Class Notes belongs to Kin 290 at 1 MDSS-SGSLM-Langley AFB Advanced Education in General Dentistry 12 Months taught by Dr. Satern in Spring 2016. Since its upload, it has received 26 views. For similar materials see Anatomy & Physiology in Kinesiology at 1 MDSS-SGSLM-Langley AFB Advanced Education in General Dentistry 12 Months.
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Date Created: 04/11/16
Chapter 9 Chapter 9 – Part A Muscles and Muscle Tissue Why This Matters • Understanding skeletal muscle tissue helps you to treat strained muscles effectively with RICE 9.1 Overview of Muscle Tissue • Nearly half of body’s mass • Can transform chemical energy (ATP) into directed mechanical energy, which is capable of exerting force • To investigate muscle, we look at: • Types of muscle tissue • Characteristics of muscle tissue • Muscle functions Types of Muscle Tissue • Terminologies: Myo, mys, and sarco are prefixes for muscle • Example: sarcoplasm: muscle cell cytoplasm • Three types of muscle tissue • Skeletal • Cardiac • Smooth • Only skeletal and smooth muscle cells are elongated and referred to as muscle fibers Types of Muscle Tissue (cont.) (Table 9.3 – p. 310) • Skeletal muscle • Skeletal muscle tissue is packaged into skeletal muscles: organs that are attached to bones and skin • Skeletal muscle fibers are longest of all muscle and have striations (stripes) • Also called voluntary muscle: can be consciously controlled • Contract rapidly; tire easily; powerful • Key words for skeletal muscle: skeletal, striated, and voluntary Types of Muscle Tissue (cont.) • Cardiac muscle • Cardiac muscle tissue is found only in heart • Makes up bulk of heart walls • Striated • Involuntary: cannot be controlled consciously • Contracts at steady rate due to heart’s own pacemaker, but nervous system © 2016 Pearson Education, Inc. 1 Chapter 9 can increase rate • Key words for cardiac muscle: cardiac, striated, and involuntary Types of Muscle Tissue (cont.) • Smooth muscle • Smooth muscle tissue: found in walls of hollow organs • Examples: stomach, urinary bladder, and airways • Not striated • Involuntary: cannot be controlled consciously • Can contract on its own without nervous system stimulation Characteristics of Muscle Tissue • All muscles share four main characteristics: • Excitability (responsiveness): ability to receive and respond to stimuli • Contractility: ability to shorten forcibly when stimulated • Extensibility: ability to be stretched • Elasticity: ability to recoil to resting length Muscle Functions • Four important functions 1. Produce movement: responsible for all locomotion and manipulation • Example: walking, digesting, pumping blood 2. Maintain posture and body position 3. Stabilize joints 4. Generate heat as they contract • Additional functions • Protect organs, form valves, control pupil size, cause “goosebumps” 9.2 Skeletal Muscle Anatomy • Skeletal muscle is an organ made up of different tissues with three features: nerve and blood supply, connective tissue sheaths, and attachments Nerve and Blood Supply • Each muscle receives a nerve, artery, and veins • Consciously controlled skeletal muscle has nerves supplying every fiber to control activity • Contracting muscle fibers require huge amounts of oxygen and nutrients • Also need waste products removed quickly Connective Tissue Sheaths (Figure 9.1 – p. 281 & Table 9.1 – p. 286) • Each skeletal muscle, as well as each muscle fiber, is covered in connective tissue • Support cells and reinforce whole muscle © 2016 Pearson Education, Inc. 2 Chapter 9 • Sheaths from external to internal: • Epimysium: dense irregular connective tissue surrounding entire muscle; may blend with fascia • Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers) • Endomysium: fine areolar connective tissue surrounding each muscle fiber Attachments • Muscles span joints and attach to bones • Muscles attach to bone in at least two places • Insertion: attachment to movable bone • Origin: attachment to immovable or less movable bone • Attachments can be direct or indirect • Direct (fleshy): epimysium fused to periosteum of bone or perichondrium of cartilage • Indirect: connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis 9.3 Muscle Fiber Microanatomy and Sliding Filament Model • Skeletal muscle fibers are long, cylindrical cells that contain multiple nuclei • Sarcolemma: muscle fiber plasma membrane • Sarcoplasm: muscle fiber cytoplasm • Contains many glycosomes for glycogen storage, as well as myoglobin for O 2 storage • Modified organelles • Myofibrils • Sarcoplasmic reticulum • T tubules Myofibrils (Figure 9.2a & b – p. 283) • Myofibrils are densely packed, rodlike elements • Single muscle fiber can contain 1000s • Accounts for ~80% of muscle cell volume • Myofibril features • Striations • Sarcomeres • Myofilaments • Molecular composition of myofilaments Myofibrils (cont.) • Striations: stripes formed from repeating series of dark and light bands along length of each myofibril © 2016 Pearson Education, Inc. 3 Chapter 9 • A bands: dark regions • H zone: lighter region in middle of dark A band • M line: line of protein (myomesin) that bisects H zone vertically • I bands: lighter regions • Z disc (line): coinshaped sheet of proteins on midline of light I band Myofibrils (cont.) • Sarcomere (Figure 9.2c – p. 283) • Smallest contractile unit (functional unit) of muscle fiber • Contains A band with half of an I band at each end • Consists of area between Z discs • Individual sarcomeres align end to end along myofibril, like boxcars of train Myofibrils (cont.) • Myofilaments (Figure 9.2d & 3 – p. 283) • Orderly arrangement of actin and myosin myofilaments within sarcomere • Actin myofilaments: thin filaments • Extend across I band and partway in A band • Anchored to Z discs • Myosin myofilaments: thick filaments • Extend length of A band • Connected at M line • Sarcomere cross section shows hexagonal arrangement of one thick filament surrounded by six thin filaments Myofibrils (cont.) • Molecular composition of myofilaments (Figure 9.3 – p. 284) • Thick filaments: composed of protein myosin that contains two heavy and four light polypeptide chains • Heavy chains intertwine to form myosin tail • Light chains form myosin globular head • During contraction, heads link thick and thin filaments together, forming cross bridges • Myosins are offset from each other, resulting in staggered array of heads at different points along thick filament Myofibrils (cont.) • Molecular composition of myofilaments (cont.) – Thin filaments: composed of fibrous protein actin • Actin is polypeptide made up of kidneyshaped G actin (globular) subunits – G actin subunits bears active sites for myosin head attachment during contraction © 2016 Pearson Education, Inc. 4 Chapter 9 • G actin subunits link together to form long, fibrous F actin (filamentous) • Two F actin strands twist together to form a thin filament – Tropomyosin and troponin: regulatory proteins bound to actin Myofibrils (cont.) • Molecular composition of myofilaments (cont.) (Figure 9.4 – p. 284) – Other proteins help form the structure of the myofibril • Elastic filament: composed of protein titin • Holds thick filaments in place; helps recoil after stretch; resists excessive stretching Dystrophin • Links thin filaments to proteins of sarcolemma Nebulin, myomesin, C proteins bind filaments or sarcomeres together • Maintain alignment of sarcomere Sarcoplasmic Reticulum and T Tubules (Figure 9.5 – p. 287) • Sarcoplasmic reticulum: network of smooth endoplasmic reticulum tubules surrounding each myofibril – Most run longitudinally – Terminal cisterns form perpendicular cross channels at the A–I band junction – SR functions in regulation of intracellular Ca levels 2+ – Stores and releases Ca Sarcoplasmic Reticulum and T Tubules (cont.) • T tubules – Tube formed by protrusion of sarcolemma deep into cell interior Increase muscle fiber's surface area greatly Lumen continuous with extracellular space Allow electrical nerve transmissions to reach deep into interior of each muscle fiber – Tubules penetrate cell's interior at each A–I band junction between terminal cisterns Triad: area formed from terminal cistern of one sarcomere, T tubule, and terminal cistern of neighboring sarcomere Sarcoplasmic Reticulum and T Tubules (cont.) • Triad relationships – T tubule contains integral membrane proteins that protrude into intermembrane space (space between tubule and muscle fiber sarcolemma) Tubule proteins act as voltage sensors that change shape in response to an electrical current – SR cistern membranes also have integral membrane proteins that protrude into intermembrane space © 2016 Pearson Education, Inc. 5 Chapter 9 SR integral proteins control opening of calcium channels in SR cisterns Sarcoplasmic Reticulum and T Tubules (cont.) • Triad relationships (cont.) – When an electrical impulse passes by, T tubule proteins change shape, causing SR proteins to change shape, causing release of calcium into cytoplasm Sliding Filament Model of Contraction (Figure 9.6a – p. 288) • Contraction: the activation of cross bridges to generate force • Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening • Contraction ends when cross bridges become inactive Sliding Filament Model of Contraction (cont.) • In the relaxed state, thin and thick filaments overlap only slightly at ends of A band • Sliding filament model of contraction states that during contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more • Neither thick nor thin filaments change length, just overlap more • When nervous system stimulates muscle fiber, myosin heads are allowed to bind to actin, forming cross bridges, which cause sliding (contraction) process to begin Sliding Filament Model of Contraction (cont.) (Figure 9.6b – p. 288) • Cross bridge attachments form and break several times, each time pulling thin filaments a little closer toward center of sarcome in a ratcheting action • Causes shortening of muscle fiber • Z discs are pulled toward M line • I bands shorten • Z discs become closer • H zones disappear • A bands move closer to each other 9.4 Muscle Fiber Contraction (Figure 9.7 – p. 289) • Four steps must occur for skeletal muscle to contract: 1. Nerve stimulation 2. Action potential, an electrical current, must be generated in sarcolemma 3. Action potential must be propagated along sarcolemma 4. Intracellular Ca levels must rise briefly • Steps 1 and 2 occur at neuromuscular junction • Steps 3 and 4 link electrical signals to contraction, so referred to as excitation contraction coupling The Nerve Stimulus and Events at the Neuromuscular Junction (Focus Figure 9.1 © 2016 Pearson Education, Inc. 6 Chapter 9 – p. 290) • Skeletal muscles are stimulated by somatic motor neurons • Axons (long, threadlike extensions of motor neurons) travel from central nervous system to skeletal muscle • Each axon divides into many branches as it enters muscle • Axon branches end on muscle fiber, forming neuromuscular junction or motor end plate – Each muscle fiber has one neuromuscular junction with one motor neuron The Nerve Stimulus and Events at the Neuromuscular Junction (cont.) • Axon terminal (end of axon) and muscle fiber are separated by gelfilled space called synaptic cleft • Stored within axon terminals are membranebound synaptic vesicles – Synaptic vesicles contain neurotransmitter acetylcholine (ACh) • Infoldings of sarcolemma, called junctional folds, contain millions of ACh receptors • NMJ consists of axon terminals, synaptic cleft, and junctional folds The Nerve Stimulus and Events at the Neuromuscular Junction (cont.) (See also A&P Flix : Events at the Neuromuscular Junction in Mastering A & P) • Events at the neuromuscular junction – Nerve impulse arrives at axon terminal, causing ACh to be released into synaptic cleft – ACh diffuses across cleft and binds with receptors on sarcolemma – ACh binding leads to electrical events that ultimately generate an action potential through muscle fiber – ACh is quickly broken down by enzyme acetylcholinesterase, which stops contractions Generation of an Action Potential Across the Sarcolemma • Resting sarcolemma is polarized, meaning a voltage exists across membrane – Inside of cell is negative compared to outside • Action potential is caused by changes in electrical charges • Occurs in three steps 1. End plate potential 2. Depolarization 3. Repolarization Generation of an Action Potential Across the Sarcolemma (cont.) 1. End plate potential (Figure 9.8 – p. 291) – ACh released from motor neuron binds to ACh receptors on sarcolemma – Causes chemically gated ion channels (ligands) on sarcolemma to open © 2016 Pearson Education, Inc. 7 Chapter 9 + – Na diffuses into muscle fiber Some K diffuses outward, but not much + – Because Na diffuses in, interior of sarcolemma becomes less negative (more positive) – Results in local depolarization called end plate potential Generation of an Action Potential Across the Sarcolemma (cont.) 2. Depolarization: generation and propagation of an action potential (AP) – If end plate potential causes enough change in membrane voltage to reach + critical level called threshold, voltagegated Na channels in membrane will open – Large influx of Na through channels into cell triggers AP that is unstoppable and will lead to muscle fiber contraction – AP spreads across sarcolemma from one voltagegated Na channel to next one in adjacent areas, causing that area to depolarize Generation of an Action Potential Across the Sarcolemma (cont.) 3. Repolarization: restoration of resting conditions (Figure 9.9 – p. 291) – Na voltagegated channels close, and voltagegated K channels open + – K efflux out of cell rapidly brings cell back to initial resting membrane voltage – Refractory period: muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete – Ionic conditions of resting state are restored by Na K pump + + + Na that came into cell is pumped back out, and K that flowed outside is pumped back into cell ExcitationContraction (EC) Coupling (Focus Figure 9.2 – pp. 292293) (See also A&P Flix : ExcitationContraction Coupling in Mastering A & P) • Excitationcontraction (EC) coupling: events that transmit AP along sarcolemma (excitation) are coupled to sliding of myofilaments (contraction) • AP is propagated along sarcolemma and down into T tubules, where voltage sensitive proteins in tubules stimulate Ca release from SR – Ca release leads to contraction • AP is brief and ends before contraction is seen Channels Involved in Initiating Muscle Contraction (Focus Figure 9.1 – p. 290) • Nerve impulse travels down axon of motor neuron • Whe2+impulse reaches axon terminal, voltagegated calcium channels open, and Ca enters axon terminal • Ca influx causes synaptic vesicle to exocytose Ach into synaptic cleft + + • ACh binds to receptors on sarcolemma, causing chemically gated Na K channels to open and initiate an end plate potential + • When threshold is reached, voltagegated Na channels open, initiating an AP © 2016 Pearson Education, Inc. 8 Chapter 9 Muscle Fiber Contraction: Cross Bridge Cycling (Focus Figure 9.2 – pp. 292293) • At low intracellular Ca concentration: – Tropomyosin blocks active sites on actin – Myosin heads cannot attach to actin – Muscle fiber remains relaxed • Voltagesensitive proteins in T tubules change shape, causing SR to release Ca to 2+ cytosol Muscle Fiber Contraction: Cross Bridge Cycling (cont.) • At higher intracellular Ca concentrations, Ca binds to troponin • Troponin changes shape and moves tropomyosin away from myosinbinding sites • Myosin heads is then allowed to bind to actin, forming cross bridge • Cycling is initiated, causing sarcomere shortening and muscle contraction 2+ • When nervous stimulation ceases, Ca is pumped back into SR, and contraction ends Muscle Fiber Contract™on: Cross Bridge Cycling (cont.) (Focus Figure 9.2 – p. 293) (See also A&P Flix : Cross Bridge Cycle in Mastering A & P) • Four steps of the cross bridge cycle 1. Cross bridge formation: highenergy myosin head attaches to actin thin filament active site 2. Working (power) stroke: myosin head pivots and pulls thin filament toward M line 3. Cross bridge detachment: ATP attaches to myosin head, causing cross bridge to detach 4. Cocking of myosin head: energy from hydrolysis of ATP “cocks” myosin head into highenergy state This energy will be used for power stroke in next cross bridge cycle 9.5 Whole Muscle Contraction 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 9.5 Whole Muscle Contraction Force and duration of contraction vary in response to stimuli of different frequencies and intensities Each muscle is served by at least one motor nerve – Motor nerve contains axons of up to hundreds of motor neurons © 2016 Pearson Education, Inc. 9 Chapter 9 – Axons branch into terminals, each of which forms NMJ with single muscle fiber Motor unit is the nervemuscle functional unit The Motor Unit (Figure 9.10 – p. 296) Motor unit consists of the motor neuron and all muscle fibers (four to several hundred) it supplies – Smaller the fiber number, the greater the fine control Muscle fibers from a motor unit are spread throughout the whole muscle, so stimulation of a single motor unit causes only weak contraction of entire muscle The Muscle Twitch (Figure 9.11 – p. 297) Muscle twitch: simplest contraction resulting from a muscle fiber’s response to a single action potential from motor neuron – Muscle fiber contracts quickly, then relaxes Twitch can be observed and recorded as a myogram – Tracing: line recording contraction activity The Muscle Twitch (cont.) Three phases of muscle twitch – Latent period: events of excitationcontraction coupling No muscle tension seen – Period of contraction: cross bridge formation Tension increases – Period of relaxation: Ca reentry into SR Tension declines to zero Muscle contracts faster than it relaxes The Muscle Twitch (cont.) Differences in strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles – Example: eye muscles contraction are rapid and brief, whereas larger, fleshy muscles (calf muscles) contract more slowly and hold it longer 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 – Required for proper control of skeletal movement Responses are graded by: – Changing frequency of stimulation – Changing strength of stimulation © 2016 Pearson Education, Inc. 10 Chapter 9 Graded Muscle Responses (cont.) (Figure 9.12a – p. 298) Muscle response to changes in stimulus frequency – Single stimulus results in single contractile response (i.e., muscle twitch) Graded Muscle Responses (cont.) (Figure 9.12b – p. 298) Muscle response to changes in stimulus frequency (cont.) – Wave (temporal) summation results if two stimuli are received by a muscle in rapid succession Muscle fibers do not have time to completely relax between stimuli, so twitches incre2+e in force with each stimulus Additional Ca that is released with second stimulus stimulates more shortening Graded Muscle Responses (cont.) Muscle response to changes in stimulus frequency (cont.) – Wave (temporal) summation results if two stimuli are received by a muscle in rapid succession (cont.) Produces smooth, continuous contractions that add up (summation) Further increase in stimulus frequency causes muscle to progress to sustained, quivering contraction referred to as unfused (incomplete) tetanus Graded Muscle Responses (cont.) (Figure 9.12c – p. 298) Muscle response to changes in stimulus frequency (cont.) – If stimuli frequency increases, muscle tension reaches maximum Referred to as fused (complete) tetanus because contractions “fuse” into one smooth sustained contraction plateau Prolonged muscle contractions lead to muscle fatigue Graded Muscle Responses (cont.) (Figure 9.13 – p. 299) Muscle response to changes in stimulus strength – Recruitment (or multiple motor unit summation): stimulus is sent to more muscle fibers, leading to more precise control – Types of stimulus involved in recruitment: Subthreshold stimulus: stimulus not strong enough, so no contractions seen Threshold stimulus: stimulus is strong enough to cause first observable contraction Maximal stimulus: strongest stimulus that increases maximum contractile force – All motor units have been recruited © 2016 Pearson Education, Inc. 11 Chapter 9 Graded Muscle Responses (cont.) (Figure 9.14 – p. 299) Muscle response to changes in stimulus strength (cont.) – Recruitment works on size principle Motor units with smallest muscle fibers are recruited first Motor units with larger and larger fibers are recruited as stimulus intensity increases Largest motor units are activated only for most powerful contractions Motor units in muscle usually contract asynchronously – Some fibers contract while others rest – Helps prevent fatigue Muscle Tone Constant, slightly contracted state of all muscles Due to spinal reflexes – Groups of motor units are alternately activated in response to input from stretch receptors in muscles Keeps muscles firm, healthy, and ready to respond Isotonic and Isometric Contractions (Figure 9.15 – p. 300) Isotonic contractions: muscle changes in length and moves load – Isotonic contractions can be either concentric or eccentric: Concentric contractions: muscle shortens and does work – Example: biceps contract to pick up a book Eccentric contractions: muscle lengthens and generates force – Example: laying a book down causes biceps to lengthen while generating a force Isotonic and Isometric Contractions (cont.) Isometric contractions – Load is greater than the maximum tension muscle can generate, so muscle neither shortens nor lengthens Isotonic and Isometric Contractions (cont.) Electrochemical and mechanical events are same in isotonic or isometric contractions, but results are different – In isotonic contractions, actin filaments shorten and cause movement – In isometric contractions, cross bridges generate force, but actin filaments do not shorten Myosin heads “spin their wheels” on same actin binding site 9.6 Energy for Contraction and ATP Providing Energy for Contraction © 2016 Pearson Education, Inc. 12 Chapter 9 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 excitationcontraction 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 Providing Energy for Contraction ATP is regenerated quickly by three mechanisms: – Direct phosphorylation of ADP by creatine phosphate (CP) – Anaerobic pathway: glycolysis and lactic acid formation – Aerobic respiration Providing Energy for Contraction (cont.) (Figure 9.16a – p. 302) Direct phosphorylation of ADP by creatine phosphate (CP) – Creatine phosphate is a unique molecule located in muscle fibers that donates a phosphate to ADP to instantly form ATP Creatine kinase is enzyme that carries out transfer of phosphate Muscle fibers have enough ATP and CP reserves to power cell for about 15 seconds Creatine phosphate + ADP creatine + ATP Providing Energy for Contraction (cont.) (Figure 9.16b – p. 302) Anaerobic pathway: glycolysis and lactic acid formation – ATP can also be generated by breaking down and using energy stored in glucose Glycolysis: first step in glucose breakdown – Does not require oxygen – Glucose is broken into 2 pyruvic acid molecules – 2 ATPs are generated for each glucose broken down Low oxygen levels prevent pyruvic acid from entering aerobic respiration phase Providing Energy for Contraction (cont.) Anaerobic pathway: glycolysis and lactic acid formation (cont.) – Normally, pyruvic acid enters mitochondria to start aerobic respiration phase; however, at high intensity activity, oxygen is not available Bulging muscles compress blood vessels, impairing oxygen delivery © 2016 Pearson Education, Inc. 13 Chapter 9 – In the absence of oxygen, referred to as anaerobic glycolysis, pyruvic acid is converted to lactic acid Providing Energy for Contraction (cont.) Anaerobic pathway: glycolysis and lactic acid formation (cont.) – Lactic acid Diffuses into bloodstream Used as fuel by liver, kidneys, and heart Converted back into pyruvic acid or glucose by liver – Anaerobic respiration yields only 5% as much ATP as aerobic respiration, but produces ATP 2½ times faster Providing Energy for Contraction (cont.) (Figure 9.16c – p. 302) Aerobic respiration – Produces 95% of ATP during rest and lighttomoderate exercise Slower than anaerobic pathway – Consists of series of chemical reactions that occur in mitochondria and require oxygen Breaks glucose into CO ,2H O2 and large amount ATP (32 can be produced) – Fuels used include glucose from glycogen stored in muscle fiber, then bloodborne glucose, and free fatty acids Fatty acids are main fuel after 30 minutes of exercise Providing Energy for Contraction (cont.) (Figure 9.17 – p. 303) Energy systems used during sports – Aerobic endurance Length of time muscle contracts using aerobic pathways – Lighttomoderate activity, which can continue for hours – Anaerobic threshold Point at which muscle metabolism converts to anaerobic pathway Muscle Fatigue Physiological inability to contract despite continued stimulation Usually occurs when there are ionic imbalances – Levels of K , Ca , P cai interfere with E-C coupling 2+ – Prolonged exercise may also damage SR and interferes with Ca regulation and release Lack of ATP is rarely a reason for fatigue, except in severely stressed muscles Excess Postexercise Oxygen Consumption For a muscle to return to its preexercise state: © 2016 Pearson Education, Inc. 14 Chapter 9 – Oxygen reserves are replenished – Lactic acid is reconverted to pyruvic acid – Glycogen stores are replaced – ATP and creatine phosphate reserves are resynthesized All replenishing steps require extra oxygen, so this is referred to as excess postexercise oxygen consumption (EPOC) – Formerly referred to as “oxygen debt” 9.7 Factors of Muscle Contraction (Figure 9.18 – p. 304) Force of Muscle Contractions Force of contraction depends on number of cross bridges attached, which is affected by four factors: 1. Number of muscle fibers stimulated (recruitment): the more motor units recruited, the greater the force. 2. Relative size of fibers: the bulkier the muscle, the more tension it can develop Muscle cells can increase in size (hypertrophy) with regular exercise Force of Muscle Contractions (cont.) (Figure 9.19 – p. 305) 3. Frequency of stimulation: the higher the frequency, the greater the force Stimuli are added together 4. Degree of muscle stretch: muscle fibers with sarcomeres that are 80–120% their normal resting length generate more force If sarcomere is less than 80% resting length, filaments overlap too much, and force decreases If sarcomere is greater than 120% of resting length, filaments do not overlap enough so force decreases Velocity and Duration of Contraction How fast a muscle contracts and how long it can stay contracted is influenced by: – Muscle fiber type – Load – Recruitment Velocity and Duration of Contraction (cont.) Muscle fiber type – Classified according to two characteristics 1. Speed of contraction – slow or fast fibers according to: – Speed at which myosin ATPases split ATP – Pattern of electrical activity of motor neurons 2. Metabolic pathways used for ATP synthesis – Oxidative fibers: use aerobic pathways – Glycolytic fibers: use anaerobic glycolysis © 2016 Pearson Education, Inc. 15 Chapter 9 Velocity and Duration of Contraction (cont.) Muscle fiber type (cont.) – Based on these two criteria, skeletal muscle fibers can be classified into three types: Slow oxidative fibers, fast oxidative fibers, or fast glycolytic fibers – Most muscles contain mixture of fiber types, resulting in a range of contractile speed and fatigue resistance All fibers in one motor unit are the same type Genetics dictate individual’s percentage of each Velocity and Duration of Contraction (cont.) (Figure 9.20 – p. 305 & Table 9.2 – p. 306)) Muscle fiber type (cont.) – Different muscle types are better suited for different jobs Slow oxidative fibers: lowintensity, endurance activities – Example: maintaining posture Fast oxidative fibers: mediumintensity activities – Example: sprinting or walking Fast glycolytic fibers: shortterm intense or powerful movements – Example: hitting a baseball Velocity and Duration of Contraction (cont.) (Figure 9.21 – p. 307) Load and recruitment – Load: muscles contract fastest when no load is added The greater the load, the shorter the duration of contraction The greater the load, the slower the contraction – Recruitment: the more motor units contracting, the faster and more prolonged the contraction 9.8 Adaptation to Exercise Aerobic (Endurance) Exercise Aerobic (endurance) exercise, such as jogging, swimming, biking leads to increased: Muscle capillaries Number of mitochondria Myoglobin synthesis – Results in greater endurance, strength, and resistance to fatigue – May convert fast glycolytic fibers into fast oxidative fibers Resistance Exercise Resistance exercise (typically anaerobic), such as weight lifting or isometric © 2016 Pearson Education, Inc. 16 Chapter 9 exercises, leads to – Muscle hypertrophy Due primarily to increase in fiber size – Increased mitochondria, myofilaments, glycogen stores, and connective tissue – Increased muscle strength and size Clinical – Homeostatic Imbalance 9.3 Muscles must be active to remain healthy Disuse atrophy (degeneration and loss of mass) – Due to immobilization or loss of neural stimulation – Can begin almost immediately. Muscle strength can decline 5% per day Paralyzed muscles may atrophy to onefourth initial size Fibrous connective tissue replaces lost muscle tissue Rehabilitation is impossible at this point 9.9 Smooth Muscle Found in walls of most hollow organs, except heart – Heart contains cardiac muscle Microscopic Structure (Figure 9.22 – p. 308) Spindleshaped fibers: thin and short compared with skeletal muscle fibers – Only one nucleus, no striations Lacks connective tissue sheaths – Contains endomysium only Microscopic Structure (cont.) All but smallest blood vessels contain smooth muscle organized into two layers of opposing sheets of fibers – Longitudinal layer: fibers run parallel to long axis of organ Contraction causes organ to shorten – Circular layer: fibers run around circumference of organ Contraction causes lumen of organ to constrict Allows peristalsis: alternating contractions and relaxations of layers mix and squeeze substances through lumen of hollow organs Microscopic Structure (cont.) (Figure 9.23 – p. 309) No neuromuscular junction, as in skeletal muscle Instead, autonomic nerve fibers innervate smooth muscle – Contain varicosities (bulbous swellings) of nerve fibers – Varicosities store and release neurotransmitters into a wide synaptic cleft referred to as a diffuse junction © 2016 Pearson Education, Inc. 17 Chapter 9 Microscopic Structure (cont.) (Figure 9.24a – p. 309) Smooth muscle does not contain sarcomeres, myofibrils, or T tubules SR is less developed than in skeletal muscle 2+ – SR does store intracellular Ca , but most calcium used for contraction has extracellular origins Sarcolemma contains pouchlike infoldings called caveolae – Caveolae contain numerous Ca channels that open to allow rapid influx of 2+ extracellular Ca Microscopic Structure (cont.) Smooth muscle also differs from skeletal muscle in following ways: – Thick filaments are fewer and have myosin heads along entire length Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2) Thick filaments have heads along entire length, making smooth muscle as powerful as skeletal muscle – No troponin complex Does contain tropomyosin, but not troponin Protein calmodulin binds Ca 2+ Microscopic Structure (cont.) (Figure 9.24b – p. 309) – Thick and thin filaments arranged diagonally Myofilaments are spirally arranged, causing smooth muscle to contract in corkscrew manner – Intermediate filament–dense body network Contain latticelike arrangement of noncontractile intermediate filaments that resist tension Dense bodies: proteins that anchor filaments to sarcolemma at regular intervals – Correspond to Z discs of skeletal muscle During contraction, areas of sarcolemma between dense bodies bulge outward – Make muscle cell look puffy See Also Table 9.3 – pp. 310311 for comparison of Skeletal & Smooth Muscle Contraction of Smooth Muscle Mechanism of contraction – Slow, synchronized contractions – Cells electrically coupled by gap junctions Action potentials transmitted from fiber to fiber © 2016 Pearson Education, Inc. 18 Chapter 9 – Some cells are selfexcitatory (depolarize without external stimuli) Act as pacemakers for sheets of muscle Rate and intensity of contraction may be modified by neural and chemical stimuli Contraction of Smooth Muscle (cont.) Mechanism of contraction (cont.) – Contraction in smooth muscle is similar to skeletal muscle contraction in following ways: Actin and myosin interact by sliding filament mechanism Final trigger is increased intracellular Ca level ATP energizes sliding process Contraction stops when Ca is no longer available Contraction of Smooth Muscle (cont.) Mechanism of contraction (cont.) – Contraction in smooth muscle is different from skeletal muscle in following ways: 2+ Some Ca still obtained from SR, but mostly comes from extracellular space Ca binds to calmodulin, not troponin Activated calmodulin then activates myosin kinase (myosin light chain kinase) Activated myosin kinase phosphorylates myosin head, activating it – Leads to crossbridge formation with actin Contraction of Smooth Muscle (cont.) Energy efficiency of smooth muscle contraction – Slower to contract and relax but maintains contraction for prolonged periods with little energy cost Slower ATPases Myofilaments may latch together to save energy – Most smooth muscle maintain moderate degree of contraction constantly without fatiguing Referred to as smooth muscle tone – Makes ATP via aerobic respiration pathways Contraction of Smooth Muscle (cont.) Regulation of contraction – Controlled by nerves, hormones, or local chemical changes © 2016 Pearson Education, Inc. 19
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