Contrac7on Ch. 9 A- 2 Notes The genera7on of Does not necessarily cause shortening of the fiber Shortening occurs when tension generated by cross bridges on the thin filaments forces opposing shortening Sliding Filament Model of Contrac7on n the state, thin and thick filaments overlap only slightly During contrac7on, heads bind to ac7n, detach, and bind again, to propel the thin filaments toward the line As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle H A 1 Fully relaxed sarcomere of a muscle fiber A 2 Fully contracted sarcomere of a muscle fiber Figure 9.6 Requirements for Skeletal Muscle Contrac7on 1. : neural s7mula7on at a neuromuscular junc7on 2. Excita7on- contrac7on coupling: Genera7on and propaga7on of an ac7on poten7al along the sarcolemma Final trigger: a brief rise in intracellular levels Events at the Neuromuscular Junc7on muscles are s7mulated by soma7c motor neurons Axons s travel from the nervous system via nerves to skeletal muscles Each axon forms several as it enters a muscle Each axon ending forms a neuromuscular junc7on with a single muscle 1
Neuromuscular Junc7on 1 arrives at axon terminal. 2 Voltage-gated Ca 2 s open and Ca 2 enters the axon terminal. Action potential (AP) Nucleus Ca 2 Myelinated axon Ca 2 Fusing synaptic vesicles of neuromuscular junction of the muscle fiber vesicle containing Mitochondrion Situated along the length of a muscle fiber and muscle fiber are separated by a gel- filled space called the clex Synap7c vesicles of axon terminal contain the neurotransmiyer acetylcholine ( ) Junc7onal folds of the sarcolemma contain Figure 9.8 Events at the Neuromuscular Junc7on Nerve arrives at axon terminal is d and binds with receptors on the Electrical events lead to the genera7on of an 1 arrives at axon terminal. 2 Voltage-gated Ca 2 s open and Ca 2 enters the axon terminal. Ca 2 entry causes some synaptic vesicles to their contents (acetylcholine) by exocytosis. Acetylcholine, a neurotransmitter, diffuses across the synaptic and binds to receptors in the sarcolemma. 5 binding opens ion s that allow simultaneous passage of Na into the muscle fiber and K out of the muscle fiber. Myelinated axon Action potential (AP) of Nucleus neuromuscular junction of the muscle fiber Ca Ca 2 2 vesicle containing Mitochondrion Fusing synaptic vesicles Junctional folds of sarcolemma Sarcoplasm of muscle fiber Na Postsynaptic membrane K ion opens; ions pass. 6 effects are terminated by its enzymatic breakdown in the synaptic by acetylcholinesterase. Ach Degraded Na Acetylcholinesterase K Postsynaptic membrane ion closed; ions cannot pass. Figure 9.8 Destruc7on of Acetylcholine effects are quickly terminated by the enzyme acetylcholinesterase con7nued muscle fiber contrac7on in the absence of addi7onal s7mula7on 1. Local (end plate poten7al): binding opens chemically (ligand) gated ion s Simultaneous diffusion of Na (inward) and K (outward) More Na diffuses, so the interior of the sarcolemma becomes less nega7ve Local depolariza7on end plate poten7al 2
2. Genera7on and of an ac7on poten7al: End plate poten7al spreads to adjacent membrane areas Voltage- gated Na s open Na influx decreases the membrane voltage toward a cri7cal threshold f threshold is reached, an ac7on poten7al is generated Local depolariza7on wave con7nues to spread, changing the of the sarcolemma Voltage- regulated Na s open in the adjacent patch, causing it to to threshold. : Na s close and voltage- gated K s open K efflux rapidly restores the res7ng polarity Fiber cannot be s7mulated and is in a refractory period un7l repolariza7on is complete onic condi7ons of the res7ng state are restored by the Na - K pump Na K 1 Local depolarization: generation of the end plate potential on the sarcolemma Na K W a ve o f de po l ariza tio n Open Na Na Closed K K 2 Generation and propagation of the action potential (AP) Closed Na Open K Na K Repolarization Figure 9.9 Open Na Na Closed K Open Na Na Closed K Na K Na K W ave o f d epol ariza tio n 1 Local depolarization: generation of the end plate potential on the sarcolemma K Na K Na K W ave o f d epol ariza tio n 1 Local depolarization: generation of the end plate potential on the sarcolemma K 2 Generation and propagation of the action potential (AP) Figure 9.9, step 1 Figure 9.9, step 2
Closed Na Na Open K Open Na Na Closed K Repolarization K Na K Na K 1 Local depolarization: generation of the end plate potential on the sarcolemma W a ve o f de po l ariza tio n K 2 Generation and propagation of the action potential (AP) Closed Na Open K Na K Repolarization Figure 9.9, step Figure 9.9 Depolarization due to Na entry Na s open Na s close, K s open Repolarization due to K exit Threshold K s close Excita7on- Contrac7on (E- C) Coupling Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the period: Time when E- C coupling events occur Time between AP ini7a7on and the beginning of contrac7on Figure 9.10 Events of Excita7on- Contrac7on (E- C) Coupling AP is propagated along sarcomere to Voltage- sensi7ve proteins s7mulate Ca 2 from Ca 2 is necessary for contrac7on is generated Muscle fiber Setting the stage Ca 2 Triad One sarcomere Figure 9.11, step 1
Steps in E-C Coupling: Ca 2 Ca 2 1 is propagated along the sarcolemma and down the s. 2 Calcium ions are d. Steps in E-C Coupling: 1 is propagated along the sarcolemma and down the s. Ca 2 Troponin Tropomyosin blocking active sites Active sites exposed and ready for myosin binding Calcium binds to troponin and removes the blocking action of tropomyosin. Ca 2 Contraction begins cross bridge The aftermath Ca 2 Figure 9.11, step 2 Figure 9.11, step Steps in E-C Coupling: 1 is propagated along the sarcolemma and down the s. Ca 2 2 Calcium ions are d. Ca 2 cross bridge Troponin Tropomyosin blocking active sites Active sites exposed and ready for myosin binding Calcium binds to troponin and removes the blocking action of tropomyosin. Contraction begins Ca 2 The aftermath Figure 9.11, step Figure 9.11, step 7 Steps in E-C Coupling: 1 is propagated along the sarcolemma and down the s. Role of Calcium (Ca 2 ) in Contrac7on Ca 2 Ca 2 Ca 2 Troponin Tropomyosin blocking active sites 2 Calcium ions are d. At low intracellular Ca 2 concentra7on: blocks the ac7ve sites on ac7n heads cannot ayach to Muscle fiber Calcium binds to troponin and removes the blocking action of tropomyosin. Active sites exposed and ready for myosin binding Contraction begins cross bridge The aftermath Figure 9.11, step 8 5
Role of Calcium (Ca 2 ) in Contrac7on At higher intracellular concentra7ons: Ca 2 binds to troponin Troponin changes shape and moves tropomyosin away from ac7ve sites Events of the cross bridge cycle occur When nervous s7mula7on ceases, Ca 2 is pumped back into the and contrac7on Cross Bridge Cycle Con7nues as long as the signal and adequate are present Cross bridge forma7on high- energy myosin head to thin filament Working (power) stroke myosin head pivots and pulls thin filament M line Cross Bridge Cycle Ca 2 Thin filament Cross bridge detachment ayaches to myosin and the cross bridge detaches of the myosin head energy from hydrolysis of cocks the myosin head into the high- energy state P i cross bridge Thick filament 1 Cross bridge formation. P i P i hydrolysis Cocking of myosin head. 2 The power (working) stroke. Cross bridge detachment. Figure 9.12 Ca 2 Thin filament cross bridge Pi Thick filament Pi 1 Cross bridge formation. 2 The power (working) stroke. Figure 9.12, step 1 Figure 9.12, step 6
Pi hydrolysis Cross bridge detachment. Cocking of myosin head. Figure 9.12, step Figure 9.12, step 5 Thin filament Ca 2 P i cross bridge Thick filament 1 Cross bridge formation. P i hydrolysis P i Cocking of myosin head. 2 The power (working) stroke. Cross bridge detachment. Figure 9.12 7