 For one heartbeat electrical activity arises in sinoatrial node, spreads to atria via inter nodal atrial pathways, travels via cardiac conduction pathways that is to atrio ventricular node, bundle of his, its right and left branches and the purkinje fibers. The spread of electrical activity is basically depolarization of the heart muscle. Now before going into the details let us see a bit of anatomy. SA node is located at junction of superior vena keva with the right atrium and AV node is located near the inter atrial septum. SA node is connected to AV node by three bundle of fibers arranged from anterior to posterior that is the anterior inter nodal tract, middle and posterior inter nodal tract. AV node continues as bundle of his. It gives off a left bundle branch and continues as right bundle branch itself. These branches and their further small branches run below the endocardium layer of heart muscle. Actually heart muscle has three layers right? Inner most layer is endocardium, middle is myocardium, outermost is epicardium. So the fibers run below the endocardium and meet purkinje fibers. So how the electrical potential spreads throughout heart? Action potential originates at SA node moves via atrial inter nodal tracts to AV node. Now since cardiac muscle cells are connected to each other by gap junctions the potential starts spreading to other cardiac cells also since ions can pass via gap junctions to other cells. So potential spreads via the conduction pathway and also along the atrial muscle cells via gap junction. It takes only about 0.1 seconds for the activity to spread to atria. Now conduction in AV node is slow and there is a delay of about 0.1 second here. This is known as AV nodal delay. After this the depolarization spreads via bundle and its branches first to the left side of interventricular septum at midpoint of septum moves to right and then further spreads down the septum to apex of the heart. Then it turns back and reaches atrioventricular groove. Because of this last part of heart to be depolarized is posterior basal part of left ventricle. This whole process of ventricular depolarization finishes in 0.08 to 0.1 second. Also if we consider the layers of heart remember that these fibers run subendocardially? So electrical activity moves from endocardium to epicardium. During all this process do not forget that heart is a sensation because of gap junctions and electrical activity spreads to other muscle cells via gap junctions. Then you may ask that if potential can pass via muscle cells then what is the need of the conduction pathways? Well there are two things. The spread is faster through this conduction pathway so it facilitates rapid depolarization of heart that is rapid spread of electrical activity. Secondly from atrioventricles the activity can spread only via this conduction pathway since atrial and ventricular fibers are separated by a fibrous tissue ring. This is also important physiologically because it helps in spacing the depolarization of atria and ventricles. Otherwise both of them will contract together which we don't want in heart. These fundamentals we will again consider in another video on ECG and then in arrhythmias but before finishing we will also consider abnormalities in conduction pathways and diseases which may occur. Now remember that apart from SA node each part of conduction pathway is capable of generating electrical impulse but SA node generates impulses at fastest rate that's why it is known as pacemaker. Also in abnormal situations even myocardium can generate electrical impulses. Secondly also remember that one spread of action potential means one heart beat one contraction. Now let us discuss some abnormalities. Diseases may occur either due to abnormality in generation of impulse or in conduction of impulse or both. So it is occurring at various levels. First let us see SA node. Diseases may affect SA node leading to decreased generation of nerve impulse and may cause severe bradycardia. Second there may be conduction block from SA node to AV node and below. Now there are levels of this block. When there is simply slowing of conduction from atria to ventricle it is known as incomplete heart block. Another level is when there is complete block of conduction from atria to ventricle that means no impulse which is generating at atria is reaching the ventricles that is known as complete heart block. Now incomplete heart block also may be of different grades. The first grade in which it causes prolonged duration of conduction only but all the impulses are being conducted to ventricle. Second one there may be missed beats. So like two impulses are getting transmitted to ventricles but the third one may be missed or there may be another pattern like three impulses are getting transmitted but every fourth impulse is getting missed. So these are two different grades of incomplete heart block. Now if we combine these incomplete and complete heart block we can say that heart block has different grades. Only prolonged duration of conduction is known as first degree heart block. When some beats are missed in between it is known as second degree heart block and when there is complete block of conduction from atria to ventricles it is known as third degree heart block. Now incomplete block ventricles beat at their own slow rhythm known as idioventricular rhythm. This may occur either due to AV node disease or a block below the node. The block occurring due to AV nodal disease is known as AV nodal block and that occurring below the node is infranodal block. In some cases rate of heart beating may be so low that it leads to decrease in blood supply to brain causing fainting attacks. This condition is known as Stokes-Adams syndrome. Heart blocks can also occur due to bundle branch block where the conduction till bundle of his is normal but below the conduction is affected. Apart from this abnormalities in generation and conduction of impulses can occur if there is an abnormal focus apart from SA node which is irritable and generates action potential. Now here since myocardium is a syncesium that potential will also spread to the heart causing extra beat. If abnormal focus is too much active and its frequency of generation of action potential is higher than SA node it will cause tachycardias. These are known as peroxismal tachycardias and depending on the location of the focus they can be atria ventricular or nodal tachycardias. Next there can be reentry of potential. See normally there is a particular direction in which the electrical activity travels and you see once a spread occurs it does not spread back because the muscle cells are refractory that is during an electrical activity they do not respond to second stimulus for some time so they are refractory. Now if there is a channel established such that current flows in one direction travels via another path comes to original pace after some time such that the cells are no longer refractory then it will depolarize the cells again and it will keep moving in a round round circle that is known as circus movement of the electric current. That ends our video on generation and conduction of electrical impulses in heart and the abnormalities which can be associated with it so if you like the video please do subscribe to the channel and don't forget to hit the bell button there down.