Skeletal Muscle Tissue

Skeletal Muscle Tissue ArrangementMyofibrils contractile elements of muscle tissue

Skeletal Muscle Cont.Muscle fiber Muscle cell; composed of several myofibrils

Skeletal Muscle Cont.Each muscle fiber is surrounded by a thin sheath of areolar connective tissue called endomysium

Muscle Tissue Cont.Fascicles A bundle of muscle fibers. There are usually between 10 to 100 muscle fibers in a fascicle.

Muscle Tissue Cont.Each fascicle is surrounded by a layer of dense irregular connective tissue called perimysium

Muscle Tissue Cont.Whole muscle made up of several fascicles

Muscle Tissue Cont.The whole muscle is surrounded by a dense irregular connective tissue called epimysium

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

Muscle TissueTendon Composed of dense regular connective tissue that attaches muscle to the periosteum of the bone

General Features of Skeletal MuscleStriated

General Features of Skeletal MuscleVoluntary

General Features of Skeletal MuscleMultinucleated

General Features of Skeletal MuscleControlled by the somatic (voluntary) division of the nervous system

Microscopic Anatomy of Muscle FibersMuscle Fiber = Muscle Cell

Microscopic Anatomy cont.Sarcolema plasma membrane of muscle cells or muscle fibers

Microscopic Anatomy cont.The multiple nuclei of each muscle fiber is located beneath the sarcolema

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.Sarcoplasm cytoplasm of a muscle fiber

Microscopic Anatomy cont.Sarcoplasmic reticulum fluid filled system of membranous sacs. Calcium is stored here.

Microscopic Anatomy cont.Dilated ends of SR are called terminal cisterns

Microscopic Anatomy cont.Myofibrils are composed of functional units called sarcomeres responsible for the striations

Microscopic Anatomy cont.Each sarcomere is separated from the next by z discs

Microscopic Anatomy cont.Sarcomeres are composed of thick (myosin) and thin (actin) filaments

Microscopic anatomy cont.A band is the part of the sarcomere composed of thick (myosin) and thin (actin) filaments

Microscopic anatomy cont.The A band is the dark striation seen under the microscope

Microscopic Anatomy cont.I Band is the part of the sarcomere that contains only thin (actin) filaments

Microscopic Anatomy cont.I Band is the light striation seen underneath the microscope

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

Microscopic AnatomyM line is the middle of the sarcomere and is composed of supporting proteins that hold the thick filaments together

How does a nerve initiate contraction?Neuromuscular junction the region of contact between a motor neuron and a skeletal muscle fiber

Initiation of ContractionSynaptic cleft the region between the neuron and muscle fiber

Initiation of ContractionThe tips of axon terminals are called synaptic end bulbs

Initiation of ContractionSynaptic vessicles membrane enclosed sacs that contain the neurotransmitter acetylcholine (Ach) located in the synaptic end bulb

Initiation of ContractionMotor end plate the region of the sarcolema opposite of the synaptic end bulb

Initiation of ContractionEach motor end plate contains between 30 to 40 million Ach receptors

Initiation of Contraction / 4 Steps1. Once the nerve impulse arrives at the synaptic end bulb, the synaptic vesicles release Ach via exocytosis.

Initiation of Contraction / 4 Steps2. 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 Steps3. 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 Steps4. ACh is broken down by acetylcholinesterase in the extracellular matrix of the synaptic cleft.

Calciums RoleOnce 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.

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

Contraction / 4 Steps1. ATP hydrolysis ATP is hydrolyzed into ADP and a phospate by ATPase on a myosin head

Contraction / 4 Steps2. Attachment of myosin to actin to form crossbridges myosin binds to actin on the myosin binding site and the phosphate is released.

Contraction / 4 Steps3. Power stroke The myosin pushes the thin filament past the thick filament toward the M line releasing ADP.

Contraction / 4 Steps4. Detachment of myosin from actin When ATP binds to the myosin head, the myosin head detaches from actin.

ContractionAs the muscle contracts the I band and H zone decreases

RelaxtionOnce nerve impulses stop;Acetylcholinesterase breaks down the remaining acetylcholineMuscle action potentials stopCalcium levels in cytosol decreasesContraction stops

How do calcium levels decrease?Ca2+ release channels closeCa2+ active transport pumps move Ca2+ back into the SRIn the SR calsequestrin binds to Ca2+ enabling more Ca2+ to be sequestered within the SR

Rigor MortisCalcium 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)

AtrophyMuscle fibers decrease in size due to loss of myofibrils

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

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

ATP and Muscle ATP is made by;Creatine phosphateAnaerobic cellular respirationAerobic cellular respiration

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

Creatine PhosphateATP + Creatine ADP + Creatine PhosphateThis reaction is catalyzed by creatine kinase

Creatine PhosphateWhen a muscle contracts CK tranfers a phosphate from creatine phosphate to ADP forming ATP and creatine.

Creatine PhosphateCreatine Phosphate + ADP Creatine and ATPThis reaction is catalyzed by CK

Anaerobic Cellular RespirationDoes not require oxygenATP is formed by a process called glycolysisA glucose is converted into two pyruvic acid molecules

Anaerobic RespirationGlycolysis uses two ATP but forms 4 ATP for a net gain of twoPyruvic acid is converted into lactic acid

Anerobic RespirationMuscle fibers attain their glucose via diffusion from the blood and glycogen stored within muscle fibers

Aerobic RespirationRequires oxygenTakes place in mitochondriaThe two molecules of pyruvic acid produced in glycolysis enter the kreb cycle.Aerobic respiration results in a net gain of 36 ATP.

Aerobic RespirationIn aerobic respiration oxygen is attained via the diffusion of oxygen from blood and oxygen released by myoglobin

Aerobic RespirationMyoglobin is a protein found in muscle cells that binds oxygen

Motor UnitsThere is only one neuromuscular junction per fiber.

Motor UnitsA somatic motor neuron branches out and forms neuromuscular junctions with many muscle fibers.

Motor UnitsA motor unit consists of a somatic motor neuron plus all the skeletal muscle fibers it stimulates

Motor UnitsAll muscle fibers in a motor unit contract in unison

Motor UnitMuscles that produce precise movements are made up of small motor units.

Red Muscle FibersHave a high myoglobin content

White Muscle FibersHave a low myoglobin content

3 Main Types of Skeletal Muscle FibersSlow Oxidative FibersFast Oxidative-Glycolytic FibersFast Glycolytic Fibers

Slow Oxidative FibersSmallest in diameterContain large amounts of myoglobinGenerate ATP by aerobic cellular respirationLarge amounts of mitochondrial and blood capillariesATPase in the myosin head hydrolyzes ATP slowly

Fast Oxidative-Glycolytic FibersIntermediate in diameterHigh myoglobin contentGenerates ATP by aerobic and anaerobic respirationHigh content of mitochondria and blood capillariesATPase hydrolyzes ATP quickly

Fast Glycolytic FibersLargest in diameterLow myoglobin contentFew blood capillaries and mitochondriaGenerate ATP by anaerobic respirationATPase hydrolyzes ATP quickly

Motor UnitMuscle fibers of a single motor unit are of the same type

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

Origin and InsertionWhen a muscle contracts, it draws one articulating bone toward the other.

Origin and InsertionThe attachment of the stationary bone is the origin.

Origin and InsertionThe attachment of the movable bone is the insertion

Twitch contractionThe contraction of all the muscle fibers in a motor unit in response to a single action potential

MyogramA record of a muscle contraction

Myogram of a Twitch ContractionLatent periodContraction periodRelaxation period

Myogram of a Twitch ContractionLatent period Lasts two millisecondsCalcium ions are released from SR

Myogram of a Twitch ContractionContraction period 10 100 msec

Myogram of a Twitch ContractionRelaxation Period 10 100 msecActive transport of calcium into SR

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

Frequency of StimulationUnfused 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 StimulationFused 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 RecruitmentNot all motor units in a muscle are not stimulated at once to prevent fatigue.

Concenteric Isotonic ContractionA muscle shortens and pulls on a tendon, which produces movement and reduces the angle at a joint.

Eccenteric Isotonic ContractionThe length of a muscle increases during contraction.

Isometeric ContractionsThe muscle doesnt shorten because the force of the load equals muscle tension.