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 CyclePower 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 formedRelaxation 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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