The Muscular System

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The Muscular SystemMuscles are responsible

for all types of body movement

Three basic muscle types are found in the body

–Skeletal muscle–Cardiac muscle–Smooth muscle

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Characteristics of MusclesMuscle cells are elongated (muscle cell = muscle fiber)Contraction of muscles is due to the movement of microfilamentsAll muscles share some terminology– Prefix myo refers to muscle– Prefix mys refers to muscle– Prefix sarco refers to flesh

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Smooth Muscle CharacteristicsHas no striationsSpindle-shaped cellsSingle nucleusInvoluntary – no conscious controlFound mainly in the walls of hollow organs

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Cardiac Muscle CharacteristicsHas striationsUsually has a single nucleusJoined to another muscle cell at an intercalated discInvoluntaryFound only in the heart

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Classification of muscleVoluntary Involuntary

Limbs Heart Viscera

Striated Non-striated

Skeletal Cardiac Smooth

Note: Control, Location and Structure6

Muscle Control

Type of muscle

Nervouscontrol

Type of control

Example

SkeletalSkeletal Controlled by CNS

Voluntary Lifting a glass

Cardiac Regulated by ANS

Involuntary Heart beating

Smooth Controlled by ANS

Involuntary Peristalsis

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There are about 650 muscles in the human body. They enable us to move, maintain posture and generate heat. In this section we will only study a sample of the major muscles.

Skeletal Muscle

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Function of Skeletal MusclesProduce movementMaintain postureStabilize jointsGenerate heat

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Skeletal Muscle Characteristics

Most are attached by tendons to bones

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Skeletal Muscle CharacteristicsCells are multinucleateStriated – have visible bandingVoluntary – subject to conscious controlCells are surrounded and bundled by connective tissue

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Connective Tissue Wrappings of Skeletal Muscle

Endomysium – around single muscle fiberPerimysium – around a fascicle (bundle) of fibers

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Connective Tissue Wrappings of Skeletal Muscle

Epimysium – covers the entire skeletal muscleFascia – on the outside of the epimysium

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Skeletal Muscle AttachmentsEpimysium blends into a connective tissue attachment– Tendon – cord-like structure– Aponeuroses – sheet-like

structure

Sites of muscle attachment– Bones– Cartilages– Connective tissue coverings

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Microscopic Anatomy of Skeletal Muscle

Cells are multinucleateNuclei are just beneath the sarcolemma

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Microscopic Anatomy of Skeletal Muscle

Sarcolemma – specialized plasma membraneSarcoplasmic reticulum – specialized smooth endoplasmic reticulum

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Microscopic Anatomy of Skeletal Muscle

Myofibril– Bundles of myofilaments– Myofibrils are aligned to give distinct

bandsI band =light bandA band = dark band

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Microscopic Anatomy of Skeletal Muscle

Sarcomere– Contractile unit of a muscle fiber

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Microscopic Anatomy of Skeletal Muscle

Organization of the sarcomere– Thick filaments = myosin filaments

Composed of the protein myosinHas ATPase enzymes

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Microscopic Anatomy of Skeletal Muscle

Organization of the sarcomere– Thin filaments = actin filaments

Composed of the protein actin

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Microscopic Anatomy of Skeletal Muscle

Myosin filaments have heads (extensions, or cross bridges)Myosin and actin overlap somewhat

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Microscopic Anatomy of Skeletal Muscle

At rest, there is a bare zone that lacks actin filamentsSarcoplasmic reticulum (SR) – for storage of calcium

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To Summarize . . .

Skeletal muscle tissue is made of cells called muscle fibers.Muscle fibers contain small cylinders called myofibrils.Myofibrils are made of sarcomeres linked end-to-end.

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Muscle Contraction

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Properties of Skeletal Muscle Activity

Irritability – ability to receive and respond to a stimulusContractility – ability to shorten when an adequate stimulus is received

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Nerve Stimulus to MusclesSkeletal muscles must be stimulated by a nerve to contractMotor unit– One neuron– Muscle cells

stimulated by that neuron

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Nerve Stimulus to MusclesNeuromuscular junctions – association site of nerve and muscle

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Nerve Stimulus to MusclesSynaptic cleft – gap between nerve and muscle– Nerve and muscle

do not make contact

– Area between nerve and muscle is filled with interstitial fluid

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Transmission of Nerve Impulse to Muscle

Neurotransmitter – chemical released by nerve upon arrival of nerve impulse– The neurotransmitter for skeletal muscle

is acetylcholine

Neurotransmitter attaches to receptors on the sarcolemmaSarcolemma becomes permeable to sodium (Na+)

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Transmission of Nerve Impulse to Muscle

Sodium rushing into the cell generates an action potentialOnce started, muscle contraction cannot be stopped

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When a muscle is relaxed, myosin and actin filaments are not attached.

The Steps in Muscle Contraction

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1. Myosin attaches to a binding site on an actin filament. Calcium is required to make a binding site available for myosin.

The Steps in Muscle Contraction

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The Sliding Filament Theory

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2. The myosin head rotates and causes the actin filament to slide along the myosin filament. The sliding causes the filaments to overlap more, and the sarcomere becomes shorter.

The Steps in Muscle Contraction

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During contraction, myosin attaches to binding sites on actin, forming cross-bridges. Using ATP, the cross-bridges pull the actin toward the center of the sarcomere.

The Steps in Muscle Contraction

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3. After the myosin head has rotated as far as it can, it must let go of the actin fiber. ATP is required for myosin to detach from actin. The myosin head snaps back into its original position, using the energy in the ATP. The ATP becomes ADP and releases a phosphate ion.

The Steps in Muscle Contraction

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The Steps in Muscle Contraction

4. Calcium exposes a new actin binding site and myosin reattaches to actin. Steps 1 through 3 happen again.

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The cross-bridges break, myosin binds to another site, and the cycle begins again until the muscle fiber is contracted.

The Steps in Muscle Contraction

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To summarize . . .

Myosin filaments bind to actin filaments, actin filaments move inward, and sarcomeres shorten to cause muscle contraction.

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Contraction of a Skeletal MuscleMuscle fiber contraction is “all or none”Within a skeletal muscle, not all fibers may be stimulated during the same intervalDifferent combinations of muscle fiber contractions may give differing responsesGraded responses – different degrees of skeletal muscle shortening

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Types of Graded Responses

Twitch– Single, brief contraction– Not a normal muscle function

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Types of Graded Responses

Tetanus (summing of contractions)– One contraction is immediately followed

by another– The muscle does

not completely return to a resting state

– The effects are added

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Types of Graded ResponsesUnfused (incomplete) tetanus– Some relaxation occurs between

contractions– The results are summed

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Types of Graded Responses

Fused (complete) tetanus– No evidence of relaxation before the

following contractions– The result is a sustained muscle

contraction

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Muscle Response to Strong Stimuli

Muscle force depends upon the number of fibers stimulatedMore fibers contracting results in greater muscle tensionMuscles can continue to contract unless they run out of energy

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Energy for Muscle Contraction

Initially, muscles used stored ATP for energy– Bonds of ATP are broken to release

energy– Only 4-6 seconds worth of ATP is stored

by muscles

After this initial time, other pathways must be utilized to produce ATP

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Energy for Muscle ContractionDirect phosphorylation– Muscle cells contain

creatine phosphate (CP)CP is a high-energy molecule

– After ATP is depleted, ADP is left

– CP transfers energy to ADP, to regenerate ATP

– CP supplies are exhausted in about 20 seconds

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Energy for Muscle ContractionAerobic Respiration– Series of metabolic

pathways that occur in the mitochondria

– Glucose is broken down to carbon dioxide and water, releasing energy

– This is a slower reaction that requires continuous oxygen

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Energy for Muscle Contraction

Anaerobic glycolysis– Reaction that breaks

down glucose without oxygen

– Glucose is broken down to pyruvic acid to produce some ATP

– Pyruvic acid is converted to lactic acid

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Energy for Muscle ContractionAnaerobic glycolysis (continued)– This reaction is

not as efficient, but is fast

Huge amounts of glucose are neededLactic acid produces muscle fatigue

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Begin October 18, 2010

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The Muscular System

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Five Golden Rules of Gross Muscle Activity

1. all muscles cross at least one joint2. bulk of muscles lies proximal to the

joint crossed3. all muscles have at least 2

attachments: origin & insertion4. muscles only pull/never push5. during contraction the muscle

insertion moves toward the origin

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Muscles and Body Movements

Movement is attained due to a muscle moving an attached bone

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Muscles and Body MovementsMuscles are attached to at least two points– Origin – attachment to

an immoveable bone– Insertion – attachment

to a movable bone

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The Muscular System

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Types of Muscle ContractionsIsotonic contractions– Myofilaments are able to slide past each

other during contractions– The muscle shortens

Isometric contractions– Tension in the muscles increases– The muscle is unable to shorten

Myofilaments “skidding their wheels”

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Muscle ToneSome fibers are contracted even in a relaxed muscleDifferent fibers contract at different times to provide muscle toneThe process of

stimulating various fibers is under involuntary control

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Muscle Tone

When muscles are not worked they atrophy (waste away) and become flaccid.

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Effects of Exercise on MuscleAerobics result in stronger muscles due to increase blood supply Muscle fibers increase mitochondria and oxygen storage Muscle becomes more fatigue resistantHeart enlarges to pump

more blood to bodyDoes not increase skeletal

muscle size

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Results of increased muscle use from resistance trainingIndividual muscle cells make more contractile filaments & connective tissue increases– Increase in muscle size– Increase in muscle

strength

Effects of Exercise on Muscle

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Begin October 19, 2010

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Muscle Fatigue and Oxygen DebtWhen a muscle is fatigued, it is unable to contractThe common reason for muscle fatigue is oxygen debt– Oxygen must be “repaid” to tissue to

remove oxygen debt– Oxygen is required to get rid of

accumulated lactic acidIncreasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

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The Muscular System

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Types of Ordinary Body Movements

FlexionExtensionRotationAbductionCircumduction

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Flexion & Extension

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Flexion & Extension

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Rotation

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Abduction & Adduction

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Special Movements

DorsiflexionPlantar flexionInversionEversionSupinationPronationOpposition

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Dorsiflexion & Plantar Flexion

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Inversion & Eversion

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Pronation & Supination

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Opposition

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Diseases and Disorders of the Muscular System

Myalgia: Muscle pain due to strain, tearing of muscle fibers. It also is a symptom of an immune response along with a fever.

Myositis: Inflammation of muscle tissue due to injury or disease.

Charley Horse (fibromyositis): Inflammation of muscle tissue and the tendons associated with that muscle due to injury (tear or severe bruising- contusion)

Cramps: Painful, involuntary muscle spasms78

Cerebral PalsyThis disorder is characterized by paralysis and or weakened muscles due to loss of muscle tone. It can be caused due to lack of oxygen to the region of the motor region of the cerebrum of the brain which controls conscious control of muscles. This is often attributed to complication during birth.

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PoliomyelitisPoliomyelitis: Polio is due to a viral infection which affects the motor neurons that control skeletal muscles. It often leads to paralysis and can result in death by paralysis of the diaphragm. Due to vaccine developed by Jonas Salk, the virus has been virtually eliminated in the US. However, it still poses a threat in developing countries. 80

Muscular DystrophyCongenital muscle-destroying disease affect specific muscle groupsMuscle fibers degenerate & atrophy due to an absence of dystrophin, a protein that helps keep muscle cells intactDuchenne’s M.D. Most common & serious—– Mostly in males (diagnosed between2-6 yrs)– Survival is rare beyond early 30’s– X-linked recessive

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Myasthenia gravisRare adult disease caused by antibodies to acetylcholine receptors at the neuromuscular junction which prevents the muscle contraction from occurring Drooping upper eyelids, difficulty swallowing & talking, muscle weakness & fatigueDeath occurs when respiratory muscles cease to function

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AgingConnective Tissue increasesAmount of Muscle tissue decreasesMuscles become stringier(sinewy)Body weight declines due to loss of muscle massBy age 80, muscle strength usually decrease by 50% without weight training exercises

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Types of MusclesPrime mover – muscle with the major responsibility for a certain movementAntagonist – muscle that opposes or reverses a prime moverSynergist – muscle that aids a prime mover in a movement and helps prevent rotationFixator – stabilizes the origin of a prime mover

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Naming of Skeletal Muscles

Direction of muscle fibers– Example: rectus (straight)

Relative size of the muscle– Example: maximus (largest)

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Naming of Skeletal Muscles

Location of the muscle– Example: many muscles are named for

bones (e.g., temporalis)

Number of origins– Example: triceps (three heads)

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Naming of Skeletal Muscles

Location of the muscle’s origin and insertion– Example: sterno (on the sternum)

Shape of the muscle– Example: deltoid (triangular)

Action of the muscle– Example: flexor and extensor (flexes or

extends a bone)

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Head and Neck Muscles

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Trunk Muscles

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Deep Trunk and Arm Muscles

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Muscles of the Pelvis, Hip, and

Thigh

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Muscles of the Lower

Leg

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Superficial Muscles: Anterior

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Superficial Muscles: Posterior

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