BIO 201: Muscle Tissue
BIO 201: Muscle Tissue BIO 201
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This 24 page Class Notes was uploaded by ASUNursing19 on Saturday March 26, 2016. The Class Notes belongs to BIO 201 at Arizona State University taught by Dr. Penkrot in Winter 2016. Since its upload, it has received 51 views. For similar materials see Human Anatomy/Physiology I in Biology at Arizona State University.
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Date Created: 03/26/16
Introduction to Muscle Movement is a fundamental characteristic of most living things Muscle cells are capable of converting the chemical energy of ATP into mechanical energy Types of muscle o Skeletal, cardiac, and smooth Physiology of skeletal muscle o Basis of warmup, quickness, strength, endurance and fatigue Muscle Functions Four important functions o Produce movement: responsible for all locomotion and manipulation Example: walking, digesting, pumping blood o Maintain posture and body position o Stabilize joints o Generate heat as they contract Additional functions o Protect organs, form valves, control pupil size, cause "goosebumps" Characteristics of Muscle Responsiveness (excitability) o To chemical signals, stretch and electrical changes across the plasma membrane *Conductivity* o Local electrical change triggers a wave of excitation that travels along the muscle fiber Contractility Extensibility o Capable of being stretched between contractions Elasticity o Returns to its original resting length after being stretched 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: o Types of muscle tissue o Characteristics of muscle tissue o Muscle functions Types of Muscle Tissue Terminologies: Myo mys , andsarco are prefixes for muscle o Example: sarcoplasm: muscle cell cytoplasm Three types of muscle tissue o Skeletal o Cardiac o Smooth Only skeletal and smooth muscle cells are elongated and referred to as scle fibers Skeletal muscle o Skeletal muscle tissue is packaged intoskeletal muscles: organs that are attached to bones and skin o Skeletal muscle fibers are longest of all muscle and havestriations (stripes) o Also called voluntary muscle : can be consciously controlled o Contract rapidly; tire easily; powerful o Key words for skeletal muscle: skeletal, striated, voluntary Cardiac muscle o Cardiac muscle tissue is found only in heart Makes up bulk of heart walls o Striated o Involuntary: cannot be controlled consciously Contracts at steady rate due to heart's own pacemaker (autorhythmic), but nervous system can increase rate o Key words for cardiac muscle: cardiac, striated, nd involuntary Smooth muscle o Smooth muscle tissue is found walls of hollow organs Examples: stomach, urinary bladder, and airways o Not striated o Involuntary: cannot be controlled consciously Can contract on its own without nervous system stimulation Connective Tissue Sheaths Each skeletal muscle, as well as each muscle fiber, is covered in connective tissue Support cells and reinforce whole muscle Sheath from external to internal: o Epimysium: dense irregular connective tissue surrounding entire muscle; may blend with fascia o Perimysium : fibrous connective tissue surrounding scicle (groups of muscle fibers) o Endomysium: fine areolar connective tissue surrounding each muscle Attachments Muscles span joints and attach to bones Muscles attach to bone in at least two places o Insertion: attachment to movable bone o Origin: attachment to immovable or less movable bone Attachments can be direct or indirect o Direct (fleshy): epimysium fused to periosteum of bone or perichondrium of cartilage o Indirect: connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike oneurosis 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 globin for 2 storage Modified organelles o Myofibrils o Sarcoplasmic reticulum o T tubules Myofibrils Myofibrils are densely packed, rodlike elements o Single muscle fiber can contain 1000s o Accounts for ~80% of muscle cell volume Myofibril features o Striations o Sarcomeres o Myofilaments Molecular composition of myofilaments Striations: stripes formed from repeating series of dark and light bands along length of each myofibril o A bands : d rk regions H zone: lighter region in middle of dark A band M line: line of proteins (myomesin) that bisects H zone vertically o I bands : l ghter regions Z disc (line: coinshaped sheet of proteins on midline of like I band Sarcomere o Smallest contractile unit (functional unit) of muscle fiber o Contains A band with half of an I band at each end Consists of area between Z discs o Individual sarcomeres align end to end along myofibril, like boxcars of train Myofilaments o Orderly arrangement of actin and myosin myofilaments within sarcomere o Actin myofilaments: thin filaments Extend across I band and partway in A band Anchored to Z discs o Myosin myofilaments : thick filaments Extend length of A band Connected at M line o Sarcomere cross section shows hexagonal arrangement of one thick filament surrounded by six thin filaments Molecular composition of myofilaments o 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 ross bridges Myosins are offset from each other, resulting in staggered array of heads at different points along thick filament o Thin filaments : composed of fibrous proteins actin Actin is polypeptide made up of kidneyshaped G actin (globular) subunits G actin subunits bears active sites for myosin had attachment during contraction G actin subunits link together to form long, fibrF actin (filamentous) Two F actin strands twist together to form a thin filament o Tropomyosin and troponin : regulatory proteins bound to actin o Other proteins help form the structure of the myofibril Elastic filament: composed of proteintitin Hold 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 Accessory Proteins At least seven other accessory proteins in or associated with thick or thin filaments o Anchor the myofilaments, regulate length of myofilaments, alignment of myofilaments for maximum effectiveness Dystrophin most clinically important o Links actin in outermost thin myofilaments to transmembrane proteins and eventually to fibrous endomysium surrounding the entire muscle cell o Genetic defects in dystrophin produce disabling disease: muscular dystrophy Sarcoplasmic Reticulum and T Tubules Sarcoplasmic reticulum: network of smooth endoplasmic reticulum tubules surrounding each myofibril o Most run longitudinally o Terminal cisterns form perpendicular cross channels at the AI band junction 2+ o SR functions in regulation of intracellular Ca levels o Stores and releases Ca2+ T Tubules o 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 o Tubules penetrate cell's interior at each AI band unction between terminal cisterns Triad : area formed from terminal cistern of one sarcomere, T tubule, and terminal cistern of neighboring sarcomere (ttubule and two terminal cisterns) Triad relationships o 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 o SR cistern membranes also have integral membrane proteins that protrude into intermembrane space SR integral proteins control opening of calcium channels in SR cisterns o 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 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 In the relaxed state, thin and tick filaments overlap only slightly at ends of A band liding filament model of contraction states that during contraction, thin filaments slide past thick filaments causing actin and myosin to overlap more o Neither thick not thin filaments change length, just overlap more When nervous system stimulates muscle fiber, myosin head are allowed to bind to actin, forming cross bridges , which cause sliding (contraction) process to begin Cross bridge attachments form and break several times, each time pulling thin filaments a little closer toward center of sarcomere in a ratcheting action o 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 Four steps must occur for skeletal muscle to contract: o Nerve stimulation o Action potential , an electrical current, must be generated in sarcolemma o Action potential2+ust be propagated along sarcolemma o 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 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 otor end plate o Each muscle fiber has one neuromuscular junction with one motor neuron Axon terminal (end of axon) and muscle fiber are separated by gelfilled space called ynaptic cleft Stored within axon terminals are membranebound synaptic vesicles o Synaptic vesicles contain neurotransmitter cetocholine (h ) Infoldings of sarcolemma, called junctional folds , contain millions of h receptors NMJ consists of axon terminals, synaptic cleft, and junctional folds Events at the neuromuscular junction: o Nerve impulse arrive at axon terminal, causing ACh to be released into synaptic cleft o ACh diffuses across cleft and binds with receptors on sarcolemma o ACh binding leads to electrical events that ultimately generate an action potential through muscle fiber o ACh is quickly broken down by enzyme acetylcholinesterase , which stops contractions The Neuromuscular Junction Synapse : any point where a nerve fiber meets its target cell (e.g., muscle cell, another neuron, gland, etc.) Neuromuscular junction (NMJ) : specific type of synapse where target cell is a muscle fiber Each terminal branch of the nerve fiber within the NMJ forms separate synapse with the muscle fiber One nerve fiber stimulates the muscle fiber at several points within the NMJ Clinical Homeostatic Imbalance 9.1 Many toxins, drugs, and diseases interfere with events at the neuromuscular junction o Example: myasthenia gravis : disease characterized by dropping upper eyelids, difficulty swallowing and talking, and generalized muscle weakness o Involves shortage of ACh receptors because person's ACh receptors are attacked by own antibodies o Suggests this is an autoimmune disease Neuromuscular Toxins Toxins that interfere with synaptic function can paralyze the muscles Some pesticides contain cholinesterase inhibitors o Bone to acetylcholinesterase and prevent it from degrading ACh o Spastic paralysis: a state of continual contraction of the muscles o Possible suffocation Tetanus (lockjaw) is a form of spastic paralysis caused by toxin ofClostridium tetani o Glycine in the spinal cord normally stops motor neurons from producing unwanted muscle contractions o Tetanus toxin blocks glycine release in the spinal cord and causes overstimulation and spastic paralysis of the muscles Flaccid paralysis: a state in which the muscles are limp and cannot contract o Curare : competes with ACh, for receptor sites, but do not stimulate the muscles o Plant poison used by South American natives to poison blowgun darts Botulism: type of food poisoning caused by a neuromuscular toxin secreted by the bacterium Clostridium botulinum o Blocks release of ACh causing flaccid paralysis o Botox cosmetic injections for wrinkle removal Tetanus Bacteria found in soil produces a toxin that inhibits muscular relaxation Nerve Gas Inhibits acetylcholinesterase, preventing muscular relaxation Curare Blocks ACh receptors, causing flaccid paralysis Botulinum Bacterial toxin, one of the most potent neurotoxins known (4 kg for total global lethality), blocks release of ACh vesicles Electrically Excitable Cells Muscle fibers are neurons are electrically excitable cells o Their plasma membrane exhibits voltage changes in response to stimulation Electrophysiology: the study of the electrical activity of cells Voltage (electrical potential): a difference in electrical charge from one point to another Resting membrane potential: about 90mV o Maintained by sodiumpotassium pump o Requires ATP to maintain Generation of an Action Potential Across the Sarcolemma Resting sarcolemma is polarized, meaning a voltage exists across membrane o Inside of cell is negative compared to outside Action potential is caused by changes in electrical charges Occurs in three steps o End plate potential o Depolarization o Repolarization End plate potential o Ach released from motor neuron binds to ACh receptors on sarcolemma o Causes chemically gates ion channels (ligands) on sarcolemma to open + o Na diffuses into muscle fiber Some (but not much) K diffuses outward + o Because Na diffuses in, interior of sarcolemma becomes less negative (more positive) o Results in local depolarization called end plate potential Depolarization : generation and propagation of an action potential (AP) o If end plate potential causes enough change in membrane voltage to reach + critical level threshold, voltagegated Na channels in membrane will open o Large influx of Na through channels into cell triggers AP that is unstoppable and will lead to muscle fiber contraction + o AP spreads across sarcolemma from one voltagegates Na channel to next one in adjacent areas, causing that area to depolarize polarization : restoration ofr esting conditionsr( esetting the system) + + o Na voltagegated channels close, and voltagegated K channels open o K efflux out of cell rapidly brings cell back to initial resting membrane voltage o Refractory period : muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete o 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 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 voltagesensitive proteins in tubules stimulate Ca release from SR o Ca release leads to contraction AP is brief and ends before contraction is seen Muscle Fiber Contraction: Cross Bridge Cycling At low intracellular Ca concentration: o Tropomyosin blocks active sites on actin o Myosin heads cannot attach to actin o Muscle fiber remains relaxed Voltagesensitive proteins in T tubules change shape, causing SR to release Ca to cytosol 2+ 2+ 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 sarcomere2+hortening and muscle contraction When nervous stimulation ceases, Ca is pumped back into SR, and contraction ends Four steps of the cross bridge cycle o Cross bridge formation: highenergy myosin head attaches to actin thin filament active site o Working (power) stroke: myosin head pivots and pulls thin filament toward M line o Cross bridge detachment: ATP attaches to myosin head, causing cross bridge to detach o 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 Clinical Homeostatic Imbalance 9.2 Rigor mortis o 34 hours after death, muscles begin to stiffen Peak rigidity occurs about 12 hours postmortem o Intracellular calcium levels increase because ATP is no longer being synthesized, so calcium cannot be pumped back into SR Results in cross bridge formation o ATP is also needed for cross bridge detachment Results in yosin head staying bound to actin , causing constant state of contraction o Muscles stay contracted until muscle protein break down, causing myosin to release 9.5 Whole Muscle Contraction Same principles apply to contraction of both single fibers and whole fibers Contraction produces muscle tension , the force exerted on load or object to be moved Contraction may/may not shorten muscle o Isometric contraction: no shortening; muscle tension increases but does not exceed load o Isotonic contraction: muscle shortens because muscle tension exceeds load 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 o Motor nerve contains axons of up to hundreds ofmotor neurons o Axons branch into terminals, each of which forms NMJ with single muscle fiber Motor unit is the nervemuscle functional unit The Motor Unit Motor unit consists of the motor neuron and all muscle fibers (four to several hundred) it supplies o 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 Muscle twitch : simplest contraction resulting form a muscle fiber's response to a single action potential from motor neuron o Muscle fiber contracts quickly, then relaxes Twitch can be observed and recorded as a myogram o Tracing: line recording contraction activity Three phases of muscle twitch o Latent period: events of excitationcontraction coupling No muscle tension seen o Period of contraction: cross bridge formation Tension increases o Period of relaxation: Ca reentry into SR Tension declines to zero Muscle contracts faster than it relaxes Differences in strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles o Example: eye muscles contraction are rapid and brief, whereas longer, fleshy muscles (calf muscles) contract more slowly and hold it longer Phases of a Twitch Contraction Latent period: 2 msec delay between the onset of stimulus and onset of twitch response o Time required for excitation, excitationcontraction coupling and tensing of elastic components of the muscle o Internal tension: force generated during latent period and no shortening of the muscle occurs Contraction phase : phase in which filaments slide and the muscle shortens o Once elastic components are taut, muscle begins to produce external tension in muscle that moves a load o Shortlived phase 2+ Relaxation phase : SR quickly reabsorbs Ca , myosin releases the thin filaments and tension declines o Muscle returns to resting length o Entire twitch lasts from 7 to 100 msec Contraction Strength of Twitches At subthreshold stimulus no contraction at all At threshold intensity and above a twitch is produced o Twitches caused by increased voltage are no stronger than those at threshold Not exactly true that muscle fiber obeys an allornone law (i.e., contraction to its maximum or not at all) o Electrical excitation of a muscle follows allornone law o Not true that muscle fibers follow the all or none law Different motor units can fire Twitches vary in strength depending upon: o Stimulus frequency stimuli arriving closer together produce stronger twitches o Concentration of Ca in sarcoplasm can vary the frequency o How stretched muscle was before it was stimulated o Temperature of the muscle warmedup muscle contracts more strongly enzymes work more quickly o Lower than normal pH of sarcoplasm weakens the contraction fatigue o State of hydration of muscle affects overlap of thick and thin filaments Muscles need to be able to contract with variable strengths for different tasks Graded Muscle Responses Normal muscle contraction is relatively smooth, and strength varies with needs o 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 o Required for proper control of skeletal movement Reponses are graded by: o Changing frequency of stimulation o Changing strength of stimulation Muscle response to changes in stimulus frequency o Single stimulus results in single contractile response (i.e., muscle twitch) o 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 increase in force with each stimulus Additional Ca that is released with second stimulus stimulates more shortening Produces smooth, continuous contractions that add up (summation) Further increase in stimulus frequency causes muscle to progress to sustained, quivering contraction referred to as used (incomplete) tetanus o If stimuli frequency increases, muscle tension reaches maximum Referred to as used (complete) tetanus because contractions "fuse" into on smooth sustained contraction plateau Prolonged muscle contractions lead tomuscle fatigue o Recruitment (or multiple motor unit summation ): stimulus is sent to more muscle fibers, leading to more precise control o 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 o 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 o Groups of motor units are alternately activated in response to input form stretch receptors in muscles Keeps muscles firm, healthy, and ready to respond Isotonic and Isometric Contractions Isotonic contractions: muscle changes in length and moves load o 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 Isometric contractions o Load is greater than the maximum tension muscle can generate, so muscle neither shortens nor lengthens o Electrochemical and mechanical events are same in isotonic or isometric contractions, but results are different In isotonic contractions, actin filaments slide past myosin filaments and cause movement In isometric contractions, cross bridges generate force, but actin filaments do not move Myosin heads "spin their wheels" on same actinbinding site Isometric muscle contraction o Muscle is producing internal tension while an external resistance causes it to stay the same length or become longer o Can be a prelude to movement when tension is absorbed by elastic elements o Important in postural muscle function and antagonistic muscle joint stabilization Isotonic muscle contraction o Muscle changes in length with no change in tension o Concentric contraction: muscle shortens while maintains tension o Eccentric contraction : muscle lengthens as it maintains tension 9.6 Energy for Contraction and ATP Providing Energy for Contraction ATP supplies the energy needed for the muscle fiber to: o Move and detach cross bridges o Pump calcium back into SR o Pump Na out of and K back into cell after excitationcontraction coupling Available stores of ATP depleted in 46 seconds ATP is the only source of energy for contractile activities; therefore it must be regenerated quickly Muscle Metabolism All muscle contraction depends on ATP ATP supply depends on availability of: o Oxygen o Organic energy sources such as glucose and fatty acids Two main pathways of ATP synthesis o Anaerobic fermentation Enables cells to produce ATP in the absence of oxygen Yields little ATP and toxic lactic acid, may be a major factor in muscle fatigue o Aerobic respiration Produces far more ATP Less toxic end products (CO 2and water) Requires a continual supply of oxygen Providing Energy for Contraction ATP is regenerated quickly by three mechanisms: o Direct phosphorylation of ADP by creatine phosphate (CP) o Anaerobic pathway: glycolysis and lactic acid formation o Aerobic respiration Direct phosphorylation of ADP by creatine phosphate (CP) o 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 Anaerobic pathway: glycolysis and lactic acid formation o 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 o 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 o In the absence of oxygen, referred to as aerobic glycolysis , pyruvic acid is converted to actic acid o Lactic acid Diffuses into bloodstream Used as fuel by liver, kidneys, and heart Converted back into pyruvic acid or glucose by liver o Anaerobic respiration yields only 5% as much ATP as aerobic respiration, but produces ATP 2.5 times faster Aerobic respiration o Produces 95% of ATP during rest and lighttomoderate exercise Slower than anaerobic pathway o Consists of series of chemical reactions that occur in mitochondria and require oxygen Breaks glucose into CO 2 H 2, and large amount ATP (32 can be produced) o Fuels used include glucose form glycogen stored in muscle fiber, then bloodborne glucose, and free fatty acids Fatty acids are main fuel after 30 minutes of exercise Energy systems used during sports o Aerobic endurance Length of time muscle contracts using aerobic pathways Lighttomoderate activity, which can continue for hours o Anaerobic threshold Point at which muscle metabolism converts to anaerobic pathway Fatigue Muscle fatigue: progressive weakness and loss of contractility from prolonged use of the muscles Causes of muscle fatigue o ATP synthesis declines as glycogen is consumed o ATP shortage slows down the Na K pumps o Lactic acid +owers pH of sarcoplasm o Release of K with each action potential causes the accumulation of extracellular K+ o Motor nerve fibers use up their ACh o Central nervous system, where all motor commands originate, fatigues by unknown processes, so there is less signal output to the skeletal muscles Endurance Endurance : the ability to maintain highintensity exercise for more than 4 to 5 minutes o Determined in large part by one's maximum oxygen uptake (VO max) 2 o Maximum oxygen uptake : the point at which the rate of oxygen consumption reaches a plateau and does not increase further with an added workload Proportional to body size Peaks at around age 20 Usually higher in males than in females (due to body size, which is indirectly related to lung capacity) Can be twice as great in trained endurance athletes as in untrained person Results in twice the ATP production Oxygen Debt Heavy breathing continues after strenuous exercise o Excess postexercise oxygen consumption (EPOC): the difference between the resting rate of oxygen consumption and the elevated rate following exercise o Typically about 11 liters extra is needed after strenuous exercise o Repaying the oxygen debt Needed for the following purposes: o Replace oxygen reserves depleted in the first minute of exercise o Replenishing the phosphagen system o Oxidizing lactic acid o Serving the elevated metabolic rate Beating Muscle Fatigue Taking oral creatine increases level of creatine phosphate (CP) in muscle tissue and increases speed of ATP regeneration o Useful in burst type exercise weightlifting o Risks are not well known Muscle cramping, electrolyte imbalances, dehydration, water retention, stroke Kidney disease from overloading kidney with metabolite, creatinine Carbohydrate loading dietary regimen o Packs extra glycogen into muscle cells o Extra glycogen is hydrophilic and adds 2.7 g water / g glycogen Athletes feel sense of heaviness outweighs benefits of extra available glycogen 9.7 Factors of Muscle Contraction Force of Muscle Contractions Force of contraction depends on number of cross bridges attached, which is affected by four factors: o Number of muscle fibers stimulated (recruitment): the more motor units recruited, the greater the force o Relative size of fibers: the bulkier the muscle, the more tension it can develop Muscle cells can increase in size (hypertrophy) with regular exercise o Frequency of stimulation: the higher the frequency, the greater the force Stimuli are added together o 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: o Muscle fiber type o Load o Recruitment Muscle fiber type o Classified according to two characteristics Speed of contraction slow or fast fibers according to: Speed at which myosin ATPases split ATP Pattern of electrical activity of motor neurons Metabolic pathways used for ATP synthesis Oxidative fibers: use aerobic pathways Glycolytic fibers: use anaerobic glycolysis o Based on these two criteria, skeletal muscle fibers can be classified into three types: Slow oxidative fibers, fast oxidative fibers, or fast glycolytic fibers o 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 o 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 movement Example: hitting a baseball Load and Recruitment o 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 o Recruitment : the more motor units contracting, the faster and more prolonged the contraction White and Dark Meat Poultry muscle has different "color" when cooked breast v. thigh FG and SO Muscle Fibers Fibers differ both in color and diameter 9.8 Adaptation to Exercise Aerobic (Endurance) Exercise Aerobic (endurance) exercise, such as jogging, swimming, biking leads to increased: o Muscle capillaries o Number of mitochondria o Myoglobin synthesis o Results in greater endurance, strength, and resistance to fatigue o May convert fast glycolytic fibers into fast oxidative fibers Resistance Exercise o Resistance exercise (typically anaerobic), such as weight lifting or isometric exercises, leads to o Muscle hypertrophy Due primarily to increase in fiber size o Increased mitochondria, myofilaments, glycogen stores, and connective tissue o Increased muscle strength and size Cardiac Muscle o Can contract without need for nervous stimulation o Contains a builtin pacemaker that rhythmically sets off contractions o Wave travels through the muscle and triggers contraction of heart chambers o Autorhythmic because of its ability to contract rhythmically and independently o Autonomic nervous system does send nerve fibers to the heart o Can increase or decrease heart rate and contraction strength o Very slow twitches does not exhibit quick twitches like skeletal muscle o Maintains tension for about 200 to 250 msec o Gives the heart time to expel blood o Uses aerobic respiration almost exclusively, rich in myoglobin, glycogen o Has unusually large mitochondria 25% of volume of cardiac muscle cell with large mitochondria 2% of skeletal muscle cell with smaller mitochondria o Very adaptable with respect to fuel used o Very vulnerable to interruptions of oxygen supply o Highly fatigue resistant 9.9 Smooth Muscle o Found in walls of most hollow organs, except heart o Heart contains cardiac muscle Microscopic Structures o Spindleshaped fibers: thin and short compared with skeletal muscle fibers o Only one nucleus, no striations o Lacks connective tissue sheaths o Contains endomysium only o All but smallest blood vessels contain smooth muscle organized into two layers of opposing sheets of fibers o Longitudinal layer : fibers run parallel to long axis of organ Contraction causes organ to shorten o Circular layer: fibers run around circumference of organ Contraction causes lumen of organ to constrict o Allows peristalsi: alternating contractions and relaxations of layers mix and squeeze substances through lumen of hollow organs o No neuromuscular junction, as in skeletal muscle o Instead, autonomic nerve fibers innervate smooth muscle o Contain varicositie (bulbous swellings) of nerve fibers o Varicosities store and release neurotransmitters into a wide synaptic cleft referred to as a iffuse junction o Smooth muscle does not contain sarcomeres, myofibrils, or T tubules o SR is less developed than in skeletal muscle o SR does store intracellular Ca , but most calcium used for contraction has extracellular origins o Sarcolemma contains pouchlike infoldings called aveolae o Caveolae contain numerous Ca channels that open to allow rapid influx of 2+ extracellular Ca o Smooth muscle also differs from skeletal muscle in following ways: o 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 o No troponin complex Does contain tropomyosin, but not troponin Proteincalmodulin binds Ca2+ o Thick and thin filaments arranged diagonally o Myofilaments are spirally arranged, causing smooth muscle to contract in corkscrew manner o Intermediate filamentdense body network o Contain latticelike arrangement of noncontractile intermediate filaments that resist tension o Dense bodies : proteins that anchor filaments to sarcolemma at regular intervals Correspond to Z discs of skeletal muscle o During contraction, areas of sarcolemma between dense bodes bulge outward Make muscle cell look puffy Contraction of Smooth Muscle o Mechanism of contraction o Slow, synchronized contractions o Cells electrically couple by gap junctions Action potentials transmitted from fiber to fiber o Some cells are selfexcitatory (depolarize without external stimuli) Act aspacemakers for sheets of muscle Rate and intensity of contraction may be modified by neural and chemical stimuli o Contraction in smooth muscle is similar to skeletal muscle contraction in following ways: Actin and myosin interact by sliding filament mechanism 2+ Final trigger is increased intracellular Ca level ATP energizes sliding process Contraction stops when Ca is no longer available o Contraction in smooth muscl2+is different from skeletal muscle in following ways: Some Ca still obtained from SR, but mostly comes from extracellular space Ca binds to calmodulin , not troponin Activated calmodulin then activates yosin kinase (myosin light chain kinase) Activated myosin kinase phosphorylates myosin head, activating it Leads to crossbridge formation with actin Stopping smooth muscle contraction requires more steps than skeletal muscle Relaxation requires: 2+ Ca detachment from calmodulin Active transport of Ca2+into SR and extracellularly Dephosphorylation of myosin to inactive myosin o Energy efficiency of smooth muscle contraction o Slower to contract and relax but maintains contraction for prolonged periods with little energy cost Slower ATPases Myofilaments may latch together to save energy o Most smooth muscle maintain moderate degree of contraction constantly without fatiguing Referred to as smooth muscle tone o Makes ATP via aerobic respiration pathways o Regulation of contraction o Controlled by nerves, hormones, or local chemical changes o Neural regulation Neurotransmitter binding causes either graded (local) potential or action potential Results in increases in Ca2+ concentration in sarcoplasm Response depends on neurotransmitter released and type of receptor molecules One neurotransmitter can have a stimulatory effect on smooth muscle in one organ, but an inhibitory effect in a different organ o Hormones and local chemicals Some smooth muscle cells have no nerve supply Depolarize spontaneously or in response to chemical stimuli that bind to G proteinlinked receptors Chemical factors can include hormones, high CO ,2pH, low oxygen Some smooth muscles respond to both neural and chemical stimuli o Special features of smooth muscle contraction o Response to stretch Stressrelaxation response: responds to stretch only briefly, then adapts to new length Retains ability to contract on demand Enables organs such as stomach and bladder to temporarily store contents o Length and tension changes Can contract when between half and twice is resting length Allows organ to have huge volume changes without becoming flabby when relaxed Types of Smooth Muscle o Smooth muscle varies in different organs by: o Fiber arrangement and organization o Innervation o Responsiveness to various stimuli All smooth muscle is categorized as either: o Unitary o Multiunit Unitary (singleunit) smooth muscle o Commonly referred to as visceral muscle o Found in all hollow organs except heart o Possess all common characteristics of smooth muscle: Arranged in opposing (longitudinal and circular) sheets Innervated by varicosities Often exhibit spontaneous action potentials Electrically coupled by gap junctions Respond to various chemical stimuli Multiunit smooth muscle o Located in large airways in lungs, large arteries, arrector pili muscles, and iris of eye o Very few gap junctions, and spontaneous depolarization is rare o Similar to skeletal muscle in some features Consists of independent muscle fibers Innervated by autonomic nervous system, forming motor units Graded contractions occur in response to neural stimuli that involve recruitment o Different from skeletal muscle because, like unitary smooth muscle, it is controlled by autonomic nervous system and hormones
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