Muscular System Muscle Contraction 12 06 2013 Sliding Filaments and Muscle Contraction Muscle Fiber Contraction o 1 H bands and I bands of the sarcomere get smaller o 2 Zones of Overlap get larger o 3 Z lines move closer together o 4 Width of the A band remains constant o This only makes sense if the thin filaments are sliding toward the center of each sarcomere next to the thick filaments Sliding Filament Theory o Basically the interaction between actin and myosin o Events Thin filaments of sarcomere slide toward M line Alongside thick filaments The width of A band STAYS THE SAME The width of the H band and I band shrink during contraction Z lines more closer together o The contraction weakens with the disappearance of I bands Then the Z lines are in contact with thick filament ends o During a contraction sliding occurs in every sarcomere along the myofibril Result Myofibril shortens muscle fiber also shortens Since myofibrils are attached to sarcolemma at each Z line and at either end of the muscle fiber Tips and Tricks other hand o Hold hands in front with palms toward body and thumbs straight up o Move hands together so fingers of one hand move in between fingers of the o Fingers thin and thick filaments o Thumbs Z lines o Finger length stays the same o Thumbs move closer together Skeletal Muscle Contraction o Steps in Muscle Contraction 1 Neural Stimulation of Sarcolemma Causes excitation contraction coupling 2 Muscle Fiber Contraction Interaction of thick and thin filaments 3 Produces muscle tension Motor neuron generates action potential in muscle cell travels along sarcolemma T tubule triad calcium channels open calcium moves out binds to troponin active side exposed actin and myosin can interact o Muscle cells contract pull on attached tendon fibers tension Tension An active force The pull on attached tendon fibers Energy must be expended to produce it Applied to some object pulls object toward source of tension Before movement can occur applied tension must overcome the object s load resistance Load A passive force that opposes movement o Amount of load depends on weight of object shape friction and other factors Applied Tension exceeds load Movement Compression A push applied to an object that forces object away from source of compression No movement can occur until compression exceeds load Muscle cells use energy to shorten and generate tension through thick thin filament interaction Not to lengthen and generate compression Muscles can pull NOT push o Overview of Skeletal Muscle Contraction Neural Control Contraction only occurs when skeletal muscle fibers are activated by neurons in the CNS Contracts when stimulated by motor neuron at neuromuscular junction Stimulus action potential in synaptic terminal A neuron can activate a muscle fiber by stimulating its sarcolemma Results in Excitation Contraction Coupling Excitation Contraction Coupling Calcium Release 1st Step in Excitation Contraction Coupling Release of calcium ions from the cisternae of the SR Steps o 1 Action Potential releases ACh into synaptic cleft ACh diffuses to motor end plate binds to receptors opens Na Channels action potential in sarcolemma o 2 Action Potential in Sarcolemma travels through T Tubules reaches Triads releases Ca2 ions from terminal cisternae of SR This begins the contraction cycle o Continues until ATP runs out or action potentials stop being produced at motor end plate Thick Thin Filament Interaction Calcium ions trigger interactions between thick and thin filaments Interaction Sarcomeres Shorten Muscle fiber ends get closer together Muscle Fiber Contraction Interaction muscle fiber contraction and consumption of energy ATP Entire skeletal muscle shortens produces a pull tension on the tendons at either end Tension Production Filament interactions produce active tension 10 3 Structural Components of a Sarcomere Skeletal Muscle Contraction o The process of contraction Neural stimulation of sarcolemma Cases excitation contraction coupling Muscle fiber contraction Caused by thick and thin filament interactions Tension production o Action potential that travels along the sarcolemma and down the T Tubule causes the release of Ca2 which allows for actin myosin interaction 10 4 Components of the Neuromuscular Junction The nervous system communicates with skeletal muscles at the neuromuscular junction The Control of Skeletal Muscle Activity Contraction begins with the release of internal stores of calcium ions o Under the control of the nervous system Neuromuscular junction NMJ o Where communication between the nervous system and a skeletal muscle o Special intercellular connection between nervous system and skeletal muscle fiber occurs fiber Action potential reaches axon terminal of motor neuron release of ACh into synaptic cleft ACh binds chemically gated Na channels on muscle opens Na channels Na influx to depolarize the muscle Action potentials generated in muscle fiber along inner surface of the fiber sarcolemma o A single axon can branch out and control multiple skeletal muscle fibers But each fiber only has ONE neuromuscular junction NMJ o Steps of Skeletal Muscle Innervation 1 Synaptic terminal cytoplasm contains vesicles filled with Ach The synaptic cleft and motor end plate contain AChE 2 Arrival of an action potential at the synaptic terminal Sudden change in the transmembrane potential 3 Action Potential triggers Exocytosis of ACh into synaptic cleft Exocytosis occurs as the vesicles fuse the neuron s plasma membrane 4 ACh diffuse and bind to ACh receptors on the motor end plate Binding of ACh Opening of chemically gated Na channels Sodium rushes into cell s sarcoplasm because it has a lower Na concentration than the extracellular fluid 5 Sudden inrush of Na causes an action potential in the sarcolemma AChE breaks down ACh on motor end plate and in cleft Inactivates ACh receptor sites o This is why effects are brief o Prevents overstimulation of muscle by motor neuron No further stimulus acts upon the motor end plate until another action potential arrives at the synaptic terminal Excitation Contraction Coupling Excitation contraction coupling o The link between the generation of an action potential in the sarcolemma and the start of a muscle contraction o Occurs at the Triads o Action potential reaches a triad t tubule and terminal cisternae of the sarcoplasmic reticulum o Releases Ca2 from terminal cisternae of SR First step of excitation contraction coupling o Triggers
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