mosby items and derived items © 2007, 2003 by mosby, inc.slide 1 chapter 11 physiology of the...

Mosby items and derived items © 2007, 2003 by Mosby, Inc.

Chapter 11Chapter 11Physiology of the Muscular SystemPhysiology of the Muscular System


IntroductionIntroduction

• Muscular system is responsible for moving Muscular system is responsible for moving the framework of the bodythe framework of the body

• In addition to movement, muscle tissue In addition to movement, muscle tissue performs various other functionsperforms various other functions


General FunctionsGeneral Functions

• Movement of the body as a whole or of Movement of the body as a whole or of its partsits parts

• Heat productionHeat production

• PosturePosture


Function of Skeletal Muscle TissueFunction of Skeletal Muscle Tissue

• Characteristics of skeletal muscle cellsCharacteristics of skeletal muscle cells

Excitability (irritability)—ability to be stimulatedExcitability (irritability)—ability to be stimulated

Contractility—ability to contract, or shorten, and Contractility—ability to contract, or shorten, and produce body movementproduce body movement

Extensibility—ability to extend, or stretch, allowing Extensibility—ability to extend, or stretch, allowing muscles to return to their resting lengthmuscles to return to their resting length



• Overview of the muscle cell Overview of the muscle cell (Figures 11-1 and 11-2)(Figures 11-1 and 11-2) Muscle cells are called fibers because of their Muscle cells are called fibers because of their

threadlike shapethreadlike shape

Sarcolemma—plasma membrane of muscle fibersSarcolemma—plasma membrane of muscle fibers

Sarcoplasmic reticulum (SR)Sarcoplasmic reticulum (SR)

• Network of tubules and sacs found within muscle fibersNetwork of tubules and sacs found within muscle fibers

• Membrane of the sarcoplasmic reticulum continually Membrane of the sarcoplasmic reticulum continually pumps calcium ions from the sarcoplasm and stores the pumps calcium ions from the sarcoplasm and stores the ions within its sacs for later release (Figure 11-3)ions within its sacs for later release (Figure 11-3)



• Overview of the muscle cell (cont.)Overview of the muscle cell (cont.) Muscle fibers contain many mitochondria and several nucleiMuscle fibers contain many mitochondria and several nuclei

Myofibrils—numerous fine fibers packed close together in Myofibrils—numerous fine fibers packed close together in sarcoplasmsarcoplasm

SarcomereSarcomere

• Segment of myofibril between two successive Z linesSegment of myofibril between two successive Z lines

• Each myofibril consists of many sarcomeresEach myofibril consists of many sarcomeres

• Contractile unit of muscle fibersContractile unit of muscle fibers



• Overview of the muscle cell (cont.)Overview of the muscle cell (cont.)

Striated muscle (Figure 11-4)Striated muscle (Figure 11-4)

• Dark stripes called A bands; light H zone runs across Dark stripes called A bands; light H zone runs across midsection of each dark A bandmidsection of each dark A band

• Light stripes called I bands; dark Z line extends across Light stripes called I bands; dark Z line extends across center of each light I bandcenter of each light I band



• Overview of the muscle cell (cont.)Overview of the muscle cell (cont.) T tubulesT tubules

• Transverse tubules extend across sarcoplasm at right Transverse tubules extend across sarcoplasm at right angles to long axis of muscle fiberangles to long axis of muscle fiber

• Formed by inward extensions of sarcolemmaFormed by inward extensions of sarcolemma

• Membrane has ion pumps that continually transport Ca+Membrane has ion pumps that continually transport Ca++ ions inward from sarcoplasm+ ions inward from sarcoplasm

• Allow electrical impulses traveling along sarcolemma to Allow electrical impulses traveling along sarcolemma to move deeper into cellmove deeper into cell



• Overview of the muscle cell (cont.)Overview of the muscle cell (cont.)

TriadTriad

• Triplet of tubules; a T tubule sandwiched between two Triplet of tubules; a T tubule sandwiched between two sacs of sarcoplasmic reticulum; allows an electrical sacs of sarcoplasmic reticulum; allows an electrical impulse traveling along a T tubule to stimulate the impulse traveling along a T tubule to stimulate the membranes of adjacent sacs of the sarcoplasmic membranes of adjacent sacs of the sarcoplasmic reticulumreticulum



• Myofilaments (Figures 11-5 and 11-6)Myofilaments (Figures 11-5 and 11-6)

Each myofibril contains thousands of thick and thin Each myofibril contains thousands of thick and thin myofilamentsmyofilaments



• Myofilaments (cont.)Myofilaments (cont.) Four different kinds of protein molecules make up Four different kinds of protein molecules make up

myofilamentsmyofilaments

• MyosinMyosin Makes up almost all the thick filamentMakes up almost all the thick filament

Myosin “heads” are chemically attracted to actin moleculesMyosin “heads” are chemically attracted to actin molecules

Myosin “heads” are known as cross bridges when attached Myosin “heads” are known as cross bridges when attached to actinto actin

• Actin—globular protein that forms two fibrous strands that twist Actin—globular protein that forms two fibrous strands that twist around each other to form bulk of thin filamentaround each other to form bulk of thin filament

• Tropomyosin—protein that blocks the active sites on actin Tropomyosin—protein that blocks the active sites on actin moleculesmolecules

• Troponin—protein that holds tropomyosin molecules in placeTroponin—protein that holds tropomyosin molecules in place



• Myofilaments (cont.)Myofilaments (cont.)

Thin filaments attach to both Z lines (Z disks) of a Thin filaments attach to both Z lines (Z disks) of a sarcomere and extend partway toward the centersarcomere and extend partway toward the center

Thick myosin filaments do not attach to the Z linesThick myosin filaments do not attach to the Z lines



• The mechanism of contractionThe mechanism of contraction

Excitation and contraction Excitation and contraction (Figures 11-7 through 11-12; Table 11-1)(Figures 11-7 through 11-12; Table 11-1)

• A skeletal muscle fiber remains at rest until stimulated A skeletal muscle fiber remains at rest until stimulated by a motor neuronby a motor neuron

• Neuromuscular junction—motor neurons connect to Neuromuscular junction—motor neurons connect to sarcolemma at motor endplate (Figure 11-7)sarcolemma at motor endplate (Figure 11-7)

• Neuromuscular junction is a synapse where Neuromuscular junction is a synapse where neurotransmitter molecules transmit signalsneurotransmitter molecules transmit signals



Excitation and contraction (cont.)Excitation and contraction (cont.)

• Acetylcholine—neurotransmitter released into synaptic Acetylcholine—neurotransmitter released into synaptic cleft that diffuses across gap, stimulates receptors, and cleft that diffuses across gap, stimulates receptors, and initiates impulse in sarcolemmainitiates impulse in sarcolemma

• Nerve impulse travels over sarcolemma and inward Nerve impulse travels over sarcolemma and inward along T tubules, which triggers release of calcium ionsalong T tubules, which triggers release of calcium ions

• Calcium binds to troponin, causing tropomyosin to shift Calcium binds to troponin, causing tropomyosin to shift and expose active sites on actinand expose active sites on actin



Excitation and contraction (cont.)Excitation and contraction (cont.)

• Sliding filament model (Figures 11-11 and 11-12)Sliding filament model (Figures 11-11 and 11-12)

When active sites on actin are exposed, myosin heads When active sites on actin are exposed, myosin heads bind to thembind to them

Myosin heads bend, pulling the thin filaments past themMyosin heads bend, pulling the thin filaments past them

Each head releases, binds to next active site, and Each head releases, binds to next active site, and pulls againpulls again

Entire myofibril shortensEntire myofibril shortens



• The mechanism of contraction (cont.)The mechanism of contraction (cont.)

RelaxationRelaxation

• Immediately after CaImmediately after Ca++++ ions are released, sarcoplasmic ions are released, sarcoplasmic reticulum begins actively pumping them back into sacs reticulum begins actively pumping them back into sacs (Figure 11-3)(Figure 11-3)

• CaCa++++ ions are removed from troponin molecules, shutting ions are removed from troponin molecules, shutting down contractiondown contraction



• The mechanism of contraction (cont.)The mechanism of contraction (cont.)

Energy sources for muscle contraction Energy sources for muscle contraction (Figure 11-13)(Figure 11-13)

• Hydrolysis of ATP yields energy required for muscular Hydrolysis of ATP yields energy required for muscular contractioncontraction

• Adenosine triphosphate (ATP) binds to myosin head and Adenosine triphosphate (ATP) binds to myosin head and then transfers its energy to myosin head to perform work then transfers its energy to myosin head to perform work of pulling thin filament during contractionof pulling thin filament during contraction

• Muscle fibers continually resynthesize ATP from Muscle fibers continually resynthesize ATP from breakdown of creatine phosphate (CP)breakdown of creatine phosphate (CP)



Energy sources for muscle contraction (cont.)Energy sources for muscle contraction (cont.)

• Catabolism by muscle fibers requires glucose and Catabolism by muscle fibers requires glucose and oxygenoxygen

• At rest, excess OAt rest, excess O22 in the sarcoplasm is bound to in the sarcoplasm is bound to

myoglobin (Box 11-4)myoglobin (Box 11-4)

Red fibers—muscle fibers with high levels of myoglobinRed fibers—muscle fibers with high levels of myoglobin

White fibers—muscle fibers with little myoglobinWhite fibers—muscle fibers with little myoglobin

• Aerobic respiration occurs when adequate OAerobic respiration occurs when adequate O22 is available is available



• The mechanism of contraction (cont.) The mechanism of contraction (cont.)

Energy sources for muscle contraction (cont.)Energy sources for muscle contraction (cont.)

• Anaerobic respiration occurs when low levels of OAnaerobic respiration occurs when low levels of O22 are are

available and results in formation of lactic acidavailable and results in formation of lactic acid

• Glucose and oxygen supplied to muscle fibers by blood Glucose and oxygen supplied to muscle fibers by blood capillaries (Figure 11-14)capillaries (Figure 11-14)

• Skeletal muscle contraction produces waste heat that can be Skeletal muscle contraction produces waste heat that can be used to help maintain set point body temperature (Figure 11-used to help maintain set point body temperature (Figure 11-15)15)


Function of Skeletal Muscle OrgansFunction of Skeletal Muscle Organs

• Muscles are composed of bundles of muscle fibers Muscles are composed of bundles of muscle fibers that are held together by fibrous connective tissuethat are held together by fibrous connective tissue

• Motor unit (Figure 11-16)Motor unit (Figure 11-16) Motor unit—motor neuron plus the muscle fibers to which it Motor unit—motor neuron plus the muscle fibers to which it

attachesattaches Some motor units consist of only a few muscle fibers, Some motor units consist of only a few muscle fibers,

whereas others consist of numerous fiberswhereas others consist of numerous fibers Generally, the smaller the number of fibers in a motor unit, Generally, the smaller the number of fibers in a motor unit,

the more precise the movement available; the larger the the more precise the movement available; the larger the number of fibers in a motor unit, the more powerful the number of fibers in a motor unit, the more powerful the contraction availablecontraction available

• Myography—method of graphing the changing Myography—method of graphing the changing tension of a muscle as it contracts (Figure 11-17)tension of a muscle as it contracts (Figure 11-17)



• Twitch contraction (Figure 11-18)Twitch contraction (Figure 11-18)

A quick jerk of a muscle that is produced as a A quick jerk of a muscle that is produced as a result of a single, brief threshold stimulus result of a single, brief threshold stimulus (generally occurs only in experimental situations)(generally occurs only in experimental situations)

The twitch contraction has three phasesThe twitch contraction has three phases

• Latent phase—nerve impulse travels to the sarcoplasmic Latent phase—nerve impulse travels to the sarcoplasmic reticulum to trigger release of Careticulum to trigger release of Ca++++

• Contraction phase—CaContraction phase—Ca++++ binds to troponin and sliding of binds to troponin and sliding of filaments occursfilaments occurs

• Relaxation phase—sliding of filaments ceasesRelaxation phase—sliding of filaments ceases



• Treppe—the staircase phenomenon Treppe—the staircase phenomenon (Figure 11-19, B)(Figure 11-19, B) Gradual, steplike increase in the strength of Gradual, steplike increase in the strength of

contractions seen in a series of twitch contractions contractions seen in a series of twitch contractions that occur 1 second apartthat occur 1 second apart

Eventually, the muscle responds with less forceful Eventually, the muscle responds with less forceful contractions, and relaxation phase becomes contractions, and relaxation phase becomes shortershorter

If relaxation phase disappears completely, If relaxation phase disappears completely, a contracture occursa contracture occurs



• Tetanus—smooth, sustained contractionsTetanus—smooth, sustained contractions Multiple wave summation—multiple twitch waves Multiple wave summation—multiple twitch waves

are added together to sustain muscle tension for are added together to sustain muscle tension for a longer timea longer time

Incomplete tetanus—very short periods of Incomplete tetanus—very short periods of relaxation occur between peaks of tension relaxation occur between peaks of tension (Figure 11-19, C)(Figure 11-19, C)

Complete tetanus—the stimulation is such that Complete tetanus—the stimulation is such that twitch waves fuse into a single, sustained peak twitch waves fuse into a single, sustained peak (Figure 11-19, D)(Figure 11-19, D)



• Muscle toneMuscle tone

Tonic contraction—continual, partial contraction Tonic contraction—continual, partial contraction of a muscleof a muscle

At any one time, a small number of muscle fibers within a At any one time, a small number of muscle fibers within a muscle contract, producing a tightness or muscle tonemuscle contract, producing a tightness or muscle tone

Muscles with less tone than normal are flaccidMuscles with less tone than normal are flaccid

Muscles with more tone than normal are spasticMuscles with more tone than normal are spastic

Muscle tone is maintained by negative feedback Muscle tone is maintained by negative feedback mechanismsmechanisms



• Graded strength principleGraded strength principle

Skeletal muscles contract with varying degrees Skeletal muscles contract with varying degrees of strength at different timesof strength at different times

Factors that contribute to the phenomenon of Factors that contribute to the phenomenon of graded strength (Figure 11-23)graded strength (Figure 11-23)

• Metabolic condition of individual fibersMetabolic condition of individual fibers

• Number of muscle fibers contracting simultaneously; the Number of muscle fibers contracting simultaneously; the greater the number of fibers contracting, the stronger the greater the number of fibers contracting, the stronger the contractioncontraction

• Number of motor units recruitedNumber of motor units recruited



• Factors that contribute to the phenomenon of graded Factors that contribute to the phenomenon of graded strength (cont.)strength (cont.)

• Intensity and frequency of stimulation (Figure 11-20)Intensity and frequency of stimulation (Figure 11-20)

• Length-tension relationship (Figure 11-21)Length-tension relationship (Figure 11-21) Maximal strength that a muscle can develop bears a direct Maximal strength that a muscle can develop bears a direct

relationship to the initial length of its fibersrelationship to the initial length of its fibers

A shortened muscle’s sarcomeres are compressed; therefore, the A shortened muscle’s sarcomeres are compressed; therefore, the muscle cannot develop much tensionmuscle cannot develop much tension

An overstretched muscle cannot develop much tension because An overstretched muscle cannot develop much tension because the thick myofilaments are too far from the thin myofilamentsthe thick myofilaments are too far from the thin myofilaments

Strongest maximal contraction is possible only when the skeletal Strongest maximal contraction is possible only when the skeletal muscle has been stretched to its optimal lengthmuscle has been stretched to its optimal length



Factors that contribute to the phenomenon of Factors that contribute to the phenomenon of graded strength (cont.)graded strength (cont.)

• Stretch reflex (Figure 11-22)Stretch reflex (Figure 11-22)

The load imposed on a muscle influences the strength of The load imposed on a muscle influences the strength of a skeletal contractiona skeletal contraction

Stretch reflex—the body tries to maintain a constancy of Stretch reflex—the body tries to maintain a constancy of muscle length in response to increased loadmuscle length in response to increased load

Maintains a relatively constant length as load is increased Maintains a relatively constant length as load is increased up to a maximum sustainable levelup to a maximum sustainable level



• Isotonic and isometric contractions Isotonic and isometric contractions (Figure 11-24)(Figure 11-24) Isotonic contractionIsotonic contraction

• Contraction in which the tone or tension within a muscle Contraction in which the tone or tension within a muscle remains the same as the length of the muscle changesremains the same as the length of the muscle changes

Concentric—muscle shortens as it contractsConcentric—muscle shortens as it contracts

Eccentric—muscle lengthens while contractingEccentric—muscle lengthens while contracting

• Isotonic—literally means “same tension”Isotonic—literally means “same tension”

• All of the energy of contraction is used to pull on thin All of the energy of contraction is used to pull on thin myofilaments and thereby change the length of a fiber’s myofilaments and thereby change the length of a fiber’s sarcomeressarcomeres



• Isotonic and isometric contractions (cont.)Isotonic and isometric contractions (cont.)

Isometric contractionIsometric contraction

• Contraction in which muscle length remains the same Contraction in which muscle length remains the same while the muscle tension increaseswhile the muscle tension increases

• Isometric—literally means “same length”Isometric—literally means “same length”

Most body movements occur as a result of both Most body movements occur as a result of both types of contractionstypes of contractions


Function of Cardiac Function of Cardiac and Smooth Muscle Tissueand Smooth Muscle Tissue

• Cardiac muscle (Figure 11-25)Cardiac muscle (Figure 11-25)

Found only in the heart, forming the bulk of the Found only in the heart, forming the bulk of the wall of each chamberwall of each chamber

Also known as striated involuntary muscleAlso known as striated involuntary muscle

Contracts rhythmically and continuously to provide Contracts rhythmically and continuously to provide the pumping action needed to maintain a constant the pumping action needed to maintain a constant blood flowblood flow



Cardiac muscle resembles skeletal muscle but has Cardiac muscle resembles skeletal muscle but has specialized features related to its role in continuously specialized features related to its role in continuously pumping bloodpumping blood

• Each cardiac muscle contains parallel myofibrils Each cardiac muscle contains parallel myofibrils (Figure 11-25)(Figure 11-25)

• Cardiac muscle fibers form strong, electrically coupled junctions Cardiac muscle fibers form strong, electrically coupled junctions (intercalated disks) with other fibers; individual cells also exhibit (intercalated disks) with other fibers; individual cells also exhibit branchingbranching

• Syncytium—continuous, electrically coupled massSyncytium—continuous, electrically coupled mass

• Cardiac muscle fibers form a continuous, contractile band Cardiac muscle fibers form a continuous, contractile band around the heart chambers that conducts a single impulse around the heart chambers that conducts a single impulse across a virtually continuous sarcolemmaacross a virtually continuous sarcolemma



• Cardiac muscle (cont.)Cardiac muscle (cont.)

• T tubules are larger and form diads with a rather sparse T tubules are larger and form diads with a rather sparse sarcoplasmic reticulumsarcoplasmic reticulum

• Cardiac muscle sustains each impulse longer than in Cardiac muscle sustains each impulse longer than in skeletal muscle; therefore, impulses cannot come rapidly skeletal muscle; therefore, impulses cannot come rapidly enough to produce tetanus (Figure 11-26)enough to produce tetanus (Figure 11-26)

• Cardiac muscle does not run low on ATP and does not Cardiac muscle does not run low on ATP and does not experience fatigueexperience fatigue

• Cardiac muscle is self-stimulatingCardiac muscle is self-stimulating



• Smooth muscleSmooth muscle Smooth muscle is composed of small, tapered cells Smooth muscle is composed of small, tapered cells

with single nuclei (Figure 11-27)with single nuclei (Figure 11-27)

No T tubules are present, and only a loosely organized No T tubules are present, and only a loosely organized sarcoplasmic reticulum is presentsarcoplasmic reticulum is present

CaCa++++ comes from outside the cell and binds to comes from outside the cell and binds to calmodulin instead of troponin to trigger a contractioncalmodulin instead of troponin to trigger a contraction

No striations, because thick and thin myofilaments are No striations, because thick and thin myofilaments are arranged differently than in skeletal or cardiac muscle arranged differently than in skeletal or cardiac muscle fibers; myofilaments are not organized into sarcomeresfibers; myofilaments are not organized into sarcomeres



• Smooth muscle (cont.)Smooth muscle (cont.) Two types of smooth muscle tissue (Figure 11-28):Two types of smooth muscle tissue (Figure 11-28):

• Single-unit (visceral)Single-unit (visceral) Gap junctions join smooth muscle fibers into large, continuous Gap junctions join smooth muscle fibers into large, continuous

sheetssheets Most common type; forms a muscular layer in the walls of hollow Most common type; forms a muscular layer in the walls of hollow

structures such as the digestive, urinary, and reproductive tractsstructures such as the digestive, urinary, and reproductive tracts Exhibits autorhythmicity, producing peristalsisExhibits autorhythmicity, producing peristalsis

• MultiunitMultiunit Does not act as a single unit but is composed of many Does not act as a single unit but is composed of many

independent cell unitsindependent cell units Each fiber responds only to nervous inputEach fiber responds only to nervous input


The Big Picture: The Big Picture: Muscle Tissue and the Whole BodyMuscle Tissue and the Whole Body

• Function of all three major types of muscle Function of all three major types of muscle is integral to the function of the entire bodyis integral to the function of the entire body

• All three types of muscle tissue provide the All three types of muscle tissue provide the movement necessary for survivalmovement necessary for survival

• Relative constancy of the body’s internal Relative constancy of the body’s internal temperature is maintained by “waste” heat temperature is maintained by “waste” heat generated by muscle tissuegenerated by muscle tissue

• Maintains the body in a relatively stable Maintains the body in a relatively stable positionposition

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