Skeletal Muscle Tissue

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Skeletal Muscle TissueSkeletal Muscle Tissue

Skeletal Muscle Tissue Skeletal Muscle Tissue ArrangementArrangement

• Myofibrils – contractile elements of muscle tissue

Skeletal Muscle Cont.Skeletal Muscle Cont.

• Muscle fiber – Muscle cell; composed of several myofibrils

Skeletal Muscle Cont.Skeletal Muscle Cont.

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

Muscle Tissue Cont.Muscle Tissue Cont.

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

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

• Whole muscle – made up of several fascicles

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

Muscle TissueMuscle Tissue

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

Muscle TissueMuscle Tissue

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

General Features of Skeletal General Features of Skeletal MuscleMuscle

• Striated

• Voluntary

• Multinucleated

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

Microscopic Anatomy of Muscle Microscopic Anatomy of Muscle FibersFibers

• Muscle Fiber = Muscle Cell

Microscopic Anatomy cont.Microscopic Anatomy cont.

• Sarcolema – plasma membrane of muscle cells or muscle fibers

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

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

• Sarcoplasm – cytoplasm of a muscle fiber

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

• Dilated ends of SR are called terminal cisterns

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

• Each sarcomere is separated from the next by z discs

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

Microscopic anatomy cont.Microscopic anatomy cont.

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

Microscopic anatomy cont.Microscopic anatomy cont.

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

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

• I Band is the light striation seen underneath the microscope

Microscopic AnatomyMicroscopic Anatomy

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

Microscopic AnatomyMicroscopic Anatomy

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

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

Initiation of ContractionInitiation of Contraction

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

• The tips of axon terminals are called synaptic end bulbs

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

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

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

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.

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.

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.

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

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.

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.

Contraction / 4 StepsContraction / 4 Steps

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

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

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

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

ContractionContraction

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

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

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

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)

AtrophyAtrophy

• Muscle fibers decrease in size due to loss of myofibrils

HypertrophyHypertrophy

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

ATP and MuscleATP and Muscle

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

ATP and Muscle ATP and Muscle

• ATP is made by;

1. Creatine phosphate

2. Anaerobic cellular respiration

3. Aerobic cellular respiration

Creatine PhosphateCreatine Phosphate

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

ATP + Creatine → ADP + Creatine Phosphate

This reaction is catalyzed by creatine kinase

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

• Creatine Phosphate + ADP → Creatine and ATP

This reaction is catalyzed by CK

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

Anaerobic RespirationAnaerobic Respiration

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

• Pyruvic acid is converted into lactic acid

Anerobic RespirationAnerobic Respiration

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

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.

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

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

Motor UnitsMotor Units

• There is only one neuromuscular junction per fiber.

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

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

• All muscle fibers in a motor unit contract in unison

Motor UnitMotor Unit

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

Red Muscle FibersRed Muscle Fibers

• Have a high myoglobin content

White Muscle FibersWhite Muscle Fibers

• Have a low myoglobin content

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

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

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

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

Motor UnitMotor Unit

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

Origin and InsertionOrigin and Insertion

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

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

• The attachment of the stationary bone is the origin.

• The attachment of the movable bone is the insertion

Twitch contractionTwitch contraction

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

MyogramMyogram

• A record of a muscle contraction

Myogram of a Twitch ContractionMyogram of a Twitch Contraction

1. Latent period

2. Contraction period

3. Relaxation period

1. Latent period –

Lasts two milliseconds

Calcium ions are released from SR

2. Contraction period –

10 – 100 msec

3. Relaxation Period –

10 – 100 msec

Active transport of calcium into SR

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.

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.

• 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.

Motor Unit RecruitmentMotor Unit Recruitment

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

Concenteric Isotonic ContractionConcenteric Isotonic Contraction

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

Eccenteric Isotonic ContractionEccenteric Isotonic Contraction

• The length of a muscle increases during contraction.

Isometeric ContractionsIsometeric Contractions

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

Skeletal Muscle Tissue

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