BSC2085 Final Exam Study Guide You Should Be Able To 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 spill into the synaptic cleft a Vesicles filled with ACh fuse with the motor neuron s plasma membrane 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 cleft a AChE quickly breaks down ACh on the motor end plate in the synaptic 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 actin 1 BSC2085 Final Exam Study Guide You Should Be Able To This allows interaction between actin s active sites the energized myosin heads 3 Cross bridge formation 4 Myosin head pivoting Energized myosin heads bind to active sites on actin forming cross bridges 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 Calcium builds up in the sarcoplasm triggers a sustained Ion pumps cease to function 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 2 BSC2085 Final Exam Study Guide You Should Be Able To 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 tetanus 3 BSC2085 Final Exam Study Guide You Should Be Able To 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 unit recruitment increases muscle tension by increasing the size of the motor units and or the number of motor units stimulated as the movement progresses This causes a smooth steady increase in muscular tension During a sustained contraction motor units are activated on a rotating basis This allows some of them to rest recover while others are actively contracting asynchronous motor unit summation Explain the difference between isotonic and isometric contraction In an isotonic contraction tension rises the skeletal muscle s length
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