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 changes in In isotonic contractions, the muscle changes in length (decreasing the angle of the joint) and length (decreasing the angle of the joint) and moves the loadmoves the load

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

does workdoes work Eccentric contractions – the muscle contracts as it Eccentric contractions – the muscle contracts as it

lengthenslengthens

Isotonic ContractionsIsotonic Contractions

Figure 9.19a

Isometric ContractionsIsometric Contractions

Tension increases to the muscle’s capacity, but Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthensthe muscle neither shortens nor lengthens

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

Isometric ContractionsIsometric Contractions

Figure 9.19b

Muscle Metabolism: Energy for Muscle Metabolism: Energy for ContractionContraction

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

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

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

Muscle Metabolism: Energy for Muscle Metabolism: Energy for ContractionContraction

Figure 9.20

Muscle Metabolism: Anaerobic Muscle Metabolism: Anaerobic GlycolysisGlycolysis

When muscle contractile activity reaches 70% When muscle contractile activity reaches 70% of maximum: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: Anaerobic Muscle Metabolism: Anaerobic GlycolysisGlycolysis

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

and heartand heart Is converted back into pyruvic acid by the liverIs converted back into pyruvic acid by the liver

Muscle FatigueMuscle Fatigue

Muscle fatigue – the muscle is in a state of Muscle fatigue – the muscle is in a state of physiological inability to contractphysiological inability to contract

Muscle fatigue occurs when:Muscle fatigue occurs when: ATP production fails to keep pace with ATP useATP production fails to keep pace with ATP use 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 muscle fatigue Intense exercise produces rapid muscle fatigue (with rapid recovery)(with rapid recovery)

NaNa++-K-K++ pumps cannot restore ionic balances pumps cannot restore ionic balances quickly enoughquickly enough

Low-intensity exercise produces slow-Low-intensity exercise produces slow-developing fatiguedeveloping fatigue

SR is damaged and CaSR is damaged and Ca2+2+ regulation is disrupted regulation is disrupted

Oxygen DebtOxygen Debt

Vigorous exercise causes dramatic changes in Vigorous exercise causes dramatic changes in muscle chemistrymuscle 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 DebtOxygen Debt

Oxygen debt – the extra amount of O2 needed Oxygen debt – the extra amount of O2 needed for the above restorative processesfor the above restorative processes

Heat Production During Muscle Heat Production During Muscle ActivityActivity

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

The remaining 60% is given off as heat The remaining 60% is given off as heat Dangerous heat levels are prevented by Dangerous heat levels are prevented by

radiation of heat from the skin and sweatingradiation of heat from the skin and 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 the more motor fibers in a muscle, the stronger the contractioncontraction

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

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

Muscle Fiber Type: Functional Muscle Fiber Type: Functional CharacteristicsCharacteristics

Speed of contraction – determined by speed in Speed of contraction – determined by speed in which ATPases split ATPwhich 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 fibers, and slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibersfast glycolytic fibers

Muscle Fiber Type: Speed of Muscle Fiber Type: Speed of ContractionContraction

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

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

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

Effects of Aerobic ExerciseEffects of Aerobic Exercise

Aerobic exercise results in an increase of:Aerobic exercise results in an increase of: Muscle capillariesMuscle capillaries Number of mitochondriaNumber of mitochondria Myoglobin synthesisMyoglobin synthesis

Effects of Resistance ExerciseEffects of Resistance Exercise

Resistance exercise (typically anaerobic) Resistance exercise (typically anaerobic) results in:results in: Muscle hypertrophyMuscle hypertrophy Increased mitochondria, myofilaments, and Increased mitochondria, myofilaments, and

glycogen storesglycogen stores

The Overload PrincipleThe Overload Principle

Forcing a muscle to work promotes increased Forcing a muscle to work promotes increased muscular strengthmuscular strength

Muscles adapt to increased demandsMuscles adapt to increased demands Muscles must be overloaded to produce further Muscles must be overloaded to produce further

gainsgains

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 several m and lengths of several hundred hundred mm

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

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

Smooth MuscleSmooth Muscle

Found in walls of hollow organs (except the Found in walls of hollow organs (except the heart)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, the When the longitudinal layer contracts, the organ dilates and contracts organ dilates and contracts

When the circular layer contracts, the organ When the circular layer contracts, the organ elongateselongates

Peristalsis – alternating contractions and Peristalsis – alternating contractions and relaxations of smooth muscles that mix and relaxations of smooth muscles that mix and squeeze substances through the lumen of squeeze substances through the lumen of hollow organshollow organs

Innervation of Smooth MuscleInnervation of Smooth Muscle

Smooth muscle lacks neuromuscular junctionsSmooth muscle lacks neuromuscular junctions Innervating nerves have bulbous swellings Innervating nerves have bulbous swellings

called varicositiescalled varicosities Varicosities release neurotransmitters into Varicosities release neurotransmitters into

wide synaptic clefts called diffuse junctionswide synaptic clefts called diffuse junctions

Innervation of Smooth MuscleInnervation of Smooth Muscle

Figure 9.25

Microscopic Anatomy of Smooth Microscopic Anatomy of Smooth MuscleMuscle

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

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

called caveolicalled caveoli

Microscopic Anatomy of Smooth Microscopic Anatomy of Smooth MuscleMuscle

Ca2+ is sequestered in the extracellular space Ca2+ is sequestered in the extracellular space near the caveoli, allowing rapid influx when near the caveoli, allowing rapid influx when channels are openedchannels 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

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

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

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

Thick and thin filaments are arranged diagonally, Thick and thin filaments are arranged diagonally, causing smooth muscle to contract in a corkscrew causing smooth muscle to contract in a corkscrew mannermanner

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

Contraction of Smooth MuscleContraction of Smooth Muscle

Whole sheets of smooth muscle exhibit slow, Whole sheets of smooth muscle exhibit slow, synchronized contractionsynchronized 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 to Action potentials are transmitted from cell to cellcell

Contraction of Smooth MuscleContraction of Smooth Muscle

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

whole sheets of musclewhole sheets of muscle Are self-excitatory and depolarize without external Are self-excitatory and depolarize without external

stimulistimuli

Contraction MechanismContraction Mechanism

Actin and myosin interact according to the sliding Actin and myosin interact according to the sliding filament mechanismfilament mechanism

The final trigger for contractions is a rise in The final trigger for contractions is a rise in intracellular Caintracellular Ca2+2+

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

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

Smooth muscle relaxes when intracellular CaSmooth muscle relaxes when intracellular Ca2+2+ levels levels dropdrop

Special Features of Smooth Special Features of Smooth Muscle ContractionMuscle Contraction

Unique characteristics of smooth muscle Unique characteristics of smooth muscle include: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 phenomenon called Smooth muscle exhibits a phenomenon called stress-relaxation response in which: stress-relaxation response in which: Smooth muscle responds to stretch only briefly, Smooth muscle responds to stretch only briefly,

and then adapts to its new lengthand then adapts to its new length The new length, however, retains its ability to The new length, however, retains its ability to

contractcontract This enables organs such as the stomach and This enables organs such as the stomach and

bladder to temporarily store contentsbladder to temporarily store contents

HyperplasiaHyperplasia

Certain smooth muscles can divide and increase their Certain smooth muscles can divide and increase their numbers by undergoing hyperplasianumbers by undergoing hyperplasia

This is shown by estrogen’s effect on the uterusThis is shown by estrogen’s effect on the uterus At puberty, estrogen stimulates the synthesis of more At puberty, estrogen stimulates the synthesis of more

smooth muscle, causing the uterus to grow to adult sizesmooth muscle, causing the uterus to grow to adult size During pregnancy, estrogen stimulates uterine growth to During pregnancy, estrogen stimulates uterine growth to

accommodate the increasing size of the growing fetusaccommodate the increasing size of the growing fetus

Types of Smooth Muscle: Single Types of Smooth Muscle: Single UnitUnit

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 gap Are electrically coupled to one another via gap

junctionsjunctions Often exhibit spontaneous action potentialsOften exhibit spontaneous action potentials Are arranged in opposing sheets and exhibit stress-Are arranged in opposing sheets and exhibit stress-

relaxation responserelaxation 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 number of A rich nerve supply, which, with a number of

muscle fibers, forms motor unitsmuscle fibers, forms motor units Graded contractions in response to neural stimuliGraded contractions in response to neural stimuli

Muscular DystrophyMuscular Dystrophy

Muscular dystrophy – group of inherited Muscular dystrophy – group of inherited muscle-destroying diseases where muscles muscle-destroying diseases where muscles enlarge due to fat and connective tissue enlarge due to fat and connective tissue deposits, but muscle fibers atrophydeposits, but muscle fibers atrophy

Muscular DystrophyMuscular Dystrophy

Duchenne muscular dystrophy (DMD)Duchenne muscular dystrophy (DMD) Inherited, sex-linked disease carried by females Inherited, sex-linked disease carried by females

and expressed in males (1/3500)and expressed in males (1/3500) Diagnosed between the ages of 2-10Diagnosed between the ages of 2-10 Victims become clumsy and fall frequently as their Victims become clumsy and fall frequently as their

muscles failmuscles fail

Muscular DystrophyMuscular Dystrophy

Progresses from the extremities upward, and Progresses from the extremities upward, and victims die of respiratory failure in their 20svictims die of respiratory failure in their 20s

Caused by a lack of the cytoplasmic protein Caused by a lack of the cytoplasmic protein dystrophindystrophin

There is no cure, but myoblast transfer therapy There is no cure, but myoblast transfer therapy shows promiseshows promise

Developmental AspectsDevelopmental Aspects

Muscle tissue develops from embryonic Muscle tissue develops from embryonic mesoderm called myoblastsmesoderm called myoblasts

Multinucleated skeletal muscles form by fusion Multinucleated skeletal muscles form by fusion of myoblastsof myoblasts

The growth factor The growth factor agrinagrin stimulates the stimulates the clustering of ACh receptors at newly forming clustering of ACh receptors at newly forming motor end platesmotor end plates

Developmental AspectsDevelopmental Aspects

As muscles are brought under the control of the As muscles are brought under the control of the somatic nervous system, the numbers of fast and somatic nervous system, the numbers of fast and slow fibers are also determinedslow fibers are also determined

Cardiac and smooth muscle myoblasts do not Cardiac and smooth muscle myoblasts do not fuse but develop gap junctions at an early fuse but develop gap junctions at an early embryonic stageembryonic stage

Developmental Aspects: Developmental Aspects: RegenerationRegeneration

Cardiac and skeletal muscle become amitotic, Cardiac and skeletal muscle become amitotic, but can lengthen and thickenbut can lengthen and thicken

Myoblastlike satellite cells show very limited Myoblastlike satellite cells show very limited regenerative abilityregenerative ability

Cardiac cells lack satellite cellsCardiac cells lack satellite cells Smooth muscle has good regenerative abilitySmooth muscle has good regenerative ability

Developmental Aspects: After Developmental Aspects: After BirthBirth

Muscular development reflects neuromuscular Muscular development reflects neuromuscular coordinationcoordination

Development occurs head-to-toe, and Development occurs head-to-toe, and proximal-to-distalproximal-to-distal

Peak natural neural control of muscles is Peak natural neural control of muscles is achieved by midadolescenceachieved by midadolescence

Athletics and training can improve Athletics and training can improve neuromuscular controlneuromuscular control

Developmental Aspects: Male Developmental Aspects: Male and Femaleand Female

There is a biological basis for greater strength There is a biological basis for greater strength in men than in womenin men than in women

Women’s skeletal muscle makes up 36% of Women’s skeletal muscle makes up 36% of their body masstheir body mass

Men’s skeletal muscle makes up 42% of their Men’s skeletal muscle makes up 42% of their body massbody mass

Developmental Aspects: Male Developmental Aspects: Male and Femaleand Female

These differences are due primarily to the These differences are due primarily to the male sex hormone testosteronemale sex hormone testosterone

With more muscle mass, men are generally With more muscle mass, men are generally stronger than womenstronger than women

Body strength per unit muscle mass, however, Body strength per unit muscle mass, however, is the same in both sexesis the same in both sexes

Developmental Aspects: Age Developmental Aspects: Age RelatedRelated

With age, connective tissue increases and With age, connective tissue increases and muscle fibers decreasemuscle fibers decrease

Muscles become stringier and more sinewyMuscles become stringier and more sinewy By age 80, 50% of muscle mass is lost By age 80, 50% of muscle mass is lost

(sarcopenia)(sarcopenia)

Developmental Aspects: Age Developmental Aspects: Age RelatedRelated

Regular exercise reverses sarcopeniaRegular exercise reverses sarcopenia Aging of the cardiovascular system affects Aging of the cardiovascular system affects

every organ in the bodyevery organ in the body Atherosclerosis may block distal arteries, Atherosclerosis may block distal arteries,

leading to intermittent claudication and leading to intermittent claudication and causing severe pain in leg musclescausing severe pain in leg muscles