2Muscle is derived from Musculus, for “Mouse” Imagine a mouse running beneath the skin.Functions of Muscles:Body movementMaintain postureProduces heatPropel substances through bodyHeartbeatTypes of muscles include:Smooth muscleCardiac muscleSkeletal muscle
3Smooth Muscle Characteristics of smooth muscles Involuntary control Tapered cells with a single, central nucleusLack striations
4Smooth Muscle There are two types of smooth muscles Multi-unit Smooth Muscleunorganized cells that contract as individual cellsLocated within the iris of eye and the walls of blood vesselsVisceral (single-unit) Smooth MuscleForm sheets of muscleCells are connected by gap junctionsMuscle fibers contract as a groupRhythmic contractionsWithin walls of most hollow organs (viscera)
5Cardiac Muscle Located only in the heart Striated cells Intercalated discsMuscle fibers branchMuscle fibers contractas a unitSelf-exciting and rhythmic
6Skeletal Muscle Usually attached to bone Voluntary control Striated (light & dark bands)Muscle fibers form bundlesSeveral peripheral nuclei
7Coverings of Skeletal Muscle FasciaDense connective tissue surrounding skeletal musclesTendonsDense connective tissue that attaches muscle to bonesContinuation of muscle fascia and bone periosteumAponeurosisBroad sheet of connective tissue attaching muscles to bone, or to other muscles.
9Coverings of Skeletal Muscle EpimysiumConnective tissue that covers theentire muscleLies deep to fasciaPerimysiumSurrounds organized bundles ofmuscle fibers, called fasciclesEndomysiumConnective tissue that coversindividual muscle fibers (cells)
10Figure 9.3 Scanning electron micrograph of a fascicle surrounded by its perimysium. Muscle fibers within the fascicle are surrounded by endomysium.
11Organization of Skeletal Muscle FascicleOrganized bundle of muscle fibersMuscle FiberSingle muscle cellCollection of myofibrilsMyofibrilsCollection of myofilamentsMyofilamentsActin filamentMyosin filamentFigure 9.2Skeletal muscle organization
13Skeletal Muscle Fibers SarcolemmaCell membrane of muscle fibersSarcoplasmCytoplasm of muscle fibersSarcoplasmic ReticulumModified Endoplasmic ReticulumStores large deposits of Calciumsarcolemma
14Skeletal Muscle Fibers (Transverse)T-tubules:invaginations of sarcolemma,extending into the sarcoplasm.Cisternae:enlarged region of sarcoplasmicreticulum, adjacent to the t-tubulesTriadT-tubule + adjacent cisternaeOpenings into t-tubules
15Myofibrils Myofibrils are bundles of actin and myosin filaments. Actin – thin filamentMyosin – thick filamentStriations appear from the organization of actin and myosin filamentsFigure 9.4 Organization of actin and myosin filaments
16Sarcomere A sarcomere is the functional unit of skeletal muscle A sarcomere is the area betweenadjacent Z-lines.During a muscle contraction the z-lines move together and the sarcomere shortens.
18Striations appear from alternate light and dark banding patterns. Z Line is the attachment site of actin filaments (center of I bands)I Bands (light band): consists of onlyactin filamentsA Bands (dark band) : consists of myosin filaments and the overlapping portion of actin filamentsFigure 9.5 thin and thick filaments in a sarcomere.
19filaments Thin filaments composed of actin proteins Thin filaments are associated with troponinand tropomyosin proteinsThick filamentscomposed of myosin proteinsDuring muscle contraction the heads on myosin filaments bind to actin filaments forming a Cross-bridge
20Cross-BridgesWhen a muscle is at rest, myosin heads are extended in the “cocked” position.During a contraction, myosin heads bind to actin, forming a cross-bridge and the myosin head pivot forward (Power Stroke) and back (Recovery stroke)
21Troponin-Tropomyosin Complex The troponin-tropomyosin complex prevents cross-bridge formation when the muscle is at rest.TropomyosinBlocks binding sites onactin when a muscle isat restTroponinCa2+ binds to troponinduring a muscle contraction.Troponin moves repositions the tropomyosin filaments, so the myosin and actin filaments can interact.End of section 1, chapter 9
23SynapseSynapse: Functional (not physical) junction between an axon of a neuron and another cellThe two cells are separated by a physical space, called the synaptic cleft.Neurotransmitters are stored within synaptic vesicles of the presynaptic cell and they’re released into the synapse.
24Neuromuscular Junction Neuromuscular Junction (NMJ) refers to the synapse between an axon and a muscle fiber.Motor End Plate is a highly folded region of muscle fiber at NMJ that contain abundant mitochondriaFigure 9.8a. General NMJ
25Motor Unit 1 motor unit may control between 1 and 1000 muscle fibers Motor neurons innervate effectors (muscles or glands)A motor unit includes a motor neuron and all of the muscle fibers it controls1 motor unit may control between 1 and 1000 muscle fibersFigure 9.9 two motor units. The muscle fibers of a motor unit are innervated (controlled) by a single motor neuron.
26Stimulus for Contraction Acetylcholine (ACh) is the only neurotransmitter that initiates skeletal muscle contractionSequence of ActionsA nerve impulse (Action Potential) reaches axon terminalThe impulse opens calcium channels at the axon terminalCalcium diffuse into axonThe calcium triggers the release of ACh from vesicles into synaptic cleft.
27Stimulus for Contraction Sequence of Actions…ContinuedACh diffuses across synaptic cleft & binds to receptors on motor endplate.ACh opens Na+ channels on muscleNa+ floods into the muscle, initiating a muscle impulse.A muscle impulse (action potential) is propagated across the entire muscle.
28Stimulus for a muscle impulse Stimulus for a muscle impulse. Corresponds to steps 1-7 in the previous slides.
29The muscle impulse causes the release of calcium from the SR The muscle impulse causes the release of calcium from the SR. Calcium binds to troponin and tropomyosin is repositioned exposing the actin filaments.
30Stimulus for contraction continued… 8. The muscle impulse diffuses across sarcolemma and down the t-tubules into the cisternae of sarcoplasmic reticula.9. The sarcoplasmic reticula release their calcium supplies into the sarcoplasm.10. Calcium binds to troponin and the troponin repositions the tropomyosin, so the myosin can bind to actin.11. Cross-bridge cycling causes the muscle to contract.
31Excitation-Contraction Coupling Calcium released from sarcoplasmic reticulum binds to troponin.Troponin moves tropomyosin, exposing actin filaments to myosin cross-bridges.myosin heads bind to actin, forming a cross bridge and cross-bridge cycling causes the muscle to contract.End of section 2, chapter 9
32Sliding Filament Theory of Contraction ivyanatomy.comsection 3, chapter 9Sliding Filament Theory of Contraction
33The Sliding Filament Model of Muscle Contraction During a muscle contractionThick (myosin) filaments and thin (actin) filaments slide across one anotherThe filaments do not change lengthsZ-bands move closer together causing the sarcomere to shorten.I bands appear narrowFigure 9.11a. Individual sarcomeres shorten as thick and thin filaments slide past one another.
34Cross Bridge CyclingWhen a muscle is relaxed, tropmyosin covers the binding sites on actin.A molecule of ADP and Phosphate remains attached to myosin from the previous contraction.
35Cross Bridge Cycling During a contraction, Calcium binds to troponin. Tropomyosin is repositioned, exposing the myosin binding sites on actin filaments
36Cross Bridge Cycling Myosin heads bind to actin filaments. The phosphate is released.
37Cross Bridge CyclingMyosin heads spring forward “Power Stroke” pulling the actin filaments.ADP is released from Myosin
38Cross Bridge Cycling Myosin is released from actin. A new molecule of ATP binds to myosin, causing it to be released from the actin filament.ATP is not yet broken down, but it is essential to release the cross-bridges.
39Cross Bridge CyclingATP is broken down, providing the energy to “cock” the myosin filaments (recovery stroke).Steps 1-6 are repeated several times.
40Figure 9. 10. The cross-bridge cycle Figure The cross-bridge cycle. The cycle continues as long as ATP is present, and nerve impulses release Acetylcholoine.Watch the You-Tube video “Sliding Filament” to view cross-bridge cycling in action.
41RelaxationWhen a nerve impulse ceases, two events relax muscle fibers.Acetylcholinesterase breaks down Ach in the synapse.Prevents continuous stimulation of a muscle fiber.Calcium Pumps (Ca2+ATPase) remove Ca2+ from the sarcoplasm and returns it to the SR.Without calcium, tropomyosin covers the binding sites on actin filaments.
42Relaxation * Notice that ATP is required for muscle relaxation! Rigor Mortis is a partial contraction of skeletal muscles that occurs a few hours after death.After death calcium leaks into sarcoplasm, triggering the muscle contractions.But ATP supplies are diminished after death, so ATP is not available to remove the cross-bridge linkages between actin and myosin.muscles do not relax*.Contraction is sustained until muscles begin to decompose.* Notice that ATP is required for muscle relaxation!
44Energy Sources for Contraction ivyanatomy.comsection 4, chapter 9Energy Sources for Contraction
45Energy Sources for Contraction ATP provides the energy to power the interaction between actin & myosin filaments.However, ATP is quickly spent and must be replenishedNew ATP molecules are synthesized byHydrolysis of Creatine PhosphateGlycolysis (anaerobic respiration)Aerobic Respiration
46Creatine PhosphateCreatine Phosphate can be hydrolyzed into Creatine, releasing energy that is used to make new ATP.The energy from creatine phosphate hydrolysis cannot be used to directly power muscles.Instead, it’s used to produce new ATP.
47Creatine Phosphate…continued When cellular ATP is abundant, creatine phosphate can be replenished by phosphorylating creatine.Creatine Phosphate provides energy for only about 10 seconds of a high intensity muscle contraction.
48GlycolysisAnaerobic respiration (glycolysis) occurs in the cytosol of the cell and does not require oxygen.Glucose molecules are partially broken down producing just 2 ATP for each glucose.If there isn’t sufficient oxygen available, glycolysis produces lactic acid as a byproduct.
49Oxygen debt of glycolysis Exercise and strenuous activity depends on anaerobic respiration for ATP supplies.During exercise anaerobic respiration causes lactic acid to accumulate in the cells.After exercise, when oxygen is available the O2 is used to convert lactic acid back to glucose in the liver.Oxygen debt is the amount of oxygen needed by liver cells to convert accumulated lactic acid back to glucose.
51Aerobic RespirationAerobic respiration (uses oxygen) occurs in the mitochondria and it includes the citric acid cycle & electron transport chain.Aerobic respiration is a slower reaction than glycolysis, but it produces the most ATP.MyoglobinOxygen binding protein (similar to hemoglobin) within muscles-Provides additional oxygen supply to muscles
52Aerobic RespirationAerobic respiration is used primarily at rest or during light exercise.Muscles that rely on aerobic respiration have plenty of mitochondria and a good blood supply.
53Energy Sources for Contraction Figure The oxygen required for aerobic respiration is carried in the blood and stored in myoglobin. In the absence of oxygen, anaerobic respiration uses pyruvic acid to produce lactic acid.
54Muscle Fatigue Muscle Fatigue = Inability for the muscle to contract Several factors can cause muscle fatigue:Decreased blood flowIon imbalances across the sarcolemmaLactic acid accumulation – (greatest cause of fatigue)Cramp:A cramp is a sustained, involuntary, and painful muscle contractionIt’s due to electrolyte imbalance surrounding muscle
55Heat Production End of Chapter 9, Section 4 Heat is produced as a by-product of cellular respirationMuscle cells are major source of body heatBlood transports heat throughout body coreEnd of Chapter 9, Section 4
57Muscle Response subthreshold stimulus Myograph A muscle contraction can be observed by removing a single skeletal muscle and connecting it to a device (myograph) that senses and records changes in the overall length of the muscle fiber.A threshold stimulus is the minimum stimulus that elicits a muscle fiber contractionall-or-none responseA threshold stimulus will cause the muscle fiber to contract fully and completely.A stronger stimulus does not produce a stronger contraction!musclesubthreshold stimulusMyographThe muscle fiber will not contract at all if the stimulus is less than threshold.
58Recording of a Muscle Contraction A twitch is a single contractile response to a stimulusA twitch can be divided into three periods.Latent periodbrief delay between the stimulusand the muscle contractionThe latent period is less than 2 milliseconds in humans2. Period of contraction3. Period of relaxation
59Summation series of twitches Figure 9.17aseries of twitchesIf the muscle is allowed to relax completely before each stimulus than the muscle will contract with the same force.Figure 9.17bsummationIf the muscle is stimulated again before it has completely relaxed, then the force of the next contraction increases.i.e. stimulating the muscle at a rapid frequency increases the force of contraction. This is called summation
60Summation Tetanic Contraction (c) If the muscle is stimulated at a high frequency the contractions fuse together and cannot be distinguished.A tetanic contraction results in a maximal sustained contraction without relaxationFigure 9.17c
61Recruitment of Motor Units all-or-none responseA muscle that is stimulated with threshold potential contracts completely and fully.A stronger stimulus does not produce a stronger contraction!Instead, the strength of a muscle is increased by recruitment of additional motor units.
62Recruitment of Motor Units Recruitment – progressive activation of motor units to increase the force of a muscle contraction.Recall that a motor unit is a motor neuron plus all of the fibers it controls.Muscles are composed of many motor units.As a general rule, motor units are recruited in order of their sizeSmall motor units are stimulated with light activities, but additionalmotor units are recruited with higher intensity activity.As the intensity of stimulation increases, recruitment of motor units continues until all motor units are activated.
63Sustained Contractions The central nervous system can increase thestrength of contractions in 2 ways:RecruitmentSmaller motor units are recruited first, followed by larger motor units.The result is a sustained contraction of increasing strength.Increased firing rateA high frequency of action potentials results in summation of the muscle contractions.If the frequency is too high, however, it may produce tetanic contractions, in which case the muscle does not relax.Muscle tone is produced because some muscles are in a continuous state of partial contraction in response to repeated nerve impulses from the spinal cord.
64Types of Contractions Isotonic – muscle contracts and changes length Concentric – shortening of muscle (a)Eccentric – lengthening of muscle (b)Isometric – muscle contracts but does not change length (c)Isometric contractions stabilizes posture and holds the body uprightFigure muscle contractions
65Fast twitch and slow twitch muscle fibers Fast & Slow twitch refers to the contraction speed, and to whether muscle fibers produce ATP oxidatively (by aerobic respiration) or glycolytically (by glycolysis)Slow-twitch fibers (Type I)Always oxidative and resistant to fatigueContain myoglobin for oxygen storage “red fibers”Also have many mitochondria for aerobic respirationGood blood supply
66Slow-twitch fibers (Type I) Slow-twitch fibers are best suited for endurance exercise over a long period with little force.Slow-twitch fibers rely on aerobic respiration for energy (ATP) and are resistant to fatigue.Slow-Twitch fibers contain abundant myoglobin for oxygen storage “red fibers” and mitochondria to carry out aerobic respiration.Because of their oxygen demands, slow-twitch fibers have a good blood supply.
67Fast twitch muscle fibers – two types Fast-twitch glycolytic fibers contract rapidly and with great force, but they fatigue quickly.They are best suited for rapid contractions over a short duration.Fast-twitch glycolytic fibers (type IIa) contain very little mitochondria and myoglobin and are “white fibers”
68Fast twitch muscle fibers – two types Fast-twitch intermediate or fast oxidative fibers contain intermediate amounts of myoglobin.They contract rapidly but also have the capacity to generate energy through aerobic respiration.
69Fast twitch and slow twitch muscle fibers Migrating birds have abundant slow-twitch fibers for flying long distances, which is why their flesh is dark.Chickens that can only flap around the barnyard have abundant fast-twitch muscles and mostly white flesh.End of Chapter 9