<prompt> Natural pacemaker of heart is:AV nodePurkinjee fibresSAM nodeSA node  <response>: Ans: d SA nodeRef: Ganong, 22nd ed Pg. 547SA nodeIt is the natural pacemaker of heart.It is located at the junction of superior vena cava with right atrium.It is made of modified cardiac muscle.Blood supply - by right coronary artery 65 and left coronary artery 35.Nerve supply - by Rt vagus and Rt sympathetics.It has the highest rate of impulse generation - 70 to 80 per min.AV nodeIt is located in the right postero inferior region of interatrialseptum near the opening of coronary sinus.lt is made of modified cardiac muscle.Blood supply -by right coronary artery.Nerve supply - by Lt vagus and Lt sympathetic.Rate of impulse generation - 40 to 60 per min.Note: Conduction speed is highest in Purkinjee fibers 4msBundle of His 1 msVentricular muscle 1 msAtrial pathway 1 msSAnode 0.05 msAV node 0.02 - 0.05 ms 