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Cardiac muscle Cardiovascular system Gap junctions SR • Sources of calcium 1. Extracellular 2. Intracellular – sarcoplasmic reticulum Ryanodine Receptor T-tubule My Dihydropyridine receptor SR myoplasm 1 Ca++ Ca++ Ca++ Ca++ SR Ca++ pump Depolarization of plasma membrane Voltage sensitive Ca++ channels open (dihydropyridine receptors) Myoplasm (intracellular) Ca++ Ca++ Ca++ Ca++ interacts with troponin & causes contraction ↑ Cytosolic Ca++ Ca++ Opens Ca++ channels of sarcoplasmice reticulum (ryanodine receptors) Opened by depolarization Ca++ Things to understand: 1. Electrical activity of each muscle cell 2. Coordination of activity across the heart Cell Types 1. Contractile a) Ventricular b) Atrial ↑ Cytosolic Ca++ T-tubule (extracellular) Electrical activity of the heart Ca++ activates receptors on sarcoplasmic reticulum Muscle contraction Electrical activity of the heart The heart’s pacemaker and conducting system are shown in bright yellow. 2. Pacemaker 2 Ionic basis of ventricular myocyte action potential Action potential of ventricular myocyte Early repolarization Plateau depolarization repolarization • • • rest • The rapid opening of voltagegated sodium channels is responsible for the rapid depolarization phase. Rest - membrane potential due to K+ efflux Depolarization – Na+ influx via voltagegated Na+ channels Plateau – balance of Ca++ influx and K+ efflux through voltage-gated ion channels Repolarization – more K+ efflux through voltage-gated ion channel The prolonged “plateau” of depolarization is due to the slow but prolonged opening of voltage-gated calcium channels PLUS reduced potassium channel permeability 3 Channel events during ventricular myocyte This is the L-type action potential Ca++ Na+ voltage-gated Ca++ channel Opening of potassium channels results in the repolarization phase. Na+ K+ Ca++ K+ Please note this sequence is a little simplified, as there are at least 3 different K+ channels that contribute Ventricular Electrical activity of the heart Atrial The heart’s pacemaker and conducting system are shown in bright yellow. 4 Pacemaker Cells depolarization repolarization Pacemaker potential Ionic basis of automaticity 1. Pacemaker potential: a) Na+ channel that opens with negative potential b) Brief Ca++ channel opening (T-type) Depolarization of the pacemaker cells is: • Automatic • Rhythmic • Sinoatrial node is the natural pacemaker of the heart • Pacemaker cells do not have a steady resting potential, rather it gradually depolarizes. The action potential of an autorhythmic cardiac cell. Na+ in through hyperpolarization activated channels PLUS calcium ions moving in through the T channels cause a threshold graded depolarization. 2. Depolarziation by voltage-gated Ca++ channel opening (L-type) 3. Repolarization by voltage-gated K+ channels 5 Reopening of potassium channels PLUS closing of calcium channels are responsible for the repolarization phase. The rapid opening of voltage-gated calcium channels is responsible for the rapid depolarization phase. Repolarizing the membrane leads to the next opening of the Na+ channels Coordination of contraction in the whole heart Faster depolarization = fast HR Slower depolarization = slower HR The rate of the pacemaker potential depolarization sets the heart rate 1. SA node depolarization initiates atrial contraction 2. atrial depolarization spreads and activates AV node 3. Purkinje fibers carry excitation to the bottom of the heart 4. Ventricular contraction sweeps up from the bottom 6 Ventricle depolarization Atrial depolarization • Important points about coordination 1. Spread through the atrial muscle is by way of gap junctions 2. The only electrical connection between atria and ventricles is the AV node and conducting fibers 3. Conduction through the AV node is slow→ delay between atrial and ventricular excitation Ventricle repolarizatio The relationship between the electrocardiogram (ECG), recorded as the difference between currents at the left and right wrists, and an action potential typical of ventricular myocardial cells. Normal ECG: P waves (atrial depolarization) are followed faithfully by QRS (ventricular depolarization) and T waves (ventricular repolarization). Abnormal ECG: every other P wave fails to evoke QRST (partial atrioventricular block). Abnormal ECG: P waves and QRST occur independently (full atrioventricular block). 7