Skeletal Muscle Tissue. Skeletal Muscle Tissue Arrangement Myofibrils – contractile elements of...
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Transcript of Skeletal Muscle Tissue. Skeletal Muscle Tissue Arrangement Myofibrils – contractile elements of...
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.
Muscle Tissue Cont.Muscle Tissue Cont.
• Each fascicle is surrounded by a layer of dense irregular connective tissue called perimysium
Muscle Tissue Cont.Muscle Tissue Cont.
• Whole muscle – made up of several fascicles
Muscle Tissue Cont.Muscle Tissue Cont.
• 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
General Features of Skeletal General Features of Skeletal MuscleMuscle
• Voluntary
General Features of Skeletal General Features of Skeletal MuscleMuscle
• Multinucleated
General Features of Skeletal General Features of Skeletal MuscleMuscle
• 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
Microscopic Anatomy cont.Microscopic Anatomy cont.
• The multiple nuclei of each muscle fiber is located beneath the sarcolema
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
Microscopic Anatomy cont.Microscopic Anatomy cont.
• Sarcoplasm – cytoplasm of a muscle fiber
Microscopic Anatomy cont.Microscopic Anatomy cont.
• Sarcoplasmic reticulum – fluid filled system of membranous sacs. Calcium is stored here.
Microscopic Anatomy cont.Microscopic Anatomy cont.
• Dilated ends of SR are called terminal cisterns
Microscopic Anatomy cont.Microscopic Anatomy cont.
• Myofibrils are composed of functional units called sarcomeres responsible for the striations
Microscopic Anatomy cont.Microscopic Anatomy cont.
• Each sarcomere is separated from the next by z discs
Microscopic Anatomy cont.Microscopic Anatomy cont.
• 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
Microscopic Anatomy cont.Microscopic Anatomy cont.
• I Band is the part of the sarcomere that contains only thin (actin) filaments
Microscopic Anatomy cont.Microscopic Anatomy cont.
• 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
Initiation of ContractionInitiation of Contraction
• The tips of axon terminals are called synaptic end bulbs
Initiation of ContractionInitiation of Contraction
• Synaptic vessicles – membrane – enclosed sacs that contain the neurotransmitter acetylcholine (Ach) located in the synaptic end bulb
Initiation of ContractionInitiation of Contraction
• Motor end plate – the region of the sarcolema opposite of the synaptic end bulb
Initiation of ContractionInitiation of Contraction
• 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.
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.
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.
Initiation of Contraction / 4 StepsInitiation of Contraction / 4 Steps
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
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.
Contraction / 4 StepsContraction / 4 Steps
3. Power stroke – The myosin pushes the thin filament past the thick filament toward the M line releasing ADP.
Contraction / 4 StepsContraction / 4 Steps
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.
Creatine PhosphateCreatine Phosphate
ATP + Creatine → ADP + Creatine Phosphate
This reaction is catalyzed by creatine kinase
Creatine PhosphateCreatine Phosphate
• When a muscle contracts CK tranfers a phosphate from creatine phosphate to ADP forming ATP and creatine.
Creatine PhosphateCreatine Phosphate
• 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.
Aerobic RespirationAerobic Respiration
• In aerobic respiration oxygen is attained via the diffusion of oxygen from blood and oxygen released by myoglobin
Aerobic RespirationAerobic Respiration
• Myoglobin is a protein found in muscle cells that binds oxygen
Motor UnitsMotor Units
• There is only one neuromuscular junction per fiber.
Motor UnitsMotor Units
• A somatic motor neuron branches out and forms neuromuscular junctions with many muscle fibers.
Motor UnitsMotor Units
• A motor unit consists of a somatic motor neuron plus all the skeletal muscle fibers it stimulates
Motor UnitsMotor Units
• 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.
Origin and InsertionOrigin and Insertion
• When a muscle contracts, it draws one articulating bone toward the other.
Origin and InsertionOrigin and Insertion
• The attachment of the stationary bone is the origin.
Origin and InsertionOrigin and Insertion
• 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
Myogram of a Twitch ContractionMyogram of a Twitch Contraction
1. Latent period –
Lasts two milliseconds
Calcium ions are released from SR
Myogram of a Twitch ContractionMyogram of a Twitch Contraction
2. Contraction period –
10 – 100 msec
Myogram of a Twitch ContractionMyogram of a Twitch Contraction
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.
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.
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.
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.