The Electrical Properties Of Heart And The Regulation Of Heart Beat In Humans


In this article the electrical activities of the human heart have been described. The generation of impulse and the spread of depolarization among the various parts of heart have been described also. Control of heart beat due to the electrical activities of heart have been also written in brief. Arrhythmias and their common types are also described in this article. Heart block and its causes have been also described.

Introduction


The human heart acts like a perfect machine
. It pumps blood continuosly from birth to a person's death. In this article the basic mechanism behind the beating of heart have been described

Conduction system of heart


The conduction system of heart comprises of-sinoatrial node(SA Node), the atrioventricular node(AV Node), the internodal atrial pathways, bundle of his and its branches and the purkinje system

Normal physiology of heart


The various parts of the conduction system are capable of generating impulse or spontaneous discharge themselves however the SA Node discharges most rapidly and depolarization spreads from it to the other parts of heart before they discharge. So, SA Node acts as the normal cardiac pacemaker. Impulse arises from SA Node, travels through internodal atrial pathways and reaches the AV Node. From AV node impulse spreads to the different parts of myocardium through the bundle of his and the purkinje system. The atria contract first leading to pumping of blood to ventricles and then the ventricles contract leading to pumping of blood to aorta and pulmonary artery

Nerve supply of SA Node and AV Node


The SA Node and AV node recieve sympathetic nerve fibres(increases heart rate) from the stellate ganglion and parasympathetic nerve fibres(decreases heart rate) from vagus nerve. This explains why our heart beat increases in cases of fear, fight and flight

Contraction of cardiac muscle cells


The resting membrane potential of an individual cardiac muscle cell is -80mV. Stimulation of these muscle cells by the impulse from the pacemaker produces a propagated action potential which is responsible for their contraction and the pumping of blood by heart

Arrhythmias


Disproportion between the pumping action of atria and ventricles leads to arrhythmia(meaning-not in a rhythmic manner). Cardiac arrhythmias are of two types-atrial arrhythmia and ventricular arrhythmia

Heart Block


When conduction system between atria and ventricles is completely interrupted, third degree heart block or complete heart block occurs. In such cases, ventricles start beating independently of atria. Such type of heart block may be due to any disease in the AV Node or in the conducting system below the node called as infranodal block. In cases of infranodal block due to any disease in the bundle of His, the ventricles beat at a slower rate which averages about 35 beats/min but it may become as low as 15 beats/min. In such persons, there may also be some periods of asystole lasting for a few minutes. This may cause cerebral ischaemia resulting into dizziness and fainting called as Stokes-Adam syndrome. In cases of incomplete heart block, the conduction system between the atria and ventricles is not completely interupted. It has two types-first and second degree heart block

Artificial pacemakers for heart


An artificial electronic pacemaker is implanted in patients having marked bradycardia due to sick sinus syndrome or due to third degree heart block. These devices are also useful for patients suffering from severe neurogenic syncope in whom due to carotid sinus stimulation gaps or pauses of more than 3 seconds occurs between the heart beats.


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