2 Muscular System Functions Body movement (Locomotion)Maintenance of postureRespirationDiaphragm and intercostal contractionsCommunication (Verbal and Facial)Constriction of organs and vesselsPeristalsis of intestinal tractVasoconstriction of b.v. and other structures (pupils)Heart beatProduction of body heat (Thermogenesis)
3 Properties of MuscleExcitability: capacity of muscle to respond to a stimulusContractility: ability of a muscle to shorten and generate pulling forceExtensibility: muscle can be stretched back to its original lengthElasticity: ability of muscle to recoil to original resting length after stretched
4 Types of Muscle Skeletal Smooth Cardiac Attached to bones Makes up 40% of body weightResponsible for locomotion, facial expressions, posture, respiratory movements, other types of body movementVoluntary in action; controlled by somatic motor neuronsSmoothIn the walls of hollow organs, blood vessels, eye, glands, uterus, skinSome functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow,In some locations, autorhythmicControlled involuntarily by endocrine and autonomic nervous systemsCardiacHeart: major source of movement of bloodAutorhythmic
5 Connective Tissue Sheaths Connective Tissue of a MuscleEpimysium. Dense regular c.t. surrounding entire muscleSeparates muscle from surrounding tissues and organsConnected to the deep fasciaPerimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicleContains b.v and nervesEndomysium. Loose connective tissue that surrounds individual muscle fibersAlso contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissueCollagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.
6 Nerve and Blood Vessel Supply Motor neuronsstimulate muscle fibers to contractNeuron axons branch so that each muscle fiber (muscle cell) is innervatedForm a neuromuscular junction (= myoneural junction)Capillary beds surround muscle fibersMuscles require large amts of energyExtensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers
8 Skeletal Muscle Long cylindrical cells Many nuclei per cell Striated VoluntaryRapid contractionsSkeletal muscle attaches to our skeleton. *The muscle cells a long and cylindrical. *Each muscle cell has many nuclei. *Skeletal muscle tissue is striated. It has tiny bands that run across the muscle cells. *Skeletal muscle is voluntary. We can move them when we want to. *Skeletal muscle is capable of rapid contractions. It is the most rapid of the muscle types.
9 Basic Features of a Skeletal Muscle Muscle attachmentsMost skeletal muscles run from one bone to anotherOne bone will move – other bone remains fixedOrigin – less movable attach- mentInsertion – more movable attach- ment
10 Basic Features of a Skeletal Muscle Muscle attachments (continued)Muscles attach to origins and insertions by connective tissueFleshy attachments – connective tissue fibers are shortIndirect attachments – connective tissue forms a tendon or aponeurosisBone markings present where tendons meet bonesTubercles, trochanters, and crests
11 Skeletal Muscle Structure Composed of muscle cells (fibers), connective tissue, blood vessels, nervesFibers are long, cylindrical, and multinucleatedTend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in lengthDevelop from myoblasts; numbers remain constantStriated appearanceNuclei are peripherally located
14 Microanatomy of Skeletal Muscle In this unit we will primarily study skeletal muscle. Each muscle cell is called a muscle fiber. Within each muscle fiber are many myofibrils.
15 Muscle Fiber Anatomy Sarcolemma - cell membrane Surrounds the sarcoplasm (cytoplasm of fiber)Contains many of the same organelles seen in other cellsAn abundance of the oxygen-binding protein myoglobinPunctuated by openings called the transverse tubules (T-tubules)Narrow tubes that extend into the sarcoplasm at right angles to the surfaceFilled with extracellular fluidMyofibrils -cylindrical structures within muscle fiberAre bundles of protein filaments (=myofilaments)Two types of myofilamentsActin filaments (thin filaments)Myosin filaments (thick filaments)At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemmaWhen myofibril shortens, muscle shortens (contracts)
16 Sarcoplasmic Reticulum (SR) SR is an elaborate, smooth endoplasmic reticulumruns longitudinally and surrounds each myofibrilForm chambers called terminal cisternae on either side of the T-tubulesA single T-tubule and the 2 terminal cisternae form a triadSR stores Ca++ when muscle not contractingWhen stimulated, calcium released into sarcoplasmSR membrane has Ca++ pumps that function to pump Ca++ out of the sarcoplasm back into the SR after contraction
19 Sarcomeres: Z Disk to Z Disk Sarcomere - repeating functional units of a myofibrilAbout 10,000 sarcomeres per myofibril, end to endEach is about 2 µm longDifferences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance.A bands: a dark band; full length of thick (myosin) filamentM line - protein to which myosins attachH zone - thick but NO thin filamentsI bands: a light band; from Z disks to ends of thick filamentsThin but NO thick filamentsExtends from A band of one sarcomere to A band of the next sarcomereZ disk: filamentous network of protein. Serves as attachment for actin myofilamentsTitin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignmentSarcomeres: Z Disk to Z Disk
21 Myosin (Thick) Myofilament Many elongated myosin molecules shaped like golf clubs.Single filament contains roughly 300 myosin moleculesMolecule 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. (Actin binding site)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 contractionMyosin (Thick) Myofilament
22 Actin (Thin) Myofilaments Thin Filament: composed of 3 major proteinsF (fibrous) actinTropomyosinTroponinTwo 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 a myosin-binding site (see yellow dot)Actin 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:Tn-A : binds to actinTn-T :binds to tropomyosin,Tn-C :binds to calcium ions.Actin (Thin) Myofilaments
23 Now, putting it all together to perform the function of muscle: Contraction Dark and light bands can be seen in the muscle fiber and also in the smaller myofibrils. An enlargement of the myofibril reveals that they are made of smaller filaments or myofilaments. *There is a thick filament called myosin and *a thin filament called actin. Note the I band, A band H zone or band and Z disc or line. These will be discussed shortly.
24 Z lineA small section of a myofibril is illustrated here. Note the thick myosin filaments are arranged between overlapping actin filaments. *The two Z lines mark the boundary of a sarcomere. The sarcomere is the functional unit of a muscle cell .We will examine how sarcomeres function to help us better understand how muscles work.
25 A myosin molecule is elongated with an enlarged head at the end.
26 Many myosin molecules form the thick myosin filament Many myosin molecules form the thick myosin filament. It has many heads projecting away from the main molecule.
27 The thinner actin filament is composed of three parts: actin, tropomyosin and troponin.
28 H BandHere is a sarcomere illustrating the thin actin and thick myosin filaments. The area of the sarcomere has only myosin is called the H band.
29 Sarcomere RelaxedHere is another diagram of a sarcomere. Note the A band. It is formed by both myosin and actin filaments. The part of the sarcomere with only actin filaments is called the I band. This is a sarcomere that is relaxed.
30 Sarcomere Partially Contracted This sarcomere is partially contracted. Notice than the I bands are getting shorter.
31 Sarcomere Completely Contracted The sarcomere is completely contracted in this slide. The I and H bands have almost disappeared.
32 Which filament has moved as the sarcomere contracted Which filament has moved as the sarcomere contracted? Note the thick myosin filaments have not changed, but the thin actin filaments have moved closer together.
33 The actin filaments are moved by the heads of the myosin filaments The actin filaments are moved by the heads of the myosin filaments. In step one the myosin head attaches to an actin filament to create a cross bridge. Step two shows that the attached myosin head bends to move the actin filament. The myosin head as expended energy to create this movement. This is a power stroke or working stroke. Step three shows that energy in the form of ATP will unhook the myosin head. In step 4 the myosin head is cocked and ready to attach to an actin filament to start another power stroke.
34 Binding Site Tropomyosin Troponin Ca2+ The string of green circles represents an actin filament. There are binding sites in the filament for the attachment of myosin heads. *In a relaxed muscle the binding sites are covered by tropomyosin. The tropomyosin has molecules of troponin attached to it. *Calcium, shown in yellow, will attach to troponin. *Calcium will change the position of the troponin, tropomyosin complex. *The troponin, tropomyosin complex has now moved so that the binding sites are longer covered by the troponin, tropomyosin complex.
35 MyosinThe binding sites are now exposed and myosin heads are able to attach to form cross bridges.*
36 This diagram shows the microanatomy of skeletal muscle tissue again This diagram shows the microanatomy of skeletal muscle tissue again. *The blue sarcoplasmic reticulum is actually the endoplasmic reticulum. It stores calcium. *The mitochondria are illustrated in orange. They generate ATP, which provides the energy for muscle contractions.
37 Excitation-Contraction Coupling Muscle contractionAlpha motor neurons release AchACh produces large EPSP in muscle fibers (via nicotinic Ach receptorsEPSP evokes action potentialAction potential (excitation) triggers Ca2+ release, leads to fiber contractionRelaxation, Ca2+ levels lowered by organelle reuptake
40 Sliding Filament Model of Contraction Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degreeIn the relaxed state, thin and thick filaments overlap only slightlyUpon stimulation, myosin heads bind to actin and sliding begins
41 How striated muscle works: The Sliding Filament Model The lever movement drives displacement of the actin filament relative to the myosinhead (~5 nm), and by deforming internal elastic structures, produces force (~5 pN).Thick and thin filaments interdigitate and “slide” relative to each other.
42 Neuromuscular Junction The next few slides will summarize the events of a muscle contraction.The nerve impulse reaches the neuromuscular junction (myoneural junction).
43 Neuromuscular Junction Region where the motor neuron stimulates the muscle fiberThe neuromuscular junction is formed by :1. End of motor neuron axon (axon terminal)Terminals have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)2. The motor end plate of a muscleA specific part of the sarcolemma that contains ACh receptorsThough exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft
45 Motor Unit: The Nerve-Muscle Functional Unit A motor unit is a motor neuron and all the muscle fibers it suppliesThe number of muscle fibers per motor unit can vary from a few (4-6) to hundreds ( )Muscles that control fine movements (fingers, eyes) have small motor unitsLarge weight-bearing muscles (thighs, hips) have large motor units
46 Motor Unit: The Nerve-Muscle Functional Unit Muscle fibers from a motor unit are spread throughout the muscleNot confined to one fascicleTherefore, contraction of a single motor unit causes weak contraction of the entire muscleStronger and stronger contractions of a muscle require more and more motor units being stimulated (recruited)
47 Motor Unit All the muscle cells controlled by one nerve cell A motor unit is all the muscle cells controlled by one nerve cell. This diagram represents two motor units. Motor unit one illustrates two muscle cells controlled by one nerve cell. When the nerve sends a message it will cause both muscle cells to contract. Motor unit two has three muscle cells innervated by one nerve cell.
48 Acetylcholine is released from the motor neuron.
49 Acetylcholine Opens Na+ Channel Acetylcholine binds with receptors in the muscle membrane to allow sodium ions to enter the muscle.
50 The influx of sodium will create an action potential in the sarcolemma The influx of sodium will create an action potential in the sarcolemma. Note: This is the same mechanism for generating action potentials for the nerve impulse. The action potential travels down a T tubule. As the action potential passes through the sarcoplamic reticulum it stimulates the release of calcium ions. Calcium binds with troponin to move tropomyosin and expose the binding sites. Myosin heads attach to the binding sites of the actin filament and create a power stroke. ATP detaches the myosin heads and energizes them for another contraction. The process will continue until the action potentials cease. Without action potentials the calcium ions will return to the sarcoplasmic reticulum.
51 Muscle Contraction Summary Nerve impulse reaches myoneural junctionAcetylcholine is released from motor neuronAch binds with receptors in the muscle membrane to allow sodium to enterSodium influx will generate an action potential in the sarcolemma
52 Muscle Contraction (Cont’d) Action potential travels down T tubuleSarcoplamic reticulum releases calciumCalcium binds with troponin to move the troponin, tropomyosin complexBinding sites in the actin filament are exposed
53 Muscle Contraction (cont’d) Myosin head attach to binding sites and create a power strokeATP detaches myosin heads and energizes them for another contactionWhen action potentials cease the muscle stop contracting
55 Refresher Course in Muscle Physiology 4/11/2003Myosin is a hexamer:2 myosin heavy chains4 myosin light chainsC terminus2 nmCoiled coil of two a helicesMyosin is a Molecular MotorMyosin head: retains all of the motor functions of myosin,i.e. the ability to produce movement and force.Myosin S1 fragmentcrystal structureRuegg et al., (2002)News Physiol Sci 17:NH2-terminal catalytic(motor) domainneck region/lever armNucleotidebinding siteCatalytic domain responsible for binding & hydrolysis of ATP, and binding actin.Neck region responsible for transport of the load.Converter responsible for energy transduction.EB2003--Susan Brooks
56 Chemomechanical coupling – conversion of chemical energy (ATP about 7 kcal x mole-1) into force/movement.ATP is unstable thermodynamicallyTwo most energetically favorable steps:1. ATP binding to myosin2. Phosphate release from myosinRate of cycling determined by M·ATPase activity and external loadAdapted from Goldman & Brenner (1987) Ann Rev Physiol 49:
57 Shortening Velocity Vependent on ATPase Activity Different myosin heavy chains (MHCs) have different ATPase activities.There are at least 7 separate skeletal muscle MHC genes…arranged in serieson chromosome 17.Two cardiac MHC genes located in tandem on chromosome 14.The slow b cardiac MHC is the predominant gene expressed in slow fibersof mammals.Goldspink (1999) J Anat 194:
58 Power Output: The Most Physiologically Relevant Marker of PerformancePower = work / time= force x distance / time= force x velocityPeak power obtained at intermediate loads and intermediatevelocities.Figure from Berne and Levy, PhysiologyMosby—Year Book, Inc., 1993.
59 Three Potential Actions During Muscle Contraction: Biceps muscle shortensduring contractionshortening(Isotonic: shortening against fixed load, speed dependent on M·ATPase activity and load)isometricMost likely to causemuscle injurylengtheningBiceps muscle lengthensduring contraction
60 Motor Unit Ratios Back muscles Finger muscles Eye muscles 1:100 1:10 Motor units come indifferent sizes. *The ratio is about one nerve cell to 100 muscle cells in the back. *Finger muscles have a much smaller ratio of 1:10. *Eye muscles have a 1:1 ratio because of the precise control needed in vision.
61 Recall The Motor Unit:motor neuron and the muscle fibers it innervatesSpinalcordThe smallest amount ofmuscle that can be activatedvoluntarily.Gradation of force in skeletalmuscle is coordinated largelyby the nervous system.Recruitment of motor unitsis the most important meansof controlling muscle tension.Since all fibers in the motorunit contract simultaneously,pressures for gene expression(e.g. frequency of stimulation,load) are identical in all fibersof a motor unit.To increase force:Recruit more M.U.sIncrease freq.(force –frequency)
62 Physiological profiles of motor units: all fibers in a motor unit are of the same fiber typeSlow motor units contain slow fibers:Myosin with long cycle time and therefore uses ATP at a slow rate.Many mitochondria, so large capacity to replenish ATP.Economical maintenance of force during isometric contractions and efficient performance of repetitive slow isotonic contractions.Fast motor units contain fast fibers:Myosin with rapid cycling rates.For higher power or when isometric force produced by slow motor units is insufficient.Type 2A fibers are fast and adapted for producing sustained power.Type 2X fibers are faster, but non-oxidative and fatigue rapidly.2X/2D not 2B.Modified from Burke and Tsairis, Ann NY Acad Sci 228: , 1974.
63 Increased use: strength training Early gains in strength appear to be predominantly due to neural factors…optimizing recruitment patterns.Long term gains almost solely the result of hypertrophy i.e. increased size.
64 The PI(3)K/Akt(PKB)/mTOR pathway is a crucial regulator of skeletal musclehypertrophy/atrophy.Application of IGF-I to C2C12 myotube cultures induced both increased width and phosphor-ylation of downstream targets of Akt (p70S6 kinase, p70S6K; PHAS-1/4E-BP1; GSK3) but did NOT activate the calcineurin pathway.Treatment with rapamycin almost completely prevented increase in width of C2C12 myotubes.Treatment with cyclosporin or FK506 does not prevent myotube growth in vitro or compensatory hypertrophy in vivoRecovery of muscle weight after following reloading is blocked by rapamycin but not cyclosporin.Rommel et al. (2001) Nature Cell Biology 3, 1009.
65 Performance Declines with Aging --despite maintenance of physical activity10080Performance (% of peak)6040Shotput/DiscusMarathon20Basketball (rebounds/game)102030405060Age (years)D.H. Moore (1975) Nature 253:NBA Register, Edition
66 Number of motor units declines during aging - extensor digitorum brevis muscle of humansAGE-ASSOCIATEDATROPHY DUE TO BOTH…Individual fiber atrophy(which may be at leastpartially preventable andreversible through exercise).Loss of fibers(which as yet appearsirreversible).Campbell et al., (1973) J Neurol Neurosurg Psych 36:
67 Motor unit remodeling with aging CentralnervoussystemMuscleMotorneuronlossAGINGFewer motor unitsMore fibers/motor unit
68 Mean Motor Unit Forces: FF motor units get smaller in old age and decrease in numberS motor units get bigger with no change in numberDecreased rate of force generation and POWER!!225200175AdultOld150125Maximum Isometric Force (mN)100755025FFFIFRSMotor Unit ClassificationKadhiresan et al., (1996)J Physiol 493:
69 Refresher Course in Muscle Physiology 4/11/2003Muscle injury may play a role in the development ofatrophy with aging.Muscles in old animals are more susceptible to contraction-induced injury than those in young or adult animals.Muscles in old animals show delayed and impaired recoveryfollowing contraction-induced injury.By way of introduction, I’ll briefly reiterate some points that John made in the previous talk…Increased susceptibilityDecreased ability to recover and prolonged deficitsBecause muscles are injured repeatedly throughout life, these two observations, along with others provide circumstantial evidence that muscle injury plays a role in the development of atrophy and weakness with aging.Muscles of animals of all ages, except perhaps the oldest-old, can continue to adapt to the habitual level of activity.Following severe injury, muscles in old animals displayprolonged, possibly irreversible, structural and functionaldeficits.EB2003--Susan Brooks
70 Disorders of Muscle Tissue Muscle tissues experience few disordersHeart muscle is the exceptionSkeletal muscle – remarkably resistant to infectionSmooth muscle – problems stem from external irritants
71 Disorders of Muscle Tissue Muscular dystrophy – a group of inherited muscle destroying diseaseAffected muscles enlarge with fat and connective tissueMuscles degenerateTypes of muscular dystrophyDuchenne muscular dystrophyMyotonic dystrophy
72 Disorders of Muscle Tissue Myofascial pain syndrome – pain is caused by tightened bands of muscle fibersFibromyalgia – a mysterious chronic-pain syndromeAffects mostly womenSymptoms – fatigue, sleep abnormalities, severe musculoskeletal pain, and headache
73 Muscular Dystrophy: A frequently fatal disease of muscle deterioration Muscular dystrophies have in the past been classified based on subjective and sometimessubtle differences in clinical presentation, such as age of onset, involvement of particularmuscles, rate of progression of pathology, mode of inheritance.Since the discovery of dystrophin, numerous genetic disease loci have been linked to proteinproducts and to cellular phenotypes, generating models for studying the pathogenesis of thedystrophies.Proteins localized in the nucleus, cytosol, cytoskeleton, sarcolemma, and ECM.Cohn and Campbell (2000) Muscle Nerve 23:
74 transmission of force to extracellular matrix Dystrophin function:transmission of force to extracellular matrixDGCdystrophindystroglycan (a and b)sarcoglycans (a, b, g, d)syntrophins (a, b1)dystrobrevins (a, b)sarcospanlaminin-a2 (merosin)(Some components ofthe dystrophin glycoproteincomplex are relativelyrecent discoveries, so onecannot assume that allplayers are yet known.)Cohn and Campbell (2000) Muscle Nerve 23:
76 ATPATP or adenosine triphosphate is the form of energy that muscles and all cells of the body use. *The chemical bond between the last two phosphates has just the right amount of energy to unhook myosin heads and energize them for another contraction. Pulling of the end phosphate from ATP will release the energy. ADP and a single phosphate will be left over. New ATP can be regenerated by reconnecting the phosphate with the ADP with energy from our food.
77 Creatine Molecule capable of storing ATP energy Creatine + ATP Creatine phosphate + ADPCreatine is a molecule capable of storing ATP energy. It can combine with ATP to produce creatine phosphate and ADP. The third phosphate and the energy from ATP attaches to creatine to form creatine phosphate.
78 Creatine Phosphate Molecule with stored ATP energy Creatine phosphate + ADPCreatine + ATPCreatine phosphate is an important chemical to muscles. *It is a molecule that is able to store ATP energy. *Creatine phosphate can combine with an ADP * to produce creatine and ATP. This process occurs faster than the synthesis of ATP from food.
79 Muscle Fatigue Lack of oxygen causes ATP deficit Lactic acid builds up from anaerobic respirationMuscle fatigue is often due to a lack of oxygen that causes ATP deficit. Lactic acid builds up from anaerobic respiration in the absence of oxygen. Lactic acid fatigues the muscle.
81 Muscle Atrophy Weakening and shrinking of a muscle May be caused ImmobilizationLoss of neural stimulationMuscle atrophy is a weakening and shrinking of a muscle. It can be caused by immobilization or loss of neural stimulation.
82 Muscle Hypertrophy Enlargement of a muscle More capillaries More mitochondriaCaused byStrenuous exerciseSteroid hormonesHypertrophy is the enlargement of a muscle. Hypertrophied muscles have more capillaries and more mitochondria to help them generate more energy. Strenuous exercise and steroid hormones can induce muscle hypertrophy. Since men produce more steroid hormones than women, they usually have more hypertrophied muscles.
83 Steroid Hormones Stimulate muscle growth and hypertrophy Steroid hormones such as testosterone stimulate muscle growth and hypertrophy.
84 Muscle Tonus Tightness of a muscle Some fibers always contracted Muscle tonus or muscle tone refers to the tightness of a muscle. In a muscle some fibers are always contracted to add tension or tone to the muscle.
85 Tetany Sustained contraction of a muscle Result of a rapid succession of nerve impulsesTetany is a sustained contraction of a muscle. It results from a rapid succession of nerve impulses delivered to the muscle.
86 TetanusThis slide illustrates how a muscle can go into a sustained contraction by rapid neural stimulation. In number four the muscle is in a complete sustained contraction or tetanus.
87 Refractory PeriodBrief period of time in which muscle cells will not respond to a stimulusThe refractory period is a brief time in which muscle cells will not respond to stimulus.
88 RefractoryThe area to the left of the red line is the refractory period for the muscle contraction. If the muscle is stimulated at any time to the left of the line, it will not respond. However, stimulating the muscle to the right of the red line will produce a second contraction on top of the first contraction. Repeated stimulations can result in tetany.
89 Refractory Periods Skeletal Muscle Cardiac Muscle Cardiac muscle tissue has a longer refractory period than skeletal muscle. This prevents the heart from going into tetany.Skeletal MuscleCardiac Muscle
90 Isometric Contraction Produces no movementUsed inStandingSittingPostureIsometric contractions produce no movement. They are used in standing, sitting and maintaining our posture. For example, when you are standing muscles in your back and abdomen pull against each other to keep you upright. They do not produce movement, but enable you to stand.
91 Isotonic Contraction Produces movement Used in Walking Moving any part of the bodyIsotonic contractions are the types that produce movement. Isotonic contractions are used in walking and moving any part of the body.
96 Developmental Aspects: Regeneration Cardiac and skeletal muscle become amitotic, but can lengthen and thickenMyoblast-like satellite cells show very limited regenerative abilityCardiac cells lack satellite cellsSmooth muscle has good regenerative abilityThere is a biological basis for greater strength in men than in womenWomen’s skeletal muscle makes up 36% of their body massMen’s skeletal muscle makes up 42% of their body mass
97 Developmental Aspects: Male and Female These differences are due primarily to the male sex hormone testosteroneWith more muscle mass, men are generally stronger than womenBody strength per unit muscle mass, however, is the same in both sexes
98 Developmental Aspects: Age Related With age, connective tissue increases and muscle fibers decreaseMuscles become stringier and more sinewyBy age 80, 50% of muscle mass is lost (sarcopenia)Decreased density of capillaries in muscleReduced staminaIncreased recovery timeRegular exercise reverses sarcopenia