Bulk is contractile muscle fibers responsible for the

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bulk is contractile muscle fibers responsible for the heart’s pumping activity events leading to contraction:1.depolarization opens a few fast voltage-gated Na+ channels in sarcolemma- Na+ enters- this initiaties a positive feedback cycle that causes the rising phase of the AP (and reversal of the membrane potential from -90 mV to nearly +30 mV)a.quick because sodium channels inactivate and Na+ influx stops 2.transmission of the depolarization wave down the T tubules causes the sarcoplasmic reticulum to release Ca+ into the sarcoplasm3.excitation-contraction coupling occurs as Ca+ provides the signal (via troponin binding) for cross bridge activation and couples the depolarization wave to the sliding of the myofilaments 10-20% of Ca_ needed to trigger contraction enters cells from extracellular space- once inside it stimulates the SR to release th other 80% of Ca+ needed slow Ca2+ channels- when Na+ membranes depolarize, the voltage change also opens channels that allow Ca+ to enter from extracellular space- then this triggers opening of Ca_ sensitive channels in the SR tubules liberating bursts of Cacalcium surge across the sarcolemma prolongs the depolarization potential briefly, producing a plateauin the AP tracing of Na+ channels few K+ channels are open, which also prolongs the plateau and prevents rapid repolarization as long as Ca+ is entering, the cells continue to contract muscle tension develops during the plateau and peaks just after the plateau ends plateau makes AP last longer- provides longer contraction needed to eject blood from the heart repolarization results from inactivation of Ca+ channels and opening of voltage-ggated K+ channels, which allows a rapid loss of potassium from the cell that restores the resting membrane potential – Ca+ is pumped back into the SR and extracellular space Energy requirementsoGreater mitochondria reflects greater dependence on oxygen for its energy metabolism oAerobic respiration oFuel molecules-> glucose and fatty acid
Heart PhysiologyoElectrical EventsAbility of muscle to depoloarize and contract is intrinsic (doesn’t depend on the nervous system) Still supplied with autonomic nerve fibers oSetting the basic rhythm: the intrinsic conduction system Independent activity of the heart is a function of the presence of gap junctions, and the activity of the heart’s in hourse conduction system Intrinsic cardiac conduction system consists of noncontractile cardiac cells specialized to initiate and distribute impulses throughout the heart, so that it depolarizes and contracts in an orderly, sequential mannerCardiac pacemaker cells(autorhythmic cells) have an unstable resting potential that continuously depolarizes, drifting slowly toward thresholdSpontaneously changing membrane potentials- pacemaker potentials or prepotentials- initiate the AP1.

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