BSC2085 Final Exam Study Guide – “You Should Be Able To” 1 Lesson 21: The Muscular System – Muscle Contraction Describe the events that occur at the neuromuscular junction during the generation of an action potential at the sarcolemma • Neuromuscular Junction (NMJ): Special intercellular connection between the nervous system & skeletal muscle fiber o Where the motor neuron interacts with the muscle fiber 1. Cytoplasm of motor neuron contains vesicles filled with acetylcholine a. Synaptic cleft & motor endplate contain acetylcholinesterase (AChE) – an enzyme that breaks down ACh 2. Arrival of an action potential at the synaptic terminal triggers the exocytosis of acetylcholine into the synaptic cleft a. Vesicles filled with ACh fuse with the motor neuron’s plasma membrane & spill into the synaptic cleft 3. Acetylcholine molecules diffuse across the synaptic cleft & bind to ACh receptors on the surface of the motor end plate a. ACh binding causes sodium (Na+) ions to rush into the sarcoplasm 4. Sudden influx of sodium ions results in the generation of an action potential in the sarcolemma a. AChE quickly breaks down ACh on the motor end plate & in the synaptic cleft b. Action potential generated at the motor end plate sweeps across the entire membrane (sarcolemma) surface Describe the steps to excitation-contraction coupling. What is the role of Ca2+? What is the role of ATP? • Excitation-contraction coupling = the link between the generation of an action potential in the sarcolemma & the start of a muscle contraction o Action potential reaches triad & triggers the release of Ca2+ from the terminal cisternae of the sarcoplasmic reticulum o Ca2+ concentration in & around the sarcomere increases o Contraction cycle can begin • Role of Ca2+ = uncover the active site on actin • Role of ATP = get myosin into a “primed” (high energy) position Explain the six steps to the contraction cycle 1. Ca2+ arrives within the zone of overlap (due to excitation-contraction coupling) 2. Active site exposure • Ca2+ ions bind to troponin i. Weakens the bond between actin & the troponin-tropomyosin complex • Troponin removes tropomyosin from the active sites on actinBSC2085 Final Exam Study Guide – “You Should Be Able To” 2 • This allows interaction between actin’s active sites & the energized myosin heads 3. Cross-bridge formation • Energized myosin heads bind to active sites on actin, forming cross-bridges 4. Myosin head pivoting • Energy released as myosin head pivots toward M line i. Power stroke • ADP & phosphate group are released 5. Cross-bridge detachment • Another ATP binds to myosin head • Link between myosin & actin broken • Active site exposed & able to form another cross-bridge 6. Myosin reactivation • Free myosin head splits (hydrolyzes) ATP into ADP & P • Energy released puts myosin head back into the high energy position • Notes: o ATP binding site exists on myosin o Multiple myosin heads interact with the thin filament at a time o As long as calcium is present for troponin to remove tropomyosin, muscles can contract Describe what happens during muscle relaxation Relaxation occurs when… 1. Calcium concentrations fall 2. Calcium ions detach from troponin 3. Troponin returns to its original position 4. The active sites on actin are re-covered by tropomyosin Explain what is rigor mortis. What causes it? Why does it end? o Rigor Mortis: A fixed muscular contraction after death Begins 2-7 hours after death • Skeletal muscle fibers run out of ATP • Ion pumps cease to function • Calcium builds up in the sarcoplasm – triggers a sustained contraction Ends 1-6 days after death • Lysosomal enzymes released by autolysis break down the Z lines & titin filaments Explain what is the optimal length for the maximum amount of tension generation in a muscle o Sarcomere works most efficiently within an optimal range of lengths With optimum overlap between thick & thin fibers…BSC2085 Final Exam Study Guide – “You Should Be Able To” 3 • Maximum number of cross bridges can form • Highest tension is produced Too little overlap or too much overlap reduce efficiency o Maximum tension is produced when the zone is overlap is large, but the thin filaments to do not extend across the sarcomere’s center o Optimal resting length = 75 – 130% of the optimal length Explain a muscle twitch. What causes it? How long does it last? What are the characteristics of a muscle twitch? What happens during the latent period? o Twitch: A single stimulus-contraction-relaxation sequence in a muscle fiber Stimulation of the muscle produces a single twitch Twitches vary in duration • Depends on: type of muscle, its location, internal & external environmental conditions, and other factors • Typically 7 – 100 milliseconds o Characteristics of Muscle Twitches: 1. Latent Period • Begins at stimulation • The action potential moves through the sarcolemma • Causes Ca2+ release from sarcoplasmic reticulum • No tension produced 2. Contraction Phase • Calcium ions bind to troponin • Active sites on actin exposed • Cross-bridge interactions occur • Tension builds to peak 3. Relaxation Phase • Ca2+ levels fall • Active sites re-covered by tropomyosin • Number of active cross-bridges declining (detaching) • Tension falls to resting levels Describe the difference between treppe and wave summation o Treppe: A stair-step increase in twitch tension o Wave Summation: Increasing tension or summation of twitches Treppe is caused by repeated stimulations immediately AFTER the relaxation phase Wave summation is caused by repeated stimulations BEFORE the end of the relaxation phase (muscle is stimulated again before the muscle has the chance to relax) Explain the difference between incomplete and complete tetanusBSC2085 Final Exam Study Guide – “You Should Be Able To” 4 In incomplete tetanus, a muscle is producing almost peak tension during rapid cycles of contraction and relaxation In complete tetanus, the muscle is producing maximum tension and the relaxation period is eliminated Describe a motor unit and explain why motor unit recruitment can increase muscle tension o Motor Unit: A single motor neuron and all the muscle fibers it controls Motor
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