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The microstructure of muscle Muscle Terminology • myofiber (muscle fiber)- a single muscle cell • sarcolemma- muscle cell membrane • sarcoplasm- muscle cell cytoplasm • myofibril- long contractile protein structure – actin and myosin • sarcomere- the contractile unit between two z-lines Connective tissue surrounding muscle The sarcoplasm with sarcoplasmic reticulum and transverse tubules More Terms • sarcoplasmic reticulum- storage and release site of calcium • transverse tubule- also involved in calcium flux Axonal Outgrowth • Prior to myofiber formation • Developing neuron forms a NMJ with a immature muscle cell (myotube) • Polyneuronal innervation of a myofiber Unanswered Questions • How do axons find the appropriate muscle? • Once an axon has found the appropriate muscle, how do they find the correct muscle fiber? Hypotheses • Motor neurons invade a muscle in a random fashion • Only the nerves that find the right muscle survive • Specificity achieved after formation of a NMJ Hypotheses Refuted • No evidence of random innervation • Outgrowing axons innervate the correct muscle • Axon outgrowth occurs before formation of muscle fibers Myogenesis: Birth of a Myofiber • Myoblast (mononuclear cell) • Myoblasts derived from somites & appear ~ 6th week of gestation • Formation of myofiber requires: • Millions of myoblasts • Aggregation and fusion of myoblasts forming a myotube Myotube Development Synaptogenesis: Birth of NMJs • Primitive NMJs first seen at 8-9 weeks of gestation • More fully developed NMJs at 19 weeks of gestation • Synaptic transmission dependent on: • Synthesis of acetylcholine • Synthesis of acetylcholine receptors Synapse Elimination • Embryonic muscle: several axons innervate a myofiber • Adult muscle: 1 axon innervates a myofiber • Synapse elimination complete before birth (~25 weeks of gestation) The Ultrastructure of Muscle The Neuromuscular Junction Neuromuscular Junction 1) Impulse travels down motor neuron 2) at end of neuron, acetylcholine released 3) Acetylcholine diffuses across synaptic cleft 4) acetylcholine binds to receptors on sarcolemma causing permeabilty 5) sodium enters cell causing depolarization and muscle contraction Muscular Contraction • functions to produce force for locomotion • force for breathing • force for postural support • heat production in cold (no force) How do skeletal muscles contract? • Sliding filament model of contraction • the interaction of actin and myosin The sliding filament theory of contraction Sliding Filament Animation • -- sliding filament animation.htm •http://intro.bio.umb.edu/111112/112s99Lect/muscle/contract.html How do the Actin and Myosin Interact? • The myosin head is unbound to the actin filament • the signal to contract initiates a strong binding state – Binding of calcium to troponin regulates this strong-weak state The Contraction Itself • during the strong binding the myosin pulls the actin past • this effectively shortens or contracts the muscle Relationship between myosin cross-bridges and Ca++ binding Where does the energy for contraction come from? • ATP is necessary for each contraction cycle to occur • each contraction cycle results in a shortening of the muscle by 1% • some muscles can shorten by up to 60 % of their resting length • therefore many shortening cycles must occur for a single contraction Crossbridge Animation • Quicktime - Actin Myosin Crossbridge 3D Animation.htm •http://www.sci.sdsu.edu/movies/actin_myosin.html ATP & the Cross-bridge Theory ATP ADP + Pi + Energy Experimental Evidence: 1. Dead muscle is stiff (rigor mortis) 2. Solution of actin and myosin No ATP: turbid solution ATP: solution clears Hydrolysis of ATP Myosin + ATP ADP + Pi – Slow dissociation of ADP & Pi • Myosin + ATP + Actin ADP + Pi Rapid dissociation of ADP and Pi! • Hydrolysis of ATP occurs after the disassociation of actin from myosin ATP in Cross-Bridge Cycle • Hydrolysis of ATP provides the energy for cross-bridge cycle • Binding of ATP to myosin dissociates actin from myosin Cytosolic Calcium # of exposed actin binding sites # of cross-bridges formed Amount of force exerted by: myofibrils myofibers muscle Steps in Excitation - Contraction coupling • at rest actin and myosin are unbound • an excitation impulse from the a motor nerve causes an end-plate potential • the potential depolarizes the muscle cell beginning at the sarcolemma The Neuromuscular Junction Excitation- Contraction cont’d • depolarization travels down the T-tubules to the sarcoplasmic reticulum • the impulse reaches the SR and calcium is released • calcium binds to troponin and causes binding of actin and myosin Excitation- Contraction (one more) • during strong binding, myosin head cocks • this action moves actin filament along myosin • Binding of ATP causes the weak binding (or release) again enabling another contraction Important Points • depolarization causes release of calcium by SR • calcium enables the strong binding state • ATP provides energy for cocking of myosin head, BUT • binding of ATP causes the weak binding state (or release) of actin and myosin A couple more important points • contraction can continue as long as calcium is available to enable strong binding AND • ATP is available for energy of cocking and release of strong binding • the signal to stop contraction is the loss of an impulse and uptake of calcium Muscle fatigue is characterized by a reduced ability to generate force Comparison of maximal shortening velocities between fiber types Type I vs Type II (velocity) • type II are fast twitch muscles – type IIa are sort of like slow twitch but faster • type I are slow twitch muscles • therefore IIb will have the fastest shortening velocity and type I will have the slowest Endurance exercise training induced changes in fiber type in skeletal muscle Isotonic vs. isometric contractions Isometric contraction • an isometric contraction is occurs when there is no change in muscle length when force is being produced • trying to push a car out of the snow • holding up a table so it can be leveled Isotonic Contraction • an isotonic contraction occurs when there is a change in muscle length • concentric when muscle shortens – bicep curl, lifting • eccentric when muscle lengthens – tug o war, negatives in weights, putting down a beer Recording of a simple twitch Relationship between stimulus strength and force of contraction Stimulus Strength vs Force of Contraction • Weak stimulus does not recruit many motor units • Stronger stimulus recruits more motor units • When all motor units are recruited, no more force can be applied regardless of stimulus strength Length-tension relationship in skeletal muscle Length Tension Relationship • There exists an optimal length of muscle at which it produces the greatest force – Typically between 100-120 % resting length • Maximal tensions at lengths longer or shorter than the optimal length will be less Progression of simple twitches, summation and tetanus Tetanus • If twitches become more frequent, greater force can be developed during summation than for a single twitch • If twitches become to frequent, tetanus will develop and the muscle will not relax • Typically results only from electrical stimulation Muscle force-velocity relationships Muscle power-velocity relationships