Knob stimulates ca channels to nerve open ca2 enter

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Unformatted text preview: n-front-of-another AP’s’ • In nervous tissue= nerve impulse or nerve signal In nerve • In muscle= moves along sarcolemma to produce contractions In to Electrically Excitable Cells Electrically Figures 11.8-11 4 phases of muscle contraction/relaxation: excitation, excitation, excitation-contraction coupling, contraction, relaxation excitation-contraction Excitation= process where AP in nerve leads to AP in process muscle muscle • Nerve signal arrives @ s.knob, stimulates Ca-channels to Nerve open, Ca2+ enter s.knob & stimulate release of ACh open, (exocytosis) into s.cleft (exocytosis) • Ach binds to receptors on sarcolemma, causes ‘ligandgated ion channel’ to open & Na+ enters/K+ exits cell • Results in end-plate potential (EPP) Results end-plate Behavior of Muscle Fibers Behavior Excitation (cont.)~ • Voltage-gated ion channels adjacent to end plate open (2nd) as a result of EPP (2 • AP is created due to separate Na & K-channels opening AP along sarcolemma along Excitation-contraction coupling= events linking AP’s to events activation of myofilaments activation • Wave of AP’s travel in all directions away from end-plate, Wave reaches T tubules & travels down them into sarcoplasm reaches • AP’s reach voltage-regulated ion gates in T tubules, AP’s voltage-regulated causing Na-gates to open causing • Ca-channels on terminal cisternae of SR (adjacent to & linked Ca-channels to T tubules) also open & Ca2+ diffuse out into cytosol to Behavior of Muscle Fibers (cont.) Behavior Excitation-contraction (cont.)~ • Ca2+ bind to troponin (on actin) causing troponin-tropomyisin complex to change shape & shift position complex • Active sites on actin filaments now exposed & ready to bind w/ Active myosin heads myosin Contraction= muscle fiber develops tension & may shorten • ‘Sliding filament theory’ discovered in 1954 • ATP bound to myosin head (to initiate process) is hydrolyzed ATP by myosin ATPase by • Released energy ‘cocks’ head to a ‘high-energy position’ • ADP & P remain on head @ this pt • Cocked head binds to actin filament, then ADP & P are Cocked released & head is ‘flexed’ to a ‘low-energy position’= power stroke stroke • Head remains bound to actin until a new ATP molecule is bound Behavior of Muscle Fibers (cont.) Behavior Contraction (cont.)~ • Once new ATP bound, head releases actin & ready to Once repeat repeat • Hydrolyze ATP, recock head (recovery stroke), attach to NEW ), active site farther down filament, & produce power stroke active • Sliding filament theory is similar to pulling an anchor out of Sliding the H2O, or a millipede walking the • Myosin heads ~5 power/recovery stokes per sec; 1 Myosin ATP/stroke ATP/stroke Relaxation= muscle fibers relax & return to resting length muscle fibers • Nerve signals stop arriving @ synaptic knob & so ACh no Nerve longer released longer • AChE breaks down Ach into fragments that are reabsorbed AChE by synaptic knob by Behavior of Whole Muscles (cont.) Behavior Relaxation (cont.)~ • Active transport pumps (uses ATP) in SR pump Ca2+ in cyto...
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