Lesson 21 BSC2085 Final Study Guide Review 1 Describe the events that occur at the neuromuscular junction during the generation of an action potential at the sarcolemma a The neuromuscular junction NMJ i Special intercellular connection between the nervous system and skeletal muscle fiber ii Action potential reaches the axon terminal of motor neuron causes release of ACh into synaptic cleft iii ACh binds chemically gated Na channels on muscle opening them Na influx to depolarize the muscle fiber iv Action potentials generated in muscle fiber along inner surface of the sarcolemma 2 Describe the steps to excitation contraction coupling What is the role of Ca2 What is the role of ATP a Action potential reaches a triad T tubule and terminal cisternae of the sarcoplasmic reticulum i Releasing Ca2 from the terminal cisternae of the sarcoplasmic reticulum first step to excitation contraction coupling ii Triggering contraction IF 1 The myosin heads are in primed high energy position 2 Used ATP energy to get into this position ATP hydrolyzed to ADP Pi on myosin head 3 Explain the six steps to the contraction cycle a 1 Contraction Cycle Begins i Begins with arrival of calcium ion within the zone of overlap b 2 Active Site Exposure i Ca ions bind to troponin weaking the bond between actin and troponin tropomyosin complex ii The troponin molecule then changes position rolling away from the active sites on actin and allowing interaction with energized myosin eads c 3 Cross Bridge Formation i Once the active sites are exposed the energized myosin heads bind to them forming cross bridges d 4 Myosin Head Pivoting i After cross bridge formation the energy that was stored in the resting state is released as the myosin head pivots toward the M line ii This action is called the power stroke 1 when it occurs the bound ADP and phosphate group are released e 5 Cross Bridge Detachment i When another ATP binds to the myosin head the link between the myosin head and the active site on the actin molecule is broken The active site is now exposed and able to form another cross bridge i Myosin reactivation occurs when the free myosin head splits f 6 Myosin Reactivation hydrolyzes ii ATP into ADP and P The energy released is used to recock the myosin head into the high energy position 4 Describe what happens during muscle relaxation a Contraction duration i Depends on 1 Duration of neural stimulus how much ACh is released by the motor neuron 2 Number of free calcium ions in sarcoplasm 3 Availability of ATP b Relaxation occurs when i Ca2 concentrations fall causing 1 Ca2 detaches from troponin 2 Active sites are re covered by tropomyosin c 5 Explain what is rigor mortis What causes it Why does it end a A fixed muscular contraction after death 2 7 hrs after death b Caused when Ion pumps cease to function ran out of ATP i ii Calcium builds up in the sarcoplasm c What causes the end of rigor mortis 1 6 days after death i Because proteins make up actin and myosin and over time the will start to break down Relaxation occurs passive return to resting length 6 Explain what is the optimal length for the maximum amount of tension generation in a muscle a As a whole a muscle fiber is either contracted or relaxed b Depends on i The number of pivoting cross bridges ii The fiber s resting length at the time of stimulation iii The frequency of stimulation c Length Tension Relationships i Number of pivoting cross bridges depends on 1 Amount of overlap between thick and thin fibers ii Optimum overlap produces greatest amount of tension 1 Too much or too little reduces efficiency why iii Normal resting sarcomere length 1 Is 75 to 130 of optimal length d The Frequency of Stimulation i A single neural stimulation produces 1 A single contraction or twitch 2 Which lasts about 7 100 msec ii Sustained muscular contractions require many repeated 7 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 stimuli a 1 Latent period i The action potential moves through sarcolemma ii Causing Ca2 release from b 2 Contraction phase i Calcium ions bind to ii Tension builds to peak c 3 Relaxation phase i Ca2 levels fall ii Active sites are covered and tension falls to resting levels 8 Describe the difference between treppe and wave summation a Treppe phase i A stair step increase in twitch tension ii Caused by repeated stimulations immediately after relaxation iii Stimulus frequency 50 second iv Causes a series of contractions with increasing tension Increase in tension caused by gradual increase in Ca2 v concentration in sarcoplasm Ca2 pumps not fast enough to pump released Ca2 back into SR b Wave summation Increasing tension or summation of twitches i ii Repeated stimulations before the end of relaxation phase iii Stimulus frequency 50 second iv Causes increasing tension or summation of twitches Wave summation Wave summation occurs when successive stimuli arrive before the relaxation phase has been completed 9 Explain the difference between incomplete and complete tetanus a Incomplete tetanus i If rapid stimulation continues and muscle is not allowed to relax twitches reach maximum level of tension b Complete tetanus i Twitches reach maximum tension ii If stimulation frequency is high enough muscle never begins to relax and is in continuous contraction c Note disease tetanus lockjaw caused by toxin from Clostridium tetani bacterium that causes over activity of skeletal muscle motor neurons Results in muscle stiffness headaches difficulty swallowing Incomplete tetanus Incomplete tetanus occurs if the stimulus frequency increases further Tension production rises to a peak and the periods of relaxation are very brief 10 Describe a motor unit and explain why motor unit recruitment can increase muscle tension a Motor units in a skeletal muscle i Contain hundreds of muscle fibers ii Contract at the same time iii Controlled by a single motor neuron b Recruitment multiple motor unit summation i In a whole muscle or group of muscles smooth motion and increasing tension are produced by slowly increasing the size or number of motor units stimulated c Maximum tension i Achieved when all motor units reach tetanus ii Can be sustained only a very short time 11 Explain the difference between isotonic and isometric contraction a Isotonic Contraction i Skeletal muscle changes length resulting in motion ii If muscle tension load resistance 1 Muscle shortens concentric
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