Muscles and Muscles and Muscle TissueMuscle Tissue

Form and Function of Form and Function of MovementMovement

Muscle TissueMuscle Tissue

Anatomy and HistologyAnatomy and Histology

Functional Characteristics Functional Characteristics of Muscle Tissueof Muscle Tissue

Excitability, or irritability – the ability Excitability, or irritability – the ability to receive and respond to stimulito receive and respond to stimuli

Contractility – the ability to shorten Contractility – the ability to shorten forciblyforcibly

Extensibility – the ability to be Extensibility – the ability to be stretched or extendedstretched or extended

Elasticity – the ability to recoil and Elasticity – the ability to recoil and resume the original resting lengthresume the original resting length

Muscle OverviewMuscle Overview The three types of muscle tissue are The three types of muscle tissue are

skeletal, cardiac, and smoothskeletal, cardiac, and smooth

These types differ in structure, These types differ in structure, location, function, and means of location, function, and means of activationactivation

Cardiac muscle Cardiac muscle cells branch, are cells branch, are striated, are striated, are uninucleate (B) and uninucleate (B) and have have intercalated intercalated discs discs (A).(A).

Locations: heartLocations: heart Function: Function:

involuntary, involuntary, rhythmic rhythmic contractioncontraction

Skeletal MuscleSkeletal Muscle Skeletal muscle cells run the Skeletal muscle cells run the

full length of a muscle. Line full length of a muscle. Line A show the width of one cell A show the width of one cell (fiber). Note the striations (fiber). Note the striations characteristics of this characteristics of this muscle type. These cells are muscle type. These cells are multicellular, B marks one multicellular, B marks one nucleus.nucleus.

Location: muscles Location: muscles associated with the skeletonassociated with the skeleton

Function: voluntary Function: voluntary movementmovement

Muscles are connected to Muscles are connected to bones by tendons. Bones bones by tendons. Bones are connected to other are connected to other bones at their joints by bones at their joints by ligaments.ligaments.

Smooth MuscleSmooth Muscle Smooth muscle cells Smooth muscle cells

are spindle shaped and are spindle shaped and uninucleate. (B).uninucleate. (B).

Locations: walls of Locations: walls of hollow organs, i.e. hollow organs, i.e. stomach, intestine, stomach, intestine, uterus, ureteruterus, ureter

Functions: involuntary Functions: involuntary movement - i.e. movement - i.e. churning of food, churning of food, movement of urine movement of urine from the kidney to the from the kidney to the bladder, partuition bladder, partuition

Skeletal MuscleSkeletal Muscle

Figure 9.2 (a)

MyofibrilsMyofibrils

Figure 9.3 (b)

SarcomeresSarcomeres

Figure 9.3 (c)

Myofilaments: Banding PatternMyofilaments: Banding Pattern

Ultrastructure of Ultrastructure of Myofilaments: Thick Myofilaments: Thick

FilamentsFilaments

Figure 9.4 (a)(b)

Ultrastructure of Ultrastructure of Myofilaments: Thick Myofilaments: Thick

FilamentsFilaments Thick filaments are composed of the Thick filaments are composed of the

protein myosinprotein myosin Each myosin molecule has a rodlike Each myosin molecule has a rodlike

tail and two globular headstail and two globular heads Tails – two interwoven, heavy Tails – two interwoven, heavy

polypeptide chainspolypeptide chains Heads – two smaller, light polypeptide Heads – two smaller, light polypeptide

chains called cross bridgeschains called cross bridges

Ultrastructure of Ultrastructure of Myofilaments: Thin Myofilaments: Thin

FilamentsFilaments

Figure 9.4 (c)

Ultrastructure of Myofilaments: Ultrastructure of Myofilaments: Thin FilamentsThin Filaments

Thin filaments are chiefly composed of Thin filaments are chiefly composed of the protein actinthe protein actin

Tropomyosin and troponin are Tropomyosin and troponin are regulatory subunits bound to actinregulatory subunits bound to actin

Arrangement of the Arrangement of the Filaments in a SarcomereFilaments in a Sarcomere

Sarcoplasmic Reticulum Sarcoplasmic Reticulum (SR)(SR)

Figure 9.5

Sarcoplasmic Reticulum Sarcoplasmic Reticulum (SR)(SR)

SR smooth endoplasmic reticulum that SR smooth endoplasmic reticulum that mostly runs longitudinally and mostly runs longitudinally and surrounds each myofibrilsurrounds each myofibril

Paired terminal cisternae form Paired terminal cisternae form perpendicular cross channelsperpendicular cross channels

Functions in the regulation of Functions in the regulation of intracellular calcium levelsintracellular calcium levels

T TubulesT Tubules T tubules are continuous with the T tubules are continuous with the

sarcolemmasarcolemma They conduct impulses to the They conduct impulses to the

deepest regions of the muscledeepest regions of the muscle These impulses signal for the release These impulses signal for the release

of Caof Ca2+2+ from adjacent terminal from adjacent terminal cisternaecisternae

Muscle Cell Muscle Cell ContractionContraction

Sliding Filament TheorySliding Filament Theory

Excitation of a Muscle FiberExcitation of a Muscle Fiber

Excitation (steps 1 & 2)Excitation (steps 1 & 2)

Nerve signal stimulates voltage-gated calcium Nerve signal stimulates voltage-gated calcium channels that result in exocytosis of synaptic channels that result in exocytosis of synaptic vesicles containing ACh = ACh releasevesicles containing ACh = ACh release

Excitation (steps 3 & 4)Excitation (steps 3 & 4)

Binding of ACh to the surface of muscle Binding of ACh to the surface of muscle cells opens Na+ and K+ channels cells opens Na+ and K+ channels resulting in an end-plate potential (EPP)resulting in an end-plate potential (EPP)

Excitation (step 5)Excitation (step 5)

Voltage change in end-plate region (EPP) opens Voltage change in end-plate region (EPP) opens nearby voltage-gated channels in plasma nearby voltage-gated channels in plasma membrane producing an action potentialmembrane producing an action potential

Excitation-Contraction Excitation-Contraction CouplingCoupling

Excitation-Contraction Excitation-Contraction CouplingCoupling(steps 6&7)(steps 6&7)

Action potential spreading over sarcolemma reaches Action potential spreading over sarcolemma reaches and enters the T tubules -- voltage-gated channels and enters the T tubules -- voltage-gated channels open in T tubules causing calcium gates to open in SRopen in T tubules causing calcium gates to open in SR

Excitation-Contraction Excitation-Contraction CouplingCoupling(steps 8&9)(steps 8&9)

Calcium released by SR binds to troponinCalcium released by SR binds to troponin Troponin-tropomyosin complex changes Troponin-tropomyosin complex changes

shape and exposes active sites on actinshape and exposes active sites on actin

Contraction Contraction (steps 10 & 11)(steps 10 & 11) Myosin ATPase in Myosin ATPase in

myosin myosin head hydrolyzes an head hydrolyzes an ATP ATP molecule, activating molecule, activating the the head and “cocking” head and “cocking” it in it in an extended positionan extended position

It binds to an active It binds to an active site on actinsite on actin

Contraction Contraction (steps 12 & 13)(steps 12 & 13) Power stroke = shows Power stroke = shows

myosin head releasing myosin head releasing the ADP & phosphate as the ADP & phosphate as it flexes pulling the thin it flexes pulling the thin filament along filament along

With the binding of more With the binding of more ATP, the myosin head ATP, the myosin head releases the thin filament releases the thin filament and extends to attach to a and extends to attach to a new active site further down new active site further down the thin filamentthe thin filament at any given moment, half at any given moment, half

of the heads are bound to of the heads are bound to a thin filament, preventing a thin filament, preventing slippageslippage

thin and thick filaments do thin and thick filaments do not become shorter, just not become shorter, just slide past each other slide past each other (sliding filament theory(sliding filament theory))

12. Power Stroke; sliding of thin filament over thick

Relaxation Relaxation (steps 14 & 15)(steps 14 & 15)

Nerve stimulation ceases and Nerve stimulation ceases and acetylcholinesterase removes ACh from receptors acetylcholinesterase removes ACh from receptors so stimulation of the muscle cell ceasesso stimulation of the muscle cell ceases

Relaxation Relaxation (step 16)(step 16)

Active transport pumps calcium from Active transport pumps calcium from sarcoplasm back into SR where it binds to sarcoplasm back into SR where it binds to calsequestrincalsequestrin

ATP is needed for muscle relaxation as ATP is needed for muscle relaxation as well as muscle contractionwell as muscle contraction

Relaxation (steps 17 & 18)Relaxation (steps 17 & 18)

Muscle ToneMuscle Tone Muscle tone:Muscle tone:

Is the constant, slightly contracted state of all Is the constant, slightly contracted state of all muscles, which does not produce active muscles, which does not produce active movementsmovements

Keeps the muscles firm, healthy, and ready to Keeps the muscles firm, healthy, and ready to respond to stimulusrespond to stimulus

Spinal reflexes account for muscle tone by:Spinal reflexes account for muscle tone by: Activating one motor unit and then anotherActivating one motor unit and then another Responding to activation of stretch receptors in Responding to activation of stretch receptors in

muscles and tendonsmuscles and tendons

Isotonic ContractionsIsotonic Contractions In isotonic contractions, the muscle In isotonic contractions, the muscle

changes in length (decreasing the changes in length (decreasing the angle of the joint) and moves the loadangle of the joint) and moves the load

The two types of isotonic contractions The two types of isotonic contractions are concentric and eccentricare concentric and eccentric Concentric contractions – the muscle Concentric contractions – the muscle

shortens and does workshortens and does work Eccentric contractions – the muscle Eccentric contractions – the muscle

contracts as it lengthenscontracts as it lengthens

Isotonic ContractionsIsotonic Contractions

Figure 9.17 (a)

Isometric ContractionsIsometric Contractions Tension increases to the muscle’s Tension increases to the muscle’s

capacity, but the muscle neither capacity, but the muscle neither shortens nor lengthensshortens nor lengthens

Occurs if the load is greater than the Occurs if the load is greater than the tension the muscle is able to developtension the muscle is able to develop

Isometric ContractionsIsometric Contractions

Figure 9.17 (b)

Muscle Metabolism: Energy Muscle Metabolism: Energy for Contractionfor Contraction

ATP is the only source used directly ATP is the only source used directly for contractile activityfor contractile activity

As soon as available stores of ATP are As soon as available stores of ATP are hydrolyzed (4-6 seconds), they are hydrolyzed (4-6 seconds), they are regenerated by:regenerated by: The interaction of ADP with creatine The interaction of ADP with creatine

phosphate (CP) phosphate (CP) Anaerobic glycolysis Anaerobic glycolysis Aerobic respirationAerobic respiration

Muscle Metabolism: Energy for Muscle Metabolism: Energy for ContractionContraction

Figure 9.18

Muscle Metabolism: Muscle Metabolism: Anaerobic GlycolysisAnaerobic Glycolysis

When muscle contractile activity When muscle contractile activity reaches 70% of maximum:reaches 70% of maximum: Bulging muscles compress blood vesselsBulging muscles compress blood vessels Oxygen delivery is impairedOxygen delivery is impaired Pyruvic acid is converted into lactic acidPyruvic acid is converted into lactic acid

Muscle Metabolism: Muscle Metabolism: Anaerobic GlycolysisAnaerobic Glycolysis

The lactic acid:The lactic acid: Diffuses into the bloodstreamDiffuses into the bloodstream Is picked up and used as fuel by the Is picked up and used as fuel by the

liver, kidneys, and heartliver, kidneys, and heart Is converted back into pyruvic acid by Is converted back into pyruvic acid by

the liverthe liver

Muscle FatigueMuscle Fatigue Muscle fatigue – the muscle is in a state Muscle fatigue – the muscle is in a state

of physiological inability to contractof physiological inability to contract Muscle fatigue occurs when:Muscle fatigue occurs when:

ATP production fails to keep pace with ATP ATP production fails to keep pace with ATP useuse

There is a relative deficit of ATP, causing There is a relative deficit of ATP, causing contracturescontractures

Lactic acid accumulates in the muscleLactic acid accumulates in the muscle Ionic imbalances are presentIonic imbalances are present

Muscle FatigueMuscle Fatigue Intense exercise produces rapid Intense exercise produces rapid

muscle fatigue (with rapid recovery)muscle fatigue (with rapid recovery) NaNa++-K-K++ pumps cannot restore ionic pumps cannot restore ionic

balances quickly enoughbalances quickly enough Low-intensity exercise produces Low-intensity exercise produces

slow-developing fatigueslow-developing fatigue SR is damaged and CaSR is damaged and Ca2+2+ regulation is regulation is

disrupteddisrupted

Oxygen DebtOxygen Debt Vigorous exercise causes dramatic changes Vigorous exercise causes dramatic changes

in muscle chemistryin muscle chemistry For a muscle to return to a resting state:For a muscle to return to a resting state:

Oxygen reserves must be replenishedOxygen reserves must be replenished Lactic acid must be converted to pyruvic acidLactic acid must be converted to pyruvic acid Glycogen stores must be replacedGlycogen stores must be replaced ATP and CP reserves must be resynthesizedATP and CP reserves must be resynthesized

Oxygen debt – the extra amount of OOxygen debt – the extra amount of O22 needed for the above restorative processesneeded for the above restorative processes

Heat Production During Heat Production During Muscle ActivityMuscle Activity

Only 40% of the energy released in Only 40% of the energy released in muscle activity is useful as workmuscle activity is useful as work

The remaining 60% is given off as The remaining 60% is given off as heatheat

Dangerous heat levels are prevented Dangerous heat levels are prevented by radiation of heat from the skin by radiation of heat from the skin and sweatingand sweating

Force of Muscle ContractionForce of Muscle Contraction The force of contraction is affected by:The force of contraction is affected by:

The number of muscle fibers contracting – the The number of muscle fibers contracting – the more motor fibers in a muscle, the stronger more motor fibers in a muscle, the stronger the contractionthe contraction

The relative size of the muscle – the bulkier The relative size of the muscle – the bulkier the muscle, the greater its strengththe muscle, the greater its strength

Degree of muscle stretch – muscles contract Degree of muscle stretch – muscles contract strongest when muscle fibers are 80-120% of strongest when muscle fibers are 80-120% of their normal resting lengththeir normal resting length

Force of Muscle ContractionForce of Muscle Contraction

Figure 9.20 (a)

Muscle Fiber Type: Muscle Fiber Type: Functional CharacteristicsFunctional Characteristics

Speed of contraction – determined by Speed of contraction – determined by speed in which ATPases split ATPspeed in which ATPases split ATP The two types of fibers are slow and fastThe two types of fibers are slow and fast

ATP-forming pathwaysATP-forming pathways Oxidative fibers – use aerobic pathwaysOxidative fibers – use aerobic pathways Glycolytic fibers – use anaerobic glycolysisGlycolytic fibers – use anaerobic glycolysis

These two criteria define three categories These two criteria define three categories – slow oxidative fibers, fast oxidative – slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibersfibers, and fast glycolytic fibers

Muscle Fiber Type: Speed Muscle Fiber Type: Speed of Contractionof Contraction

Slow oxidative fibers contract slowly, Slow oxidative fibers contract slowly, have slow acting myosin ATPases, have slow acting myosin ATPases, and are fatigue resistantand are fatigue resistant

Fast oxidative fibers contract quickly, Fast oxidative fibers contract quickly, have fast myosin ATPases, and have have fast myosin ATPases, and have moderate resistance to fatiguemoderate resistance to fatigue

Fast glycolytic fibers contract Fast glycolytic fibers contract quickly, have fast myosin ATPases, quickly, have fast myosin ATPases, and are easily fatiguedand are easily fatigued

Smooth MuscleSmooth Muscle Composed of spindle-shaped fibers with a Composed of spindle-shaped fibers with a

diameter of 2-10 diameter of 2-10 m and lengths of m and lengths of several hundred several hundred mm

Lack the coarse connective tissue sheaths Lack the coarse connective tissue sheaths of skeletal muscle, but have fine of skeletal muscle, but have fine endomysiumendomysium

Organized into two layers (longitudinal and Organized into two layers (longitudinal and circular) of closely apposed fiberscircular) of closely apposed fibers

Found in walls of hollow organs (except Found in walls of hollow organs (except the heart)the heart)

Have essentially the same contractile Have essentially the same contractile mechanisms as skeletal musclemechanisms as skeletal muscle

Smooth MuscleSmooth Muscle

Figure 9.24

PeristalsisPeristalsis When the longitudinal layer contracts, When the longitudinal layer contracts,

the organ dilates and contracts the organ dilates and contracts When the circular layer contracts, the When the circular layer contracts, the

organ elongatesorgan elongates Peristalsis – alternating contractions Peristalsis – alternating contractions

and relaxations of smooth muscles and relaxations of smooth muscles that mix and squeeze substances that mix and squeeze substances through the lumen of hollow organsthrough the lumen of hollow organs

Innervation of Smooth Innervation of Smooth MuscleMuscle

Smooth muscle lacks neuromuscular Smooth muscle lacks neuromuscular junctionsjunctions

Innervating nerves have bulbous Innervating nerves have bulbous swellings called varicositiesswellings called varicosities

Varicosities release Varicosities release neurotransmitters into wide synaptic neurotransmitters into wide synaptic clefts called diffuse junctionsclefts called diffuse junctions

Innervation of Smooth Innervation of Smooth MuscleMuscle

Figure 9.25

Microscopic Anatomy of Microscopic Anatomy of Smooth MuscleSmooth Muscle

SR is less developed than in skeletal muscle SR is less developed than in skeletal muscle and lacks a specific patternand lacks a specific pattern

T tubules are absentT tubules are absent Plasma membranes have pouchlike Plasma membranes have pouchlike

infoldings called caveoliinfoldings called caveoli CaCa2+2+ is sequestered in the extracellular is sequestered in the extracellular

space near the caveoli, allowing rapid influx space near the caveoli, allowing rapid influx when channels are openedwhen channels are opened

There are no visible striations and no There are no visible striations and no sarcomeressarcomeres

Thin and thick filaments are presentThin and thick filaments are present

Ratio of thick to thin filaments is much lower Ratio of thick to thin filaments is much lower than in skeletal musclethan in skeletal muscle

Thick filaments have heads along their entire Thick filaments have heads along their entire lengthlength

There is no troponin complexThere is no troponin complex Thick and thin filaments are arranged Thick and thin filaments are arranged

diagonally, causing smooth muscle to diagonally, causing smooth muscle to contract in a corkscrew mannercontract in a corkscrew manner

Noncontractile intermediate filament bundles Noncontractile intermediate filament bundles attach to dense bodies (analogous to Z discs) attach to dense bodies (analogous to Z discs) at regular intervalsat regular intervals

Proportion and Organization of Proportion and Organization of Myofilaments in Smooth MuscleMyofilaments in Smooth Muscle

Figure 9.26

Proportion and Organization of Proportion and Organization of Myofilaments in Smooth MuscleMyofilaments in Smooth Muscle

Contraction of Smooth Contraction of Smooth MuscleMuscle

Whole sheets of smooth muscle exhibit Whole sheets of smooth muscle exhibit slow, synchronized contractionslow, synchronized contraction

They contract in unison, reflecting their They contract in unison, reflecting their electrical coupling with gap junctionselectrical coupling with gap junctions

Action potentials are transmitted from cell Action potentials are transmitted from cell to cellto cell

Some smooth muscle cells: Some smooth muscle cells: Act as pacemakers and set the contractile pace Act as pacemakers and set the contractile pace

for whole sheets of musclefor whole sheets of muscle Are self-excitatory and depolarize without Are self-excitatory and depolarize without

external stimuliexternal stimuli

Types of Smooth Muscle: Types of Smooth Muscle: Single UnitSingle Unit

The cells of single-unit smooth muscle, The cells of single-unit smooth muscle, commonly called visceral muscle:commonly called visceral muscle: Contract rhythmically as a unitContract rhythmically as a unit Are electrically coupled to one another via Are electrically coupled to one another via

gap junctionsgap junctions Often exhibit spontaneous action Often exhibit spontaneous action

potentialspotentials Are arranged in opposing sheets and Are arranged in opposing sheets and

exhibit stress-relaxation responseexhibit stress-relaxation response

Types of Smooth Muscle: Types of Smooth Muscle: MultiunitMultiunit

Multiunit smooth muscles are found:Multiunit smooth muscles are found: In large airways to the lungsIn large airways to the lungs In large arteriesIn large arteries In arrector pili musclesIn arrector pili muscles Attached to hair folliclesAttached to hair follicles In the internal eye musclesIn the internal eye muscles

Types of Smooth Muscle: Types of Smooth Muscle: MultiunitMultiunit

Their characteristics include:Their characteristics include: Rare gap junctionsRare gap junctions Infrequent spontaneous depolarizationsInfrequent spontaneous depolarizations Structurally independent muscle fibers Structurally independent muscle fibers A rich nerve supply, which, with a A rich nerve supply, which, with a

number of muscle fibers, forms motor number of muscle fibers, forms motor unitsunits

Graded contractions in response to Graded contractions in response to neural stimulineural stimuli

Contraction MechanismContraction Mechanism Actin and myosin interact according Actin and myosin interact according

to the sliding filament mechanismto the sliding filament mechanism The final trigger for contractions is a The final trigger for contractions is a

rise in intracellular Carise in intracellular Ca2+2+

CaCa2+2+ is released from the SR and is released from the SR and from the extracellular spacefrom the extracellular space

CaCa2+2+ interacts with calmodulin and interacts with calmodulin and myosin light chain kinase to activate myosin light chain kinase to activate myosinmyosin

Role of Calcium IonRole of Calcium Ion CaCa2+2+ binds to calmodulin and activates it binds to calmodulin and activates it Activated calmodulin activates the Activated calmodulin activates the

kinase enzymekinase enzyme Activated kinase transfers phosphate Activated kinase transfers phosphate

from ATP to myosin cross bridgesfrom ATP to myosin cross bridges Phosphorylated cross bridges interact Phosphorylated cross bridges interact

with actin to produce shorteningwith actin to produce shortening Smooth muscle relaxes when Smooth muscle relaxes when

intracellular Caintracellular Ca2+2+ levels drop levels drop

Special Features of Smooth Special Features of Smooth Muscle ContractionMuscle Contraction

Unique characteristics of smooth Unique characteristics of smooth muscle include:muscle include: Smooth muscle toneSmooth muscle tone Slow, prolonged contractile activitySlow, prolonged contractile activity Low energy requirementsLow energy requirements Response to stretchResponse to stretch

Response to StretchResponse to Stretch Smooth muscle exhibits a Smooth muscle exhibits a

phenomenon called phenomenon called stress-relaxation response in which: stress-relaxation response in which: Smooth muscle responds to stretch only Smooth muscle responds to stretch only

briefly, and then adapts to its new lengthbriefly, and then adapts to its new length The new length, however, retains its The new length, however, retains its

ability to contractability to contract This enables organs such as the stomach This enables organs such as the stomach

and bladder to temporarily store contentsand bladder to temporarily store contents