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BIOC33/BIOC34 Lec 2

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Biological Sciences
Stephen Reid

BIOC34 Lec 2  Conduction system of the heart o SAnode - fire fastest;AP depolarize the rest of it; depolarization spreads to atrial contractile cells  Neuronal action potential o Resting membrane potential, slow depolarization to threshold, massive depolarization, repolarization, hyperpolarization phase, back to resting potential o These changes are due initially to increases in Na permeability which carry large increases in membrane potential. Na permeability begins to decreases, K permeability increases, so we get repolarization. Hyperpolarization is due to K permeability changes - exits from cells (slightly delayed)  K current that comes on slowly is referred to as a delayed rectifier  Pacemaker potential o Pacemaker cells have NO resting potential, onceAP ends, they depolarize again o Slow depolarization to threshold is our pacemaker potential and it is driven by a closure of “funny channels” = Na influx. Closure of K channels = inward rectifiers o Then we get to threshold, theAP itself is carried by Ca, either by T-type or L-type o Then get to depolarization phase o This is the type ofAP happening on conductive pathways o TheseAP stimulate contractile cells; which are basically skeletal muscle cells  Cardiac action potential o Cardiac action potentials last 200-400 ms, while in neuron it lasts 2-4 ms o Has a plateau phase where membrane potential remains high but relatively flat; like what we saw in pacemaker potential, once depolarization is over, Ca depolarizes cell and causes muscle cells to contract - need binding of Ca to troponin and tropomyosin complexes for thick and thin filaments to slide o Closure of K channels - inward rectifying K current; this happens early in pacemaker cells and contractile cells and standard delayed rectifier happening later on o Cardiac action potential is broken down into 5 phases  Starts at 0, not phase 1  0  -19 mv  Cardiac action potential: phase 0 o Have a rapid increase in membrane potential; initial increase is seen by Na influx, opening of voltage-gated Na channels  Due to cells being stimulated by conduction system o Opening of Na channels is a cascade phenomenon - opening one opens many like in a neuron o Reach initial peak of depolarization because of Na influx - at this point, no more Na comes into cell  Phase 1 o Remainder of depolarization is due to changes in K and Na permeability o In phase 1 - initial repolarization  Triggered by deactivation of Na channels o K and Ca also play a role here. K (inward rectifier) channels close and K+ is trapped in the cell. Positive charge leads to depolarization and maintains existing depolarization o Opening of L-type Ca channels  Ca+ come in  Increase in permeability of Ca o These 3 events contribute to maintenance of membrane potential  Phase 2 (plateau) o Plateau phase - though there is a slight decrease o Continuation of K and Ca events from Phase 1  K channels remain closed, L-type Ca stay open o Elevated membrane potential is the continued result of K in the cell and Ca entering o This varies from cell to cell o AP takes same shape in most cardiac contractile cells but depending on where they are in the heart, the length ofAP can differ  Phase 3 o Second repolarization phase o First was little yellow area where there is a small, initial decrease in membrane potential o Phase 3 - Membrane potential falls back down to resting potential o K and Ca driven phenomena - voltage gated L-type channels close therefore Ca no longer coming in o The delayed rectifying K channels open (same in neurons - those that open slowly), K leaves cell and takes + charge out; membrane potential is reduced o Inward rectifying channels also open; K also leaves by these channels therefore reduction in membrane potential o Repolarization is a reduction in Ca permeability, outside Ca channels close and increase in K permeability as inward rectifying channels which were closed are now open and delayed rectifying also start to open  Phase 4 o Resting membrane potential - cell waits for depolarization  ECG: Electrocardiogram o Auseful and diagnostic tool for looking at heart function o ECG is the measurement of electrical activity coming from the heart measured by surface electrodes on arms/chest o Can be used for many things: o 1. Use for electrical axis determination - direction overall that electrical activity is flowing through the heart  Normally flows at 60 degree angle from heart to chest  Shifts to left/right can signify problems o 2. Heart rate:  Bradycardia & tachycardia - decreases and increases in heart rate o 3. Arrhythmia - an abnormal rhythm  Can be a continuum from benign to very serious  Generally, if abnormal rhythm is generated by pacemaker activity in the atria, the heart is stable; if generated in the ventricle, because there is no conduction between atria and ventricle, then that is when there will be problems o 4. Activation sequence disorders - examples of heart block; transmission of electrical activity is either slowed, reduced, or completely abolished o Block in branch bundles - get conduction normally in one and abnormally in the other o 5. Hypertrophy - growth of the heart  Bad  Stretching/growth of the heart  If heart muscle grows, it is because the heart has been under stress for a long time • E.g. high blood pressure causes heart muscles to grow and the heart loses ability to stretch properly and fill properly • Anything that causes the heart to grow or stretch properly, will hinder its ability to contract o 6. Damage to coronary circulation; blood is supplied from blood in coronary arteries  If blood is not getting enough oxygen, there will be abnormalities in the T- wave, which tends to show a downward pattern o 7. Effects of different drugs, electrolyte imbalances, etc.  Bipolar limb leads o The first way that ECG was measured was using standard bipolar limb leads o In this configuration and method, a single electrode is put on the left arm, right arm, left leg o Can compare electrical activity between both arms and both arms and left leg o This gives a potential lead voltage o Measuring the difference in right to left arm, gives the little lead, or standard lead I  Right arm to leg = II  Left arm to leg = III o For leads II and III, leg is positive and arms are negative o This forms an equilateral triangle around the heart called Einthoven’s triangle  Einthoven’s Law o = the electrical potential in lead I + III = lead II o EI+ E III II o If you know two of the leads, can measure the third o Limb lead II is the most important because it is coming down at the orientation of 60 degrees down from the heart, which is the same as the normal electrical axis of the heart  Same orientation as standard electrical axis of the heat  Triangle and axial reference system o Can take the triangle, collapse it around the heart, and come up with positioning numbers that determine if the electrical axis is moving in an abnormal way o By convention, the direction lead I is going, is designated as 0 degrees  II = 60 degrees  III = 120 degrees • Axial reference system o Have our 3 standard limb leads and different directions that each positive end leads into  Positive and negative… o Shows ECG trace from each limb lead o They all look the same; 3 major deflections (P-wave complex) and the magnitude is slightly different o Positive and negative deflections are the result of waves of depolarization  Arrive from waves of depolarization either moving towards or away from the positive and a particular limb lead  If a wave of depolarization is going towards positive electrode in lead I (left arm) have a positive deflection in lead I • If it is going away from the left arm, will have a negative deflection in lead I o In lead II, large positive deflection means positive depolarization is moving towards left leg o If positive deflection leads away from positive electrodes, leads to negative deflection  Important to realize that positive vs. negative deflection are due to moving towards or away positive electrodes in one of the given leads o When looking at chest leads, they will look different. Looking at hearts electrical activity from a different perspective  ECG waves o Each heart beat consists of 3 specific deflections away from baseline, known as the isoelectric line o Have a P-wave (first to show up) which reflects depolarization of atria - encompasses electrical events going from initial activity through to depolarization of atrial
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