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

cardiac, and smoothcardiac, and smooth These types differ in structure, location, These types differ in structure, location,

function, and means of activationfunction, and means of activation

Muscle SimilaritiesMuscle Similarities Skeletal and smooth muscle cells are elongated Skeletal and smooth muscle cells are elongated

and are called muscle fibersand are called muscle fibers Muscle contraction depends on two kinds of Muscle contraction depends on two kinds of

myofilaments – actin and myosinmyofilaments – actin and myosin Muscle terminology is similarMuscle terminology is similar

Sarcolemma – muscle plasma membraneSarcolemma – muscle plasma membrane Sarcoplasm – cytoplasm of a muscle cellSarcoplasm – cytoplasm of a muscle cell Prefixes – myo, mys, and sarco all refer to musclePrefixes – myo, mys, and sarco all refer to muscle

Skeletal Muscle TissueSkeletal Muscle Tissue Packaged in skeletal muscles that attach to and cover Packaged in skeletal muscles that attach to and cover

the bony skeletonthe bony skeleton Has obvious stripes called striationsHas obvious stripes called striations Is controlled voluntarily (i.e., by conscious control)Is controlled voluntarily (i.e., by conscious control) Contracts rapidly but tires easilyContracts rapidly but tires easily Is responsible for overall body motilityIs responsible for overall body motility Is extremely adaptable and can exert forces ranging Is extremely adaptable and can exert forces ranging

from a fraction of an ounce to over 70 poundsfrom a fraction of an ounce to over 70 pounds

Cardiac Muscle TissueCardiac Muscle Tissue Occurs only in the heart Occurs only in the heart Is striated like skeletal muscle but is not Is striated like skeletal muscle but is not

voluntaryvoluntary Contracts at a fairly steady rate set by the Contracts at a fairly steady rate set by the

heart’s pacemakerheart’s pacemaker Neural controls allow the heart to respond to Neural controls allow the heart to respond to

changes in bodily needschanges in bodily needs

Smooth Muscle TissueSmooth Muscle Tissue Found in the walls of hollow visceral organs, Found in the walls of hollow visceral organs,

such as the stomach, urinary bladder, and such as the stomach, urinary bladder, and respiratory passagesrespiratory passages

Forces food and other substances through Forces food and other substances through internal body channelsinternal body channels

It is not striated and is involuntaryIt is not striated and is involuntary

Functional Characteristics of Functional Characteristics of Muscle TissueMuscle Tissue

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

Contractility – the ability to shorten forciblyContractility – the ability to shorten forcibly Extensibility – the ability to be stretched or Extensibility – the ability to be stretched or

extendedextended Elasticity – the ability to recoil and resume the Elasticity – the ability to recoil and resume the

original resting lengthoriginal resting length

Muscle FunctionMuscle Function Skeletal muscles are responsible for all Skeletal muscles are responsible for all

locomotionlocomotion Cardiac muscle is responsible for coursing the Cardiac muscle is responsible for coursing the

blood through the bodyblood through the body Smooth muscle helps maintain blood pressure, Smooth muscle helps maintain blood pressure,

and squeezes or propels substances (i.e., food, and squeezes or propels substances (i.e., food, feces) through organsfeces) through organs

Muscles also maintain posture, stabilize joints, Muscles also maintain posture, stabilize joints, and generate heatand generate heat

Skeletal MuscleSkeletal Muscle Each muscle is a discrete organ composed of Each muscle is a discrete organ composed of

muscle tissue, blood vessels, nerve fibers, and muscle tissue, blood vessels, nerve fibers, and connective tissueconnective tissue

Skeletal MuscleSkeletal Muscle The three connective tissue sheaths are:The three connective tissue sheaths are:

Endomysium – fine sheath of connective tissue Endomysium – fine sheath of connective tissue composed of reticular fibers surrounding each composed of reticular fibers surrounding each muscle fibermuscle fiber

Perimysium – fibrous connective tissue that Perimysium – fibrous connective tissue that surrounds groups of muscle fibers called fasciclessurrounds groups of muscle fibers called fascicles

Epimysium – an overcoat of dense regular Epimysium – an overcoat of dense regular connective tissue that surrounds the entire muscleconnective tissue that surrounds the entire muscle

Skeletal MuscleSkeletal Muscle

Figure 9.2a

Skeletal Muscle: Nerve and Skeletal Muscle: Nerve and Blood SupplyBlood Supply

Each muscle is served by one nerve, an artery, Each muscle is served by one nerve, an artery, and one or more veinsand one or more veins

Each skeletal muscle fiber is supplied with a Each skeletal muscle fiber is supplied with a nerve ending that controls contractionnerve ending that controls contraction

Contracting fibers require continuous delivery Contracting fibers require continuous delivery of oxygen and nutrients via arteriesof oxygen and nutrients via arteries

Wastes must be removed via veinsWastes must be removed via veins

Skeletal Muscle: AttachmentsSkeletal Muscle: Attachments Most skeletal muscles span joints and are Most skeletal muscles span joints and are

attached to bone in at least two placesattached to bone in at least two places When muscles contract the movable bone, the When muscles contract the movable bone, the

muscle’s insertion moves toward the muscle’s insertion moves toward the immovable bone, the muscle’s originimmovable bone, the muscle’s origin

Skeletal Muscle: AttachmentsSkeletal Muscle: Attachments Muscles attach:Muscles attach:

Directly – epimysium of the muscle is fused to the Directly – epimysium of the muscle is fused to the periosteum of a boneperiosteum of a bone

Indirectly – connective tissue wrappings extend Indirectly – connective tissue wrappings extend beyond the muscle as a tendon or aponeurosisbeyond the muscle as a tendon or aponeurosis

Microscopic Anatomy of a Microscopic Anatomy of a Skeletal Muscle FiberSkeletal Muscle Fiber

Each fiber is a long, cylindrical cell with Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemmamultiple nuclei just beneath the sarcolemma

Fibers are 10 to 100 Fibers are 10 to 100 m in diameter, and up to m in diameter, and up to hundreds of centimeters longhundreds of centimeters long

Each cell is a syncytium (multinucleate Each cell is a syncytium (multinucleate cytoplasm that is not separated into different cytoplasm that is not separated into different cells) produced by fusion of embryonic cellscells) produced by fusion of embryonic cells

Microscopic Anatomy of a Microscopic Anatomy of a Skeletal Muscle FiberSkeletal Muscle Fiber

Sarcoplasm has numerous glycosomes and a Sarcoplasm has numerous glycosomes and a unique oxygen-binding protein called unique oxygen-binding protein called myoglobinmyoglobin

Fibers contain the usual organelles, myofibrils, Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubulessarcoplasmic reticulum, and T tubules

MyofibrilsMyofibrils Myofibrils are densely packed, rodlike Myofibrils are densely packed, rodlike

contractile elements contractile elements They make up most of the muscle volume They make up most of the muscle volume The arrangement of myofibrils within a fiber is The arrangement of myofibrils within a fiber is

such that a perfectly aligned repeating series of such that a perfectly aligned repeating series of dark A bands and light I bands is evidentdark A bands and light I bands is evident

MyofibrilsMyofibrils

Figure 9.3b

SarcomeresSarcomeres The smallest contractile unit of a muscleThe smallest contractile unit of a muscle The region of a myofibril between two The region of a myofibril between two

successive Z discssuccessive Z discs Composed of myofilaments made up of Composed of myofilaments made up of

contractile proteinscontractile proteins Myofilaments are of two types – thick and thinMyofilaments are of two types – thick and thin

SarcomeresSarcomeres

Figure 9.3c

Myofilaments: Banding PatternMyofilaments: Banding Pattern Thick filaments – extend the entire length of Thick filaments – extend the entire length of

an A bandan A band Thin filaments – extend across the I band and Thin filaments – extend across the I band and

partway into the A bandpartway into the A band Z-disc – coin-shaped sheet of proteins Z-disc – coin-shaped sheet of proteins

(connectins) that anchors the thin filaments (connectins) that anchors the thin filaments and connects myofibrils to one anotherand connects myofibrils to one another

Myofilaments: Banding PatternMyofilaments: Banding Pattern Thin filaments do not overlap thick filaments Thin filaments do not overlap thick filaments

in the lighter H zonein the lighter H zone M lines appear darker due to the presence of M lines appear darker due to the presence of

the protein desminthe protein desmin

Ultrastructure of Myofilaments: Ultrastructure of Myofilaments: Thick FilamentsThick Filaments

Thick filaments are composed of the protein Thick filaments are composed of the protein myosinmyosin

Each myosin molecule has a rod-like tail and Each myosin molecule has a rod-like tail and two globular headstwo globular heads Tails – two interwoven, heavy polypeptide chainsTails – two interwoven, heavy polypeptide chains Heads – two smaller, light polypeptide chains Heads – two smaller, light polypeptide chains

called cross bridgescalled cross bridges

Ultrastructure of Myofilaments: Ultrastructure of Myofilaments: Thick FilamentsThick Filaments

Figure 9.4a,b

Ultrastructure of Myofilaments: Ultrastructure of Myofilaments: Thin FilamentsThin Filaments

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

Each actin molecule is a helical polymer of Each actin molecule is a helical polymer of globular subunits called G actinglobular subunits called G actin

The subunits contain the active sites to which The subunits contain the active sites to which myosin heads attach during contractionmyosin heads attach during contraction

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

Ultrastructure of Myofilaments: Ultrastructure of Myofilaments: Thin FilamentsThin Filaments

Figure 9.4c

Arrangement of the Filaments in Arrangement of the Filaments in a Sarcomerea Sarcomere

Longitudinal section within one sarcomereLongitudinal section within one sarcomere

Figure 9.4d

Sarcoplasmic Reticulum (SR)Sarcoplasmic Reticulum (SR) SR is an elaborate, smooth endoplasmic SR is an elaborate, smooth endoplasmic

reticulum that mostly runs longitudinally and reticulum that mostly runs longitudinally and surrounds each myofibrilsurrounds each myofibril

Paired terminal cisternae (fluid filled sac) form Paired terminal cisternae (fluid filled sac) form perpendicular cross channelsperpendicular cross channels

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

Sarcoplasmic Reticulum (SR)Sarcoplasmic Reticulum (SR) Elongated tubes called T tubules penetrate into Elongated tubes called T tubules penetrate into

the cell’s interior at each A band–I band the cell’s interior at each A band–I band junctionjunction

T tubules associate with the paired terminal T tubules associate with the paired terminal cisternae to form triadscisternae to form triads

Sarcoplasmic Reticulum (SR)Sarcoplasmic Reticulum (SR)

Figure 9.5

T TubulesT Tubules T tubules are continuous with the sarcolemmaT tubules are continuous with the sarcolemma They conduct impulses to the deepest regions They conduct impulses to the deepest regions

of the muscleof the muscle These impulses signal for the release of CaThese impulses signal for the release of Ca2+2+

from adjacent terminal cisternaefrom adjacent terminal cisternae

Triad RelationshipsTriad Relationships T tubules and SR provide tightly linked signals T tubules and SR provide tightly linked signals

for muscle contractionfor muscle contraction A double zipper of integral membrane proteins A double zipper of integral membrane proteins

protrudes into the intermembrane spaceprotrudes into the intermembrane space T tubule proteins act as voltage sensorsT tubule proteins act as voltage sensors SR proteins are receptors that regulate CaSR proteins are receptors that regulate Ca2+2+

release from the SR cisternaerelease from the SR cisternae

Sliding Filament Model of Sliding Filament Model of ContractionContraction

Thin filaments slide past the thick ones so that Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a the actin and myosin filaments overlap to a greater degreegreater degree

In the relaxed state, thin and thick filaments In the relaxed state, thin and thick filaments overlap only slightlyoverlap only slightly

Upon stimulation, myosin heads bind to actin Upon stimulation, myosin heads bind to actin and sliding beginsand sliding begins

Sliding Filament Model of Sliding Filament Model of ContractionContraction

Each myosin head binds and detaches several Each myosin head binds and detaches several times during contraction, acting like a ratchet times during contraction, acting like a ratchet to generate tension and propel the thin to generate tension and propel the thin filaments to the center of the sarcomerefilaments to the center of the sarcomere

As this event occurs throughout the As this event occurs throughout the sarcomeres, the muscle shortenssarcomeres, the muscle shortens

Skeletal Muscle ContractionSkeletal Muscle Contraction In order to contract, a skeletal muscle must:In order to contract, a skeletal muscle must:

Be stimulated by a nerve endingBe stimulated by a nerve ending Propagate an electrical current, or action potentialPropagate an electrical current, or action potential,,

along its sarcolemmaalong its sarcolemma Have a rise in intracellular CaHave a rise in intracellular Ca2+2+ levels, the final levels, the final

trigger for contractiontrigger for contraction Linking the electrical signal to the contraction Linking the electrical signal to the contraction

is excitation-contraction couplingis excitation-contraction coupling

Nerve Stimulus of Skeletal Nerve Stimulus of Skeletal MuscleMuscle

Skeletal muscles are stimulated by motor Skeletal muscles are stimulated by motor neurons of the somatic nervous systemneurons of the somatic nervous system

Axons of these neurons travel in nerves to Axons of these neurons travel in nerves to muscle cellsmuscle cells

Axons of motor neurons branch profusely as Axons of motor neurons branch profusely as they enter musclesthey enter muscles

Each axonal branch forms a neuromuscular Each axonal branch forms a neuromuscular junction with a single muscle fiberjunction with a single muscle fiber

Neuromuscular JunctionNeuromuscular Junction The neuromuscular junction is formed from:The neuromuscular junction is formed from:

Axonal endings, which have small membranous Axonal endings, which have small membranous sacs (synaptic vesicles) that contain the sacs (synaptic vesicles) that contain the neurotransmitter acetylcholineneurotransmitter acetylcholine (ACh)(ACh)

The motor end plate of a muscle, which is a The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh specific part of the sarcolemma that contains ACh receptors and helps form the neuromuscular receptors and helps form the neuromuscular junctionjunction

Neuromuscular JunctionNeuromuscular Junction Though exceedingly close, axonal ends and muscle Though exceedingly close, axonal ends and muscle

fibers are always separated by a space called the fibers are always separated by a space called the synaptic cleftsynaptic cleft

Neuromuscular JunctionNeuromuscular Junction

Figure 9.7 (a-c)

Neuromuscular JunctionNeuromuscular Junction When a nerve impulse reaches the end of an When a nerve impulse reaches the end of an

axon at the neuromuscular junction:axon at the neuromuscular junction: Voltage-regulated calcium channels open and Voltage-regulated calcium channels open and

allow Caallow Ca2+2+ to enter the axon to enter the axon CaCa2+2+ inside the axon terminal causes axonal inside the axon terminal causes axonal

vesicles to fuse with the axonal membranevesicles to fuse with the axonal membrane

Neuromuscular JunctionNeuromuscular Junction This fusion releases ACh into the synaptic cleft via This fusion releases ACh into the synaptic cleft via

exocytosisexocytosis ACh diffuses across the synaptic cleft to ACh ACh diffuses across the synaptic cleft to ACh

receptors on the sarcolemmareceptors on the sarcolemma Binding of ACh to its receptors initiates an action Binding of ACh to its receptors initiates an action

potential in the musclepotential in the muscle

Destruction of AcetylcholineDestruction of Acetylcholine ACh bound to ACh receptors is quickly ACh bound to ACh receptors is quickly

destroyed by the enzyme acetylcholinesterase destroyed by the enzyme acetylcholinesterase This destruction prevents continued muscle This destruction prevents continued muscle

fiber contraction in the absence of additional fiber contraction in the absence of additional stimulistimuli

Action PotentialAction Potential A transient depolarization event that includes A transient depolarization event that includes

polarity reversal of a sarcolemma (or nerve polarity reversal of a sarcolemma (or nerve cell membrane) and the propagation of an cell membrane) and the propagation of an action potential along the membraneaction potential along the membrane

Role of Acetylcholine (Ach)Role of Acetylcholine (Ach) ACh binds its receptors at the motor end plateACh binds its receptors at the motor end plate Binding opens chemically gated channelsBinding opens chemically gated channels NaNa++ and K and K++ diffuse out and the interior of the diffuse out and the interior of the

sarcolemma becomes less negativesarcolemma becomes less negative This event is called depolarizationThis event is called depolarization