1 Skeletal Muscle Structure – Molecular Level TitinConnects myosin to the Z-lines in the sarcomereIt is very elasticAble to stretch up to 3 times its resting lengthImportant molecule because it is responsible for muscle flexibility
3 Skeletal Muscle Contraction – Force Generation Chemical or heat energy in the body is converted to mechanical work or movement.A nerve impulse arrives at the neuromuscular junction (NMJ) and stimulates the beginning of the contraction processNMJ = synapse between a motor neuron and a skeletal muscle cellStimulation of the skeletal muscle cell triggers the release of calcium ions from the terminal cisternae of the sarcoplasmic reticulumCalcium catalyzes the contraction process
4 Skeletal Muscle Contraction – Force Generation Calcium ions bind to troponinTroponin then pushes tropomyosin away thus exposing the active site that it is covering on actinMyosin crossbridges have a strong affinity for the exposed active site on the actin moleculeMyosin binds to the exposed active siteMyosin crossbridges pull on the actin myofilament pulling it toward the center of the sarcomereThis motion physically shortens the sarcomere, the myofibril, and the muscle fiber.
8 The Sliding-Filament Model A muscle contracts because the myosin and actin myofilaments slide past each otherMyosin cross bridges attach and pull, release, reattach and pull, sliding the actin toward the center of the sarcomereResults in shortening of the I-band and the H-zoneNeither actin nor myosin actually change length even though the sarcomere is shortened in the contraction processThe A-band remains the same length (length of myosin)A single attachment of the cross bridge results in about a 1% shortening of the total muscleMuscles normally shorten 35 to 50% of their total resting length
9 The Sliding-Filament Model Each myosin cross bridge must attach and reattach many times during a single contractionCalled crossbridge cyclingPower Stroke - Attachment of the myosin cross bridge to actin requires energyBreakdown of ATP into ADP and P provides the energy required for pulling on the actin myofilamentATP-ase catalyzes the breakdown of ATPRigor – low-energy, strong bond between myosin and actinADP and P are released from the myosin head thus breaking the bond between the myosin crossbridge and actinNow the muscle is in a state of relaxationCocking - Upon completion of the pulling mechanism, another ATP attaches to the myosin crossbridgePreparation for another crossbridge cycle
13 Excitation-Contraction Coupling Sequence of events that links the nerve impulse and skeletal muscle contractionMotor Neurons – stimulates skeletal musclesExcitatory effectWhen a skeletal muscle cell receives input from a motor neuron, it depolarizesDepolarization causes the muscle cell to fire an action potential
14 Excitation-Contraction Coupling Action PotentialsLarge changes in cell membrane potential (charge)Inside of the cell becomes more positive relative to the outside of the cellFunction to transmit information over long distances
15 Excitation-Contraction Coupling Neuromuscular Junction (NMJ)The synapse between the motor neuron and the muscle cellSynaptic CleftThe extra-cellular space between the motor neuron and the muscle cell
17 Excitation-Contraction Coupling The NMJ releases a neurotransmitter from the motor neuron into the synaptic cleftThe neurotransmitter is acetylcholine (ACh)This neurotransmitter is synthesized by the nerve cell and stored in synaptic vesiclesWhen an nerve impulse reaches the NMJ, the synaptic vesicles release acetylcholine into the synaptic cleft.
19 Excitation-Contraction Coupling 4) Acetylcholine rapidly diffuses across the synaptic cleft to combine with receptors on muscle cell membrane (sarcolemma)The muscle cell is also called the motor end plate membrane5) ACh causes depolarization of the muscle cell membraneGenerates an action potential6) Acetylcholine bound to the receptor is rapidly decomposed by acetylcholinesterase (enzyme) preventing continuous stimulation of the muscle fiber.
20 Excitation-Contraction Coupling Stimulation of ContractionAction potential propogates along the sarcolemma and down the T-tubules to reach the sarcoplasmic reticulumSarcoplasmic reticulum releases calciumCalcium is actively pumped into and stored in the SR leaving a small concentration of calcium ions in the sarcoplasmThe action potential causes the calcium ions to be released from the SR into the sarcoplasm
21 Excitation-Contraction Coupling When released from the SR, calcium travels toward the myofilamentsCalcium binds with troponin on the actin myofilament causing a conformational change, which results in moving tropomyosin off the active siteMyosin heads are then able to bind to the G-actin on the active sitesThis begins the contraction process of crossbridge cycling
22 Excitation-Contraction Coupling Crossbridge cycling continues as long as there is an adequate supply of ATP and if there is stimulation from a motor neuronCrossbridge cycling stops if there is an inadequate supply of ATP or if the motor neuron impulse stopsWhen the motor neuron impulse stops, calcium ions are rapidly pumped back into the sarcoplasmic reticulum for storageThe calcium ion concentration in the sarcoplasm decreasesTropomyosin returns to its original position blocking the myosin binding site on actinThe muscle cell relaxes
24 Muscle Cell Metabolism How Muscle Cells Provide ATP to Drive the Crossbridge Cycle…The sources of ATP:Available ATP in the sarcoplasmCreatine phosphateGlucose
25 Muscle Cell Metabolism Available ATPThere is a limited supply of readily available ATPA small amount of ATP is stored in the myosin crossbridges immediately available when the muscle begins to contract.Contraction uses up this source of ATP in about 6 seconds making it necessary to have other sources of ATP available
26 Muscle Cell Metabolism Creatine Phosphate (CP)When the ATP stores in the myosin crossbridges are exhausted, ADP and CP are used to regenerate ATP.CP + ADP = ATP + Creatine.The energy available from stored ATP and from the reaction of joining ADP with CP provides only about 20 seconds worth of energyThe muscles could contract only long enough to run a 100 m dash on the energy from these sources
27 Muscle Cell Metabolism GlucoseCellular respiration of glucose is an energy source utilized to generate ATPMuscle contractions that are longer than seconds depend on cellular respiration of glucose as a source of ATP
28 Muscle Cell Metabolism RecallCells store glucose in the sarcoplasm in the form of glycogenThe cell must break apart the glycogen molecules to release the individual glucose molecules – this is called glycogenolysisThe breakdown of glucose, called glycolysis, occurs in the sarcoplasm of the muscle cell and does not require oxygen, it is anaerobicGlycolysis produces pyruvic acid, and a small amount of ATP.The majority of the ATP used by muscles is formed by aerobic processes in the mitochondria.At low intensities, the muscle cell depends on aerobic glycolysis during which oxidative phosphorylation becomes more important
29 Muscle Cell Metabolism – Changes with Exercise Intensity Anerobic MetabolismOxygen is not readily availableDuring intense exercise, when the supply of oxygen cannot keep up with metabolic demand of the cells, pyruvic acid produced during glycolysis is converted to lactic acid.Lactic acid accumulates in the muscle resulting in the burning sensation during short duration, high intensity muscular exercise such as lifting weightsLactic acid is quickly removed from the muscle and taken to the liver where it is converted to glucose
30 Muscle Cell Metabolism – Changes with Exercise Intensity Aerobic MetabolismOxygen is readily availableDuring prolonged, low-intensity exercise, the muscles are supplied with adequate oxygen by the protein myoglobinMyoglobinSimilar to hemoglobin (oxygen binding protein in the blood)Myoglobin has a high affinity for oxygen and binds to it loosely inside muscle cellsMyoglobin brings oxygen into the muscle cell and stores it temporarilyThis provides a continuous supply of oxygen even when blood flow to the muscle is reduced
31 Muscle Cell Metabolism – Changes with Exercise Intensity When exercise stops, the body's need for oxygen continues for a period of timeThe body responds to this need by continuing to breathing heavily until all the sources of ATP have been replenishedOxygen DebtThe amount of oxygen necessary to restore the resting metabolic state of the bodyA better, and more currently accepted, term to describe the events following exercise is recovery oxygen consumption
32 Muscle Cell Metabolism – Changes with Exercise Intensity Recovery oxygen consumptionIncludes the oxygen needed to:Restore muscles to their resting metabolic conditionConvert lactic acid to pyruvic acid in the liverReplenish cellular stores of glycogen, creatine phosphate, and ATPReturn resting body temperature to normalReturn the heart muscle and the muscles of respiration to normal, which need repair from the minor tissue damage that occurs due to exerciseThe amount of oxygen needed to meet recovery oxygen consumption demands depends on an individual's physical condition and the duration and intensity of the exercise session.
34 Types of Skeletal Muscle Fibers Not all muscle fibers are the same physiologicallyMuscles vary depending on:The predominant pathway utilized to synthesize ATPOxidative fibers - predominantly aerobic pathwaysOxidative phosphorylation in the mitochondriaFatigue-resistant fibersGlycolytic fibers – predominantly anaerobic pathwaysGlycolysis in the sarcoplasmFatigable fibersThe amount of myoglobinRed fibers - high amounts of myoglobinWhite fibers - small amounts of myoglobinEfficiency of ATPaseFast twitch fibers - decompose ATP rapidlySlow twitch fibers - decompose ATP slowly
35 Types of Skeletal Muscle Fibers Slow-twitch fatigue-resistant fibersSlow oxidative fibers, or red muscle fibers.Contain abundant myoglobin giving them their red color.Slow acting ATPase enzymesAbundant mitochondriaDepend upon aerobic pathways for production of ATPEndurance type musclesAble to deliver strong, prolonged contractions.Examples:Postural muscles - spinal extensorsAnti-gravity muscles - calf muscle
36 Types of Skeletal Muscle Fibers Fast-twitch fatigable fibersFast glycolytic fibers, or white muscle fibers.Contain small amounts of myoglobinFast acting ATPase enzymesAllows the muscle fiber to contract rapidlyFew mitochondriaContract for limited periods of time because fatigue rapidlyPlenty of glycogenDepends on anaerobic metabolismExtensive sarcoplasmic reticulumRapidly releases and stores calcium ions contributing to rapid contractionsBest suited for short duration, high intensity contractions
37 Types of Skeletal Muscle Fibers Intermediate FibersFast-twitch fatigue-resistant fibersFast glycolytic fibersPale muscle fibersCharacteristics lie between the red and white fibers
38 Types of Skeletal Muscle Fibers Most of the body's muscles contain a mixture of fiber types.It is the motor nerve that innervates the muscle cell that determines its typeTherefore, all of the muscle cells in a single motor unit are of the same typeMotor Unit – a motor neuron and all of the muscle fibers it innervatesExamples:Running – the motor nerve stimulates the motor units containing fast-twitch fibers.Posture – the motor nerve stimulates the motor units containing slow-twitch fibers.
39 Types of Skeletal Muscle Fibers Slow twitch fibers are recruited firstThis is because they are found in small motor unitsFast twitch fibers are recruited lastThis is because they are found in large motor units
40 Types of Skeletal Muscle Fibers People are genetically predisposed to have relatively more of one fiber type than anotherPeople who excel at marathon running have higher percentages of slow twitch fatigue resistant muscle fibersPeople who excel at sprinting have higher percentages of fast twitch fatigable fibers