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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Destruction of Acetylcholine  ACh bound to ACh receptors is quickly destroyed.

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Presentation on theme: "Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Destruction of Acetylcholine  ACh bound to ACh receptors is quickly destroyed."— Presentation transcript:

1 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Destruction of Acetylcholine  ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase  This destruction prevents continued muscle fiber contraction in the absence of additional stimuli  Events at the neuromuscular junction Events at the neuromuscular junction  Generation of an action potential

2 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Excitation-Contraction Coupling  Excitation-Contraction Coupling Excitation-Contraction Coupling

3 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.10 ADP PiPi Net entry of Na + Initiates an action potential which is propagated along the sarcolemma and down the T tubules. T tubule Sarcolemma SR tubules (cut) Synaptic cleft Synaptic vesicle Axon terminal ACh Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma. Action potential in T tubule activates voltage-sensitive receptors, which in turn trigger Ca 2+ release from terminal cisternae of SR into cytosol. Calcium ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed. Contraction; myosin heads alternately attach to actin and detach, pulling the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers the cycling process. Removal of Ca 2+ by active transport into the SR after the action potential ends. SR Tropomyosin blockage restored, blocking myosin binding sites on actin; contraction ends and muscle fiber relaxes. Ca

4 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.10 Synaptic cleft Synaptic vesicle Axon terminal ACh Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma.

5 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.10 Net entry of Na + Initiates an action potential which is propagated along the sarcolemma and down the T tubules. T tubule Sarcolemma Synaptic cleft Synaptic vesicle Axon terminal ACh Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma. 1

6 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.10 Net entry of Na + Initiates an action potential which is propagated along the sarcolemma and down the T tubules. T tubule Sarcolemma SR tubules (cut) Synaptic cleft Synaptic vesicle Axon terminal ACh Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma. Action potential in T tubule activates voltage-sensitive receptors, which in turn trigger Ca 2+ release from terminal cisternae of SR into cytosol. SR Ca

7 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.10 Net entry of Na + Initiates an action potential which is propagated along the sarcolemma and down the T tubules. T tubule Sarcolemma SR tubules (cut) Synaptic cleft Synaptic vesicle Axon terminal ACh Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma. Action potential in T tubule activates voltage-sensitive receptors, which in turn trigger Ca 2+ release from terminal cisternae of SR into cytosol. Calcium ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed. SR Ca

8 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.10 Net entry of Na + Initiates an action potential which is propagated along the sarcolemma and down the T tubules. T tubule Sarcolemma SR tubules (cut) Synaptic cleft Synaptic vesicle Axon terminal ACh Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma. Action potential in T tubule activates voltage-sensitive receptors, which in turn trigger Ca 2+ release from terminal cisternae of SR into cytosol. Calcium ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed. Contraction; myosin heads alternately attach to actin and detach, pulling the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers the cycling process. SR Ca

9 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.10 Net entry of Na + Initiates an action potential which is propagated along the sarcolemma and down the T tubules. T tubule Sarcolemma SR tubules (cut) Synaptic cleft Synaptic vesicle Axon terminal ACh Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma. Action potential in T tubule activates voltage-sensitive receptors, which in turn trigger Ca 2+ release from terminal cisternae of SR into cytosol. Calcium ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed. Contraction; myosin heads alternately attach to actin and detach, pulling the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers the cycling process. Removal of Ca 2+ by active transport into the SR after the action potential ends. SR Ca

10 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.10 ADP PiPi Net entry of Na + Initiates an action potential which is propagated along the sarcolemma and down the T tubules. T tubule Sarcolemma SR tubules (cut) Synaptic cleft Synaptic vesicle Axon terminal ACh Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma. Action potential in T tubule activates voltage-sensitive receptors, which in turn trigger Ca 2+ release from terminal cisternae of SR into cytosol. Calcium ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed. Contraction; myosin heads alternately attach to actin and detach, pulling the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers the cycling process. Removal of Ca 2+ by active transport into the SR after the action potential ends. SR Tropomyosin blockage restored, blocking myosin binding sites on actin; contraction ends and muscle fiber relaxes. Ca

11 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Sequential Events of Contraction  Cross bridge formation – myosin cross bridge attaches to actin filament Cross bridge formation  Working (power) stroke – myosin head pivots and pulls actin filament toward M line  Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches  “Cocking” of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high-energy state PLAY InterActive Physiology ®: Sliding Filament Theory, pages 3-29

12 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.12 ATP ADP ATP hydrolysis ADP ATP PiPi PiPi Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. Thin filament As ATP is split into ADP and P i, the myosin head is energized (cocked into the high-energy conformation). Inorganic phosphate (P i ) generated in the previous contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released. Myosin head (low-energy configuration) As new ATP attaches to the myosin head, the link between myosin and actin weakens, and the cross bridge detaches. Thick filament

13 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.12 ADP PiPi Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. 1

14 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.12 ADP PiPi Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. Inorganic phosphate (P i ) generated in the previous contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released. 1 2

15 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.12 ADP ATP PiPi Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. 1 2 Inorganic phosphate (P i ) generated in the previous contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released.

16 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.12 ATP ADP ATP PiPi Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. Myosin head (low-energy configuration) As new ATP attaches to the myosin head, the link between myosin and actin weakens, and the cross bridge detaches Inorganic phosphate (P i ) generated in the previous contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released.

17 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.12 ATP ADP ATP hydrolysis ADP ATP PiPi PiPi Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. Thin filament As ATP is split into ADP and P i, the myosin head is energized (cocked into the high-energy conformation). Inorganic phosphate (P i ) generated in the previous contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released. Myosin head (low-energy configuration) As new ATP attaches to the myosin head, the link between myosin and actin weakens, and the cross bridge detaches. Thick filament

18 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Figure 9.12 ATP ADP ATP hydrolysis ADP ATP PiPi PiPi Myosin head (high-energy configuration) Myosin head attaches to the actin myofilament, forming a cross bridge. Thin filament As ATP is split into ADP and P i, the myosin head is energized (cocked into the high-energy conformation). Inorganic phosphate (P i ) generated in the previous contraction cycle is released, initiating the power (working) stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line. Then ADP is released. Myosin head (low-energy configuration) As new ATP attaches to the myosin head, the link between myosin and actin weakens, and the cross bridge detaches. Thick filament

19 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Contraction of Skeletal Muscle Fibers  Contraction – refers to the activation of myosin’s cross bridges (force-generating sites)  Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening  Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced

20 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Overview – Contraction of a Skeletal Muscle  Principles of muscle mechanics:  Force exerted by a contracting muscle on an object is muscle tension.  Opposing force on the muscle by the weight of the object is called the load.  A contracting muscle does not always shorten and move the load.  Isometric vs. Isotonic  A muscle contracts with varying force and for different periods of time in response to stimuli

21 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings The Motor Unit  Each muscle is served by at least one motor nerve  Contains axons of up to hundreds of motor neurons.

22 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings The Motor Unit  A motor unit = motor neuron and all the muscle fibers it supplies.motor unit  Motor neuron fires - all of the fibers it innervates contract  May have as many as several hundred or as few as four  Fine control vs. gross (large) movements  Muscle fibers are spread out within a unit  Stimulation of a single motor unit causes a weak contraction of the entire muscle.

23 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings The Muscle Twitch  Muscle twitch = the response of the motor unit to a single action potential  The muscle fibers contract quickly and then relax 1.Latent: excitation- contraction coupling is occurring 2.Contraction: cross bridges are active 3.Relaxation: initiated by reentry of Ca 2+ into the SR

24 Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Developmental Aspects  All muscle tissues develop from embryonic mesoderm cells called myoblasts  Skeletal muscle myoblasts fuse (multiple nuclei)  Cardiac and Smooth develop gap junctions  Smooth muscle has good ability to regenerate throughout life  Muscle mass differs between sexes due to testosterone  Muscle is highly vascularized – resistant to infection


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