Presentation on theme: "Muscular System: Histology and Physiology"— Presentation transcript:
1 Muscular System: Histology and Physiology Chapter 9Muscular System: Histology and Physiology
2 Functions of the Muscular System Body movement (skeletal muscles attached to bones)Maintenance of postureRespiration (skeletal muscles of thorax are responsible for the movement necessary for respiration)Production of body heat (when skeletal muscles contact, heat is given off as a by-product)Communication (speaking, writing)Constriction of organs and vessels (contraction of smooth muscle)Heart beat (contraction of cardiac muscle)
3 General Functional Characteristics of Muscle Contractility: ability of a muscle to shorten with forceExcitability: capacity of muscle to respond to a stimulus (by nerve or hormone)Extensibility: muscle can be stretched to its normal resting length and beyond to a limited degreeElasticity: ability of muscle to recoil to original resting length after stretched
4 Types of Muscle Tissue Skeletal Smooth Cardiac Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movementVoluntarySmoothWalls of hollow organs, blood vessels, eye, glands, skinSome functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flowIn some locations, autorhythmicControlled involuntarily by endocrine and autonomic nervous systemsCardiacHeart: major source of movement of bloodAutorhythmic
6 Skeletal Muscle Structure Composed of muscle cells (fibers), connective tissue, blood vessels, nervesFibers are long, cylindrical, multinucleatedTend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in lengthDevelop from myoblasts (they are converted to muscle fibers as contractile proteins accumulate within their cytoplasm); numbers remain constant (# of muscle fibers remain constant after birth----so, enlargement of muscles is an increase in size rather than #)Striated appearance due to light and dark banding
7 Connective Tissue Coverings of Muscle LayersExternal lamina. Delicate, reticular fibers. Surrounds sarcolemma (P.M.)Endomysium. Loose C.T. with reticular fibers.Perimysium. Denser C.T. surrounding a group of muscle fibers. Each group called a fasciculusEpimysium. C.T. that surrounds a whole muscle (many fascicles)Fascia: connective tissue sheetForms layer under the skinHolds muscles together and separates them into functional groups.Allows free movements of muscles.Carries nerves (motor neurons, sensory neurons), blood vessels, and lymphatics.Continuous with connective tissue of tendons and periosteum.
8 Nerves and Blood Vessel Supply Motor neurons: stimulate muscle fibers to contract. Nerve cells with cell bodies in brain or spinal cord; axons extend to skeletal muscle fibers through nervesAxons branch so that each muscle fiber is innervatedCapillary beds surround muscle fibers
9 Muscle Fibers Nuclei just inside sarcolemma Cell packed with myofibrils within cytoplasm (sarcoplasm = cytoplasm without myofibrils)Threadlike (extends from one end of muscle fiber to the other)Composed of protein threads called myofilaments: thin (actin 8nm) and thick (myosin 12nm)Sarcomeres: actin & myosin myofilaments form highly ordered units called sarcomeres. They are joined end to end to form the myofibrils.
12 Actin (Thin) Myofilaments Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere.Composed of G actin monomers each of which has an active siteActin site can bind myosin during muscle contraction.Tropomyosin: an elongated protein winds along the groove of the F actin double helix.Troponin is composed of three subunits: one that binds to actin, a second that binds to tropomyosin, and a third that binds to calcium ions. Spaced between the ends of the tropomyosin molecules in the groove between the F actin strands.The tropomyosin/troponin complex regulates the interaction between active sites on G actin and myosin.
13 Myosin (Thick) Myofilament Many elongated myosin molecules shaped like golf clubs.Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally.Myosin headsCan bind to active sites on the actin molecules to form cross-bridges.Attached to the rod portion by a hinge region that can bend and straighten during contraction.Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction
14 Sarcomeres: Z Disk to Z Disk Z disk: filamentous network of protein. Serves as attachment for actin myofilamentsStriated appearanceI bands: from Z disks to ends of thick filamentsA bands: length of thick filamentsH zone: region in A band where actin and myosin do not overlapM line: middle of H zone; delicate filaments holding myosin in placeIn muscle fibers, A and I bands of parallel myofibrils are aligned.Titin filaments: elastic chains of amino acids; make muscles extensible and elastic
15 Sliding Filament Model Actin myofilaments sliding over myosin to shorten sarcomeresActin and myosin do not change lengthShortening sarcomeres responsible for skeletal muscle contractionDuring relaxation, sarcomeres lengthen because of some external force, like forces produced by other muscles (contraction of antagonistic muscles) or by gravity.- agonist = muscle that accomplishes a certain movement, such as flexion.- antagonist = muscle acting in opposition to agonist.
17 Physiology of Skeletal Muscle Fibers Nervous system controls muscle contractions through action potentialsResting membrane potentialsMembrane voltage difference across membranes (polarized)Inside cell more negative due to accumulation of large protein molecules. More K+ on inside than outside. K+ leaks out (through leak channels) but not completely because negative molecules hold some back.Outside cell more positive and more Na+ on outside than inside.Na+ /K+ pump maintains this situation.Must exist for action potential to occur
18 Ion Channels Types Each is specific for one type of ion Ligand-gated. Ligands are molecules that bind to receptors. Receptor: protein or glycoprotein with a receptor siteExample: neurotransmittersGate is closed until neurotransmitter attaches to receptor molecule. When Ach (acetylcholine) attaches to receptor on muscle cell, Na gate opens. Na moves into cell due to concentration gradientVoltage-gatedOpen and close in response to small voltage changes across plasma membraneEach is specific for one type of ion
19 Action Potentials Phases Depolarization: Inside of plasma membrane becomes less negative. If change reaches threshold, depolarization occursRepolarization: return of resting membrane potential. Note that during repolarization, the membrane potential drops lower than its original resting potential, then rebounds. This is because Na plus K together are higher, but then Na/K pump restores the resting potentialAll-or-none principle: like camera flash systemPropagate: Spread from one location to another. Action potential does not move along the membrane: new action potential at each successive location.Frequency: number of action potential produced per unit of time
22 Neuromuscular Junction Synapse: axon terminal resting in an invagination of the sarcolemmaNeuromuscular junction (NMJ):Presynaptic terminal: axon terminal with synaptic vesiclesSynaptic cleft: spacePostsynaptic membrane or motor end-plate
23 Function of Neuromuscular Junction Synaptic vesiclesNeurotransmitter: substance released from a presynaptic membrane that diffuses across the synaptic cleft and stimulates (or inhibits) the production of an action potential in the postsynaptic membrane.AcetylcholineAcetylcholinesterase: A degrading enzyme in synaptic cleft. Prevents accumulation of ACh
25 Excitation-Contraction Coupling Mechanism by which an action potential causes muscle fiber contractionInvolvesSarcolemmaTransverse (T) tubules: invaginations of sarcolemmaTerminal cisternaeSarcoplasmic reticulum: smooth ERTriad: T tubule, two adjacent terminal cisternaeCa2+Troponin
28 RelaxationCa2+ moves back into sarcoplasmic reticulum by active transport. Requires energyCa2+ moves away from troponin-tropomyosin complexComplex re-establishes its position and blocks binding sites.
29 Muscle TwitchMuscle contraction in response to a stimulus that causes action potential in one or more muscle fibersMuscle contraction measures as force, also called tension. Requires up to a second to occur.PhasesLag or latent (neuromuscular junction & step #1 of cross-bridge movement)Contraction (step #2 - #6 of cross-bridge movement)Relaxation (powerpoint slide # 28)
31 Stimulus Strength and Muscle Contraction All-or-none law for muscle fibersContraction of equal force in response to each action potentialSub-threshold stimulus: no action potential; no contractionThreshold stimulus: action potential; contractionStronger than threshold; action potential; contraction equal to that with threshold stimulusMotor units: a single motor neuron and all muscle fibers innervated by it
32 Contraction of the Whole Muscle Whole muscles exhibit characteristics that are more complex than those of individual muscle fibers or motor units. Instead of responding in an all-or-none fashion, whole muscles respond to stimuli in a graded fashion, which means that the strength of the contractions can range from weak to strong.Remember: There are many muscle fibers in one fasciculi and many fasciculi in one whole muscle.Strength of contraction in whole muscle is graded: ranges from weak to strong depending on stimulus strengthMultiple motor unit summation: the force in which a whole muscle contracts depends on the number of motor units stimulated to contract. (force of contraction increases as more & more motor units are stimulated). A muscle has many motor unitsSubmaximal stimuliMaximal stimulusSupramaximal stimuli
34 Stimulus Frequency and Muscle Contraction Relaxation of a muscle fiber is not required before a second action potential can stimulate a second contraction.As the frequency of action potentials increase, the frequency of contraction increasesIncomplete tetanus: muscle fibers partially relax between contractionComplete tetanus: no relaxation between contractionsMultiple-wave summation: muscle tension increases as contraction frequencies increase
35 Types of Muscle Contractions Isometric: no change in length of muscle but tension increases during contractionPostural muscles of body ex: muscles hold spine erect while person is sitting or standingIsotonic: change in length but tension constant ex: waving using computer keyboardConcentric: tension is so great it overcomes opposing resistance and muscle shortens ex: raising of a weight during a bicep curl.Eccentric: tension maintained but muscle lengthens ex: person slowly lowers a heavy weightMuscle tone: constant tension by muscles for long periods of time
36 FatigueDecreased capacity to work and reduced efficiency of performanceTypesPsychological: depends on emotional state of individual ex: burst of activity in tired athlete in response to encouragement from spectators shows how psychological fatigue can be overcomeMuscular: results from ATP depletion ex: fatigue in lower limbs of marathon runners or in upper & lower limbs of swimmersSynaptic: occurs in NMJ due to lack of acetylcholine ex: rare-----only under extreme exertion
37 Physiological Contracture and Rigor Mortis Physiological contracture: state of extreme fatigue (extreme exercise) where due to lack of ATP neither contraction nor relaxation can occurRigor mortis: development of rigid muscles several hours after death. Ca2+ leaks into sarcoplasm and attaches to myosin heads and crossbridges form but no ATP available to bind to myosin so the cross-bridges are unable to release. Rigor ends as tissues start to deteriorate.
38 Energy SourcesATP provides immediate energy for muscle contractions. Produced from three sourcesCreatine phosphateDuring resting conditions stores energy to synthesize ATPADP + Creatine phosphate Creatine + 1ATP(Creatine Kinase)Anaerobic respirationOccurs in absence of oxygen and results in breakdown of glucose to yield ATP and lactic acidAerobic respirationRequires oxygen and breaks down glucose to produce ATP, carbon dioxide and waterMore efficient than anaerobic
39 Slow and Fast Fibers Slow-twitch oxidative Fast-twitch Contract more slowly, smaller in diameter, better blood supply, more mitochondria (also called oxidative because carry out aerobic respiration), more fatigue-resistant than fast-twitch, large amount of myoglobin (dark pigment which binds oxygen & acts as a muscle reservoir for oxygen when blood does not supply adequate amount).Postural muscles, more in lower than upper limbs. Dark meat of chicken.Functions: Maintenance of posture & performance in endurance activities.Fast-twitchRespond rapidly to nervous stimulation, contain myosin that can break down ATP more rapidly than that in Type I, less blood supply, fewer and smaller mitochondria than slow-twitch (adapted to perform anaerobic respiration)Lower limbs in sprinter, upper limbs of most people. White meat in chicken.Comes in oxidative and glycolytic formsFunctions: Rapid, intense movements of short durationDistribution of fast-twitch and slow-twitchMost muscles have both but varies for each muscleExercise: weight lifting enlarges fast-twitch & aerobic training enlarges slow-twitchEffects of exercise: change in size of muscle fibersHypertrophy: increase in muscle sizeIncrease in myofibrilsIncrease in nuclei due to fusion of satellite cellsIncrease in strengthAtrophy: decrease in muscle sizeReverse except in severe situations where cells die
41 Smooth MuscleNot striated, fibers smaller than those in skeletal muscleSpindle-shaped; single, central nucleusMore actin than myosinCaveolae: indentations in sarcolemma; may act like T tubulesDense bodies instead of Z disks as in skeletal muscle; have noncontractile intermediate filamentsCa2+ required to initiate contractions; binds to calmodulin (protein). Calmodulin molecules with Ca++ bound to them activate an enzyme called myosin kinase, which transfers a phosphate group from ATP to heads of myosin molecules. Cross-bridging occursRelaxation: caused by enzyme myosin phosphatase
43 Electrical Properties of Smooth Muscle Slow waves of depolarization and repolarization transferred from cell to cellDepolarization caused by spontaneous diffusion of Na+ and Ca2+ into cellDoes not follow all-or-none lawContraction regulated by nervous system and by hormones (ex: epinephrine)
44 Regulation of Smooth Muscle Innervated by autonomic nervous system (composed of nerve fibers that send impulses from CNS to smooth muscle, cardiac muscle, glands)Neurotransmitters are acetylcholine and norepinephrine (increases cardiac output, blood glucose levels)Hormones important as epinephrine and oxytocinReceptors present on plasma membrane; which neurotransmitters or hormones bind determines response
45 Cardiac Muscle Found only in heart Striated Each cell usually has one nucleusHas intercalated disks and gap junctionsAutorhythmic cellsAction potentials of longer durationThe depolarization of cardiac muscle results from influx of Na+ and Ca2+ across the plasma membrane
46 Effects of Aging on Skeletal Muscle Reduced muscle massIncreased time for muscle to contract in response to nervous stimuliReduced staminaIncreased recovery timeLoss of muscle fibers