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BSC2085  Final Exam 12/7/13 - 10-4: The Nervous System Communicates With Skeletal Muscles at The Neuromuscular Junctiono Communication between the nervous system and a skeletal muscle fiber occur at this specialized intercellular connection: Neuromuscular Junction (myoneural junction) Here, a signal releases Ca2+ (begins the contraction)o The link between the generation of an action potential in the sarcolemma and the start of a muscle contraction is called Excitation-Contraction Coupling. occurs at triads action potential triggers release of Ca2+ from cisternae of SR SR changes permeability for 0.03 sec in and around sarcomere reaches 100x resting level Ca2+ concentration Cisternae are located at zone of overlap, thus effect is instantaneous Troponin (the lock) attached to the active sites is moved by Calcium (the key) Happens when Ca2+ binds to Troponin, which is bound to Tropomyosin strand, the whole strand and attached troponin molecule roll away from active site After this the Contraction Cycle beginso Contraction CyclePower Stroke: molecular interactions of muscle contractions each shorten sarcomere by 0.5%, and the entire muscle shortens at the same rate the Cycling Rate (number of power strokes per second) determines speed of shortening  Contraction Cycle is like tug of war, at any given time, some people are grabbing and pulling.  The amount of tension produced depends on how many (myosin heads of thick filaments) are pulling at (thin filaments) at the same timeo Skeletal Muscle Innervation Broken Down (Figure 10-11) ACh is released at synaptic terminal of neuron Changes permeability of adjacent cell to Sodium Ions and sodium rushes into sarcoplasm (this is the transmission of an action potential or electrical impulse) Effect doesn’t last too long, AChE breaks remaining ACh down in synaptic cleft Sudden rush of Na+ generates action potential in the sarcolemma. o THEN: Contraction cycle begins arrival of calcium ions at zone of overlap Active Site Exposure calcium binds to troponin weakens bond between actin and troponin-tropomyosin complex Troponin molecule changes position, revealing active sites Cross Bridge Formation Energized myosin heads bind to active sites Myosin Head Pivoting Energy in myosin head is released (releases ADP and phosphate) and they pivot towards M line: Power stroke Cross Bridge Detachment When another ATP binds to myosin head, the link to active site is broken Myosin Reactivation  happens when free myosin head splits ATP into ADP and P, this energy relocks myosin head with active siteo Why don’t both heads of the myofibril move towards the middle? One side is usually fixed, the origin the insertion is the side that usually moveso Relaxation  The length of a contraction depends on: Period of stimulation at neuromuscular junction Presence of free calcium ions in sarcoplasm Availability of ATP Contractions will only continue if additional action potentials arrive at synaptic terminal in rapid succession (under these conditions the contraction cycle will repeat over and over) Mechanisms responsible for returning cell to resting level (ending stimulus’ affect) Active Ca2+ transport across sarcolemma Active Ca2+ transport into SR (far more important, reabsorptionhappens very quickly) Then: Ca ions detach from troponin active sites on action are covered again by tropomyosin strand Sarcomeres shorten (return to original length) due to gravity and elasticity of fibers, NOT any active mechanism - 10-5: Sarcomere shortening and muscle fiber stimulationo When sarcomeres shorten during a contraction, muscle fibers within shorteno Tension produced by individual fiber can varyo Tension produced by fiber depends on: the number of pivoting cross-bridges NOT the number of sarcomeres contracting since Ca2+ are released from all triads Therefore, the fiber is either ‘On’ - producing tension or ‘Off’ - relaxedo It also depends on The fiber’s resting length at the time of stimulation which determines the degree of overlap And the frequency of stimulation  which affects the internal concentration of Ca2+ o Length-Tension Relationship: the number of crossbridges depends on the degree of overlap Only myosin heads in the zone of overlap can bind to active sites This is how tension relates to structure of individual sarcomere Optimal Range of Length  compressed and shortened or stretched and lengthened- it cannot produce as much tension  At Shortest Length thick filaments jam against Z lines cross-bridges can form BUT myosin heads cannot pivot Length Tension prevents skeletal muscles from extreme compression or stretchingo Frequency of Stimulation -  A single simulation producing a single contraction is a twitch so to increase duration, we repeat stimulation Breakdown of a single twitch:Latent Period: begins at stimulation, lasts 2 msec No tension yet! But, action potential sweeps across sarcolemma  as SR releases Ca2+Contraction %Phase: %lasts 15 msec Tension, rises to a peak! Ca2+ binds to troponin active sites are exposed cross bridges formedRelaxation Phase: lasts 25 msec Ca2+ levels fall sites are covered by tropomyosino Treppe (staircase) Immediately after relaxation phase ends, a second stimulus is sent Develops higher tension than first stimulus This increase lasts 30-50 stimulations until the tension remains constant Rise is caused by: increase in Ca2+ concentration pump in SR has no time to intake Most Skeletal Muscles DON’T undergo Treppeo Wave Summation (summation of twitches) Second stimulus arrives BEFORE relaxation phase ends Creating more powerful second contraction  Determined by: the duration of a single twitch if 20 msec (1/50 sec) Then, the number of twitches per sec > 50 This rate is called Stimulus Frequencyo Incomplete Tetanus (Tetanos: convulsive tension) A muscle producing almost peak tension during rapid cycles of contraction AND relaxation Stimulation continues so muscle is never allowed to relax Reaching Peak Value of 4x maximum produced in treppeo Complete Tetanus Higher frequency eliminates relaxation phase SR has no time to reabsorb Ca2+ This prolongs the contraction making it continuous o Tension Production by Skeletal


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