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York University
BIOL 4510
Peter Backxx

BIOL4510KINE4510 ExcitationContraction CouplingThe purpose of electrical depolarization ie action potentials is to initiate contraction This process is called excitationcontraction coupling EC couplingThe electrical changes in the 2muscle cell myocyte induce an increase in intracellular Ca leading to the development of 2contractionMuscle relaxation ensues following a decrease in intracellular CaThere are significant differences in EC coupling in skeletal cardiac and smooth muscleTop figure showing action potentials APs in Cardiac Muscle vs Skeletal Muscle In sk muscle APs are very short 35msecs and a single AP generates a twitch force lasting 50100 msecs In cardiac muscle APs are much longer 200msecsand a single AP generates a twitch force lasting 300 msecs Bottom figure shows that APs in sk muscle have a rapid recovery time short refractory time and therefore sk muscle can be rapidly and repetitively stimulated which causes fusion of twitches and the generation of tetanus In fact sk muscle contraction usually involves tetanization of individual motor units Cardiac muscle has long AP relative to forcepressure generationTherefore tetanization is not possibleCardiac muscle cannot be stimulated faster than the refractory period of the APNoterefractory periodAP duration since electrical and thus mechanical stimulation of muscle can only occur if most Na channels are available to initiate and propagate APs Therefore refractory period controlled by rate of recovery of Na channels from inactivation which requires membrane repolarizationThus long APs in cardiac muscle prevent tetanization of cardiac muscleWhy is this a critical feature of heart1 EC Coupling in the Heart AP generation occurs in order to induce muscle contractionThe process of membrane depolarization leading to muscle contraction is called excitationcontraction couplingECCthe events underlying the mechanical response muscle contraction to an electrical 2depolarization action potential In the heart ECC is dependent on a rise in Ca which catalyzes the conversion of ATP into force and mechanical work by myosin and the contractile 2proteins The Ca originates from both intracellular sarcoplasmic reticular and extracellular 2CaFigure 2shows the absence of contraction when extracellular Ca is removedNote Sk Muscle will 2still contract in the absence of extracellular Ca 2It has been calculated that there is an increase of 90120 M total Ca in the cytoplasm with 22each cardiac contraction The rise in free Ca which is about 1M The free Ca rise is the 22tipoftheiceberg The low free Ca is the result of buffering 1001 of Ca by intracellular iproteins The major buffers being troponinC 150M and calmodulin 20Mas well as NCX SERCA2a 2
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