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Physiology 5: Today we are going to talk about: -autonomic innervations of the heart -ECG Innervations of the heart: It comes from the sympathetic and parasympathetic system. Sympathetic innervations come from the cardiac plexus (T1-T4) to supply all parts of the heart. Parasympathetic innervations comes from the vagus nerve to both atria SA node and AV node.(i.e. it does not supply the ventricles or very minimal ). The sympathetic system usually stimulates the heart (i.e. cardio acceleratory center that is found in the medulla oblongata) and works at the postsynaptic junction through NE and E (epinephrine and nor epinephrine) to increase the cardiac membrane permeability to sodium and calcium which in turn increases the depolarization rate (much less time is needed to reach the threshold) thus increasing the heart rate. This effect is called positive chronotropic effect (increase of the heart rate).as it increases permeability to sodium and calcium it increase the force of contraction of the heart. This is called positive iontropic effect. It also affects the conduction of AP by the positive dromotropic effect. The parasympathetic system usually inhibits the heart decreasing its rate (HR).this system has a negative chromtropic effect and negative dromotropic effect BUT with no effect on the force of contraction (i.e no iontropic effect). This system releases ACTL that decreases the membrane permeability to sodium and calcium AND increase the permeability to potassium. This shifts the resting membrane potential from -60 mV to -70 mV, so more time is needed to reach the threshold thus the HR is decreased. Important note: Changes of membrane ion permeability do not change the peak of the AP (all or none), only manipulate with the rate of AP (either faster to slower). Example: Stimulation of the vagus nerve -----decrease of HR-----HR rate keeps on decreasing till the heart stops by the overstimulation of the parasympathetic system…after 30s the heart starts to contract again .Why? Since the ventricles (and the purkinje fibers) are not affected by the parasympathetic system the purakinje fibers will override the effect of the parasympathetic system allowing it to contract again (i.e. ventricular escape). Stock Adam s syndrome: Is a syndrome which stimulates the parasympathetic system through its pressure on the vagus nerve----decrease HR--Coma for 30s------Ventricular escape the S.A node will return to be the heart pacemaker after the stimulus or pressure on the vagus nerve is removed. --------------------------------------------------------------------------Now to the second part of our lecture. ECG: What is ECG? The recording of the electrical changes that occur in the heart during a cardiac cycle that keeps repeating itself each time. It does not record any of the mechanical changes , muscle contraction or relaxation that occurs in the heart. During this cycle, there is atrial depolarization followed by ventricular depolarization then atrial and ventricular repolarization. We say that the AP is monophasic,what does that mean? It means that we have one wave for depolarization and repolarization that occur at the same side above the isoelectric line (the positive side) The ECG is bipolar, meaning that depolarization and repolarization waves should occur on opposite sides and are two different waves. Back to ECG. In the ECG recording we have the P wave,QRS complex and the T wave. P represents the atrial depolarization. The P wave immediately precedes atrial contraction. QRS complex represents the depolarization of the ventricles. The QRS complex immediately precedes ventricular contraction. T represents the repolarization of ventricles. Notice in the ECG: --there is no repolaztion of atria because repolarzation occurs at the same time as the depolarization of ventricles which overrides it. --The T wave is on the positve side of the isoelectric line although it is a repolarizing wave ( Repolarization wave should occur opposite to depolarizing wave as mentioned )..the reason of this will be explained later on (star) Depolarization starts from the endocardium to the epicardium, from the base to the apex. Repolarization starts from the epicardium to the endocardium,from apex to base. So both of them will have a positive deflection in the graph. (repolarization will be deflected to the positive side rather than the negative one).because they started from opposite origins(star) Why do the depolarization and repolarization waves start from different origins? There are two theories that explain this: 1- There is an intrinsic property of the muscles of the epicerium so that they have a short action potential while endocardium muscle have a long action potential to the extent that repolarization of the epicardium occur before the endocarium although they were depolarized after it. 2- After depolarization and when the heart contracts,it develops pressure inside it that will affect the endocardium more than the epicardium,this pressure will change the permeability of the ions to the membrane to the extent that it will prolong the repolarization wave of the endocardium more than the epicardium. Important note:this pressure will not affect the atria considerably because its propotionaly low in atria compared to ventricles ,so depolarization and repolarization waves start from the same origin… Depolarization starts from the endocardium to the epicardium and repolarization starts from the endocardium to the epicardium.so IF we record an atrial repolarization it will be a downward deflection (negative deflection).Atrial repolarization can only be recorded when the heart rate is low,for example in AV block.