Skeletal Muscle Tissue. Skeletal Muscle Tissue Arrangement Myofibrils – contractile elements of...

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Skeletal Muscle Skeletal Muscle Tissue Tissue

Transcript of Skeletal Muscle Tissue. Skeletal Muscle Tissue Arrangement Myofibrils – contractile elements of...

Page 1: Skeletal Muscle Tissue. Skeletal Muscle Tissue Arrangement Myofibrils – contractile elements of muscle tissue.

Skeletal Muscle TissueSkeletal Muscle Tissue

Page 2: Skeletal Muscle Tissue. Skeletal Muscle Tissue Arrangement Myofibrils – contractile elements of muscle tissue.

Skeletal Muscle Tissue Skeletal Muscle Tissue ArrangementArrangement

• Myofibrils – contractile elements of muscle tissue

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Skeletal Muscle Cont.Skeletal Muscle Cont.

• Muscle fiber – Muscle cell; composed of several myofibrils

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Skeletal Muscle Cont.Skeletal Muscle Cont.

• Each muscle fiber is surrounded by a thin sheath of areolar connective tissue called endomysium

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Muscle Tissue Cont.Muscle Tissue Cont.

• Fascicles – A bundle of muscle fibers. There are usually between 10 to 100 muscle fibers in a fascicle.

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Muscle Tissue Cont.Muscle Tissue Cont.

• Each fascicle is surrounded by a layer of dense irregular connective tissue called perimysium

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Muscle Tissue Cont.Muscle Tissue Cont.

• Whole muscle – made up of several fascicles

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Muscle Tissue Cont.Muscle Tissue Cont.

• The whole muscle is surrounded by a dense irregular connective tissue called epimysium

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Muscle TissueMuscle Tissue

• All three connective tissues (endomysium, perimysium, epimysium) extend beyond the muscle fiber to form a tendon.

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Muscle TissueMuscle Tissue

• Tendon – Composed of dense regular connective tissue that attaches muscle to the periosteum of the bone

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General Features of Skeletal General Features of Skeletal MuscleMuscle

• Striated

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General Features of Skeletal General Features of Skeletal MuscleMuscle

• Voluntary

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General Features of Skeletal General Features of Skeletal MuscleMuscle

• Multinucleated

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General Features of Skeletal General Features of Skeletal MuscleMuscle

• Controlled by the somatic (voluntary) division of the nervous system

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Microscopic Anatomy of Muscle Microscopic Anatomy of Muscle FibersFibers

• Muscle Fiber = Muscle Cell

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Microscopic Anatomy cont.Microscopic Anatomy cont.

• Sarcolema – plasma membrane of muscle cells or muscle fibers

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Microscopic Anatomy cont.Microscopic Anatomy cont.

• The multiple nuclei of each muscle fiber is located beneath the sarcolema

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Microscopic Anatomy cont.Microscopic Anatomy cont.

• T (tranverse tubules) – Invagination of the sarcolema that tunnel in from the surface to the center of each muscle fiber

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Microscopic Anatomy cont.Microscopic Anatomy cont.

• Sarcoplasm – cytoplasm of a muscle fiber

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Microscopic Anatomy cont.Microscopic Anatomy cont.

• Sarcoplasmic reticulum – fluid filled system of membranous sacs. Calcium is stored here.

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Microscopic Anatomy cont.Microscopic Anatomy cont.

• Dilated ends of SR are called terminal cisterns

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Microscopic Anatomy cont.Microscopic Anatomy cont.

• Myofibrils are composed of functional units called sarcomeres responsible for the striations

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Microscopic Anatomy cont.Microscopic Anatomy cont.

• Each sarcomere is separated from the next by z discs

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Microscopic Anatomy cont.Microscopic Anatomy cont.

• Sarcomeres are composed of thick (myosin) and thin (actin) filaments

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Microscopic anatomy cont.Microscopic anatomy cont.

• A band is the part of the sarcomere composed of thick (myosin) and thin (actin) filaments

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Microscopic anatomy cont.Microscopic anatomy cont.

• The A band is the dark striation seen under the microscope

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Microscopic Anatomy cont.Microscopic Anatomy cont.

• I Band is the part of the sarcomere that contains only thin (actin) filaments

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Microscopic Anatomy cont.Microscopic Anatomy cont.

• I Band is the light striation seen underneath the microscope

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Microscopic AnatomyMicroscopic Anatomy

• The H zone is the part of the A band that contains only thick filaments (myosin)

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Microscopic AnatomyMicroscopic Anatomy

• M line is the middle of the sarcomere and is composed of supporting proteins that hold the thick filaments together

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How does a nerve initiate How does a nerve initiate contraction?contraction?

• Neuromuscular junction – the region of contact between a motor neuron and a skeletal muscle fiber

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Initiation of ContractionInitiation of Contraction

• Synaptic cleft – the region between the neuron and muscle fiber

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Initiation of ContractionInitiation of Contraction

• The tips of axon terminals are called synaptic end bulbs

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Initiation of ContractionInitiation of Contraction

• Synaptic vessicles – membrane – enclosed sacs that contain the neurotransmitter acetylcholine (Ach) located in the synaptic end bulb

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Initiation of ContractionInitiation of Contraction

• Motor end plate – the region of the sarcolema opposite of the synaptic end bulb

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Initiation of ContractionInitiation of Contraction

• Each motor end plate contains between 30 to 40 million Ach receptors

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Initiation of Contraction / 4 StepsInitiation of Contraction / 4 Steps

1. Once the nerve impulse arrives at the synaptic end bulb, the synaptic vesicles release Ach via exocytosis.

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Initiation of Contraction / 4 StepsInitiation of Contraction / 4 Steps

2. When two ACh molecules bind to the ACh receptors at the motor end plate it opens the cation channel and Na+ can flow across the membrane.

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Initiation of Contraction / 4 StepsInitiation of Contraction / 4 Steps

3. Once the inside of the muscle fiber is more positively charged, a muscle action potential is triggered, which propogates along the sarcolema and into the T tubule system.

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Initiation of Contraction / 4 StepsInitiation of Contraction / 4 Steps

4. ACh is broken down by acetylcholinesterase in the extracellular matrix of the synaptic cleft.

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Calcium’s RoleCalcium’s Role

• Once the action potential propagates along the sarcolema and into the T tubules Ca2+ release channels in the SR membrane open causing Ca2+ to flow out of the SR into the cytosol.

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Calcium’s RoleCalcium’s Role

• Calcium binds to troponin on the actin filaments causing the troponin-tropomyosin complexes to move away from the myosin binding sites on actin.

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Contraction / 4 StepsContraction / 4 Steps

1. ATP hydrolysis – ATP is hydrolyzed into ADP and a phospate by ATPase on a myosin head

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Contraction / 4 StepsContraction / 4 Steps

2. Attachment of myosin to actin to form crossbridges – myosin binds to actin on the myosin binding site and the phosphate is released.

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Contraction / 4 StepsContraction / 4 Steps

3. Power stroke – The myosin pushes the thin filament past the thick filament toward the M line releasing ADP.

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Contraction / 4 StepsContraction / 4 Steps

4. Detachment of myosin from actin – When ATP binds to the myosin head, the myosin head detaches from actin.

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ContractionContraction

• As the muscle contracts the I band and H zone decreases

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RelaxtionRelaxtion

Once nerve impulses stop;

1. Acetylcholinesterase breaks down the remaining acetylcholine

2. Muscle action potentials stop

3. Calcium levels in cytosol decreases

4. Contraction stops

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How do calcium levels decrease?How do calcium levels decrease?

• Ca2+ release channels close

• Ca2+ active transport pumps move Ca2+ back into the SR

• In the SR calsequestrin binds to Ca2+ enabling more Ca2+ to be sequestered within the SR

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Rigor MortisRigor Mortis

• Calcium leaks out of the SR therefore allowing myosin heads to bind to actin.

• ATP production ceases so myosin cannot detach form actin.

• Muscles therefore become rigid (cannot contract or stretch)

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AtrophyAtrophy

• Muscle fibers decrease in size due to loss of myofibrils

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HypertrophyHypertrophy

• Muscle fibers increase in diameter due to the production of more myofibrils.

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ATP and MuscleATP and Muscle

• Muscle fibers need ATP for powering the contraction cycle and to pump Ca2+ into the SR.

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ATP and Muscle ATP and Muscle

• ATP is made by;

1. Creatine phosphate

2. Anaerobic cellular respiration

3. Aerobic cellular respiration

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Creatine PhosphateCreatine Phosphate

• When the muscle is relaxed creatine kinase (CK) transfers a phosphate from ATP to creatine forming creatine phosphate and ADP.

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Creatine PhosphateCreatine Phosphate

ATP + Creatine → ADP + Creatine Phosphate

This reaction is catalyzed by creatine kinase

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Creatine PhosphateCreatine Phosphate

• When a muscle contracts CK tranfers a phosphate from creatine phosphate to ADP forming ATP and creatine.

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Creatine PhosphateCreatine Phosphate

• Creatine Phosphate + ADP → Creatine and ATP

This reaction is catalyzed by CK

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Anaerobic Cellular RespirationAnaerobic Cellular Respiration

• Does not require oxygen

• ATP is formed by a process called glycolysis

• A glucose is converted into two pyruvic acid molecules

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Anaerobic RespirationAnaerobic Respiration

• Glycolysis uses two ATP but forms 4 ATP for a net gain of two

• Pyruvic acid is converted into lactic acid

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Anerobic RespirationAnerobic Respiration

• Muscle fibers attain their glucose via diffusion from the blood and glycogen stored within muscle fibers

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Aerobic RespirationAerobic Respiration

• Requires oxygen

• Takes place in mitochondria

• The two molecules of pyruvic acid produced in glycolysis enter the kreb cycle.

• Aerobic respiration results in a net gain of 36 ATP.

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Aerobic RespirationAerobic Respiration

• In aerobic respiration oxygen is attained via the diffusion of oxygen from blood and oxygen released by myoglobin

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Aerobic RespirationAerobic Respiration

• Myoglobin is a protein found in muscle cells that binds oxygen

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Motor UnitsMotor Units

• There is only one neuromuscular junction per fiber.

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Motor UnitsMotor Units

• A somatic motor neuron branches out and forms neuromuscular junctions with many muscle fibers.

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Motor UnitsMotor Units

• A motor unit consists of a somatic motor neuron plus all the skeletal muscle fibers it stimulates

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Motor UnitsMotor Units

• All muscle fibers in a motor unit contract in unison

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Motor UnitMotor Unit

• Muscles that produce precise movements are made up of small motor units.

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Red Muscle FibersRed Muscle Fibers

• Have a high myoglobin content

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White Muscle FibersWhite Muscle Fibers

• Have a low myoglobin content

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3 Main Types of Skeletal Muscle 3 Main Types of Skeletal Muscle FibersFibers

1. Slow Oxidative Fibers

2. Fast Oxidative-Glycolytic Fibers

3. Fast Glycolytic Fibers

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Slow Oxidative FibersSlow Oxidative Fibers

• Smallest in diameter

• Contain large amounts of myoglobin

• Generate ATP by aerobic cellular respiration

• Large amounts of mitochondrial and blood capillaries

• ATPase in the myosin head hydrolyzes ATP slowly

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Fast Oxidative-Glycolytic FibersFast Oxidative-Glycolytic Fibers

• Intermediate in diameter

• High myoglobin content

• Generates ATP by aerobic and anaerobic respiration

• High content of mitochondria and blood capillaries

• ATPase hydrolyzes ATP quickly

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Fast Glycolytic FibersFast Glycolytic Fibers

• Largest in diameter

• Low myoglobin content

• Few blood capillaries and mitochondria

• Generate ATP by anaerobic respiration

• ATPase hydrolyzes ATP quickly

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Motor UnitMotor Unit

• Muscle fibers of a single motor unit are of the same type

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Origin and InsertionOrigin and Insertion

• Most muscles cross at least one joint and are attached to the articulating bones that form the joint.

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Origin and InsertionOrigin and Insertion

• When a muscle contracts, it draws one articulating bone toward the other.

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Origin and InsertionOrigin and Insertion

• The attachment of the stationary bone is the origin.

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Origin and InsertionOrigin and Insertion

• The attachment of the movable bone is the insertion

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Twitch contractionTwitch contraction

• The contraction of all the muscle fibers in a motor unit in response to a single action potential

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MyogramMyogram

• A record of a muscle contraction

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Myogram of a Twitch ContractionMyogram of a Twitch Contraction

1. Latent period

2. Contraction period

3. Relaxation period

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Myogram of a Twitch ContractionMyogram of a Twitch Contraction

1. Latent period –

Lasts two milliseconds

Calcium ions are released from SR

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Myogram of a Twitch ContractionMyogram of a Twitch Contraction

2. Contraction period –

10 – 100 msec

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Myogram of a Twitch ContractionMyogram of a Twitch Contraction

3. Relaxation Period –

10 – 100 msec

Active transport of calcium into SR

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Frequency of StimulationFrequency of Stimulation

Wave summation –

When a second stimulus occurs before the muscle has relaxed, the second contraction is stronger than the first.

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Frequency of StimulationFrequency of Stimulation

Unfused tetanus –

When a skeletal muscle is stimulated at a rate of 20 to 30 times per second, it can only partially relax between stimuli resulting in a sustained but wavering contraction.

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Page 132: Skeletal Muscle Tissue. Skeletal Muscle Tissue Arrangement Myofibrils – contractile elements of muscle tissue.

Frequency of StimulationFrequency of Stimulation

• Fused tetanus –

When a skeletal muscle is stimulated at a rate of 80 to 100 stimuli per second, a sustained contraction results in which individual twitches cannot be discerned.

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Motor Unit RecruitmentMotor Unit Recruitment

• Not all motor units in a muscle are not stimulated at once to prevent fatigue.

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Concenteric Isotonic ContractionConcenteric Isotonic Contraction

• A muscle shortens and pulls on a tendon, which produces movement and reduces the angle at a joint.

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Eccenteric Isotonic ContractionEccenteric Isotonic Contraction

• The length of a muscle increases during contraction.

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Isometeric ContractionsIsometeric Contractions

• The muscle doesn’t shorten because the force of the load equals muscle tension.

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