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The SA node and the AV node

SA node:
Sino-Atrial node is located at the junction of superior vena cava with right atrium, Just deep to the epicardium, near the sulcus terminalis. The SA node is made of modified cardiac muscle that has fewer strations and indistinct boundaries. These cells are small round with few organelles which are connected by gap junctions. These cells are called P cells (pacemaker cells). SA node is the fastest discharging part of the myocardium (70 to 80/min)

Blood supply of SA node- Right coronary artery supplies the SA node in 65% of the cases.
Nerve supply- SA node develops from the structures on the right side of the embryo, therefore SA node is supplied by Right vagus nerve and Right sympathetics. Noradrenergic nerve fibers are epicardial while vagal fibers are endocardial.

AV node:
Atrio-ventricular node is located in the right postero-infero region of the interatrial septum near the opening of coronary sinus. There are 3 bundles of fibers that connect the SA node with the AV node. Namely Anterior internodal tract of Bachman, Middle internodal tract of Winckebach and Posterior internodal tract of Thorel.
Blood supply- Right coronary artery.
Nerver supply- AV node develops from structures in the left side of the enbryo. Therefore it is supplied by Left vagus and left sympathetics.

The depolarization of SA and AV nodes depends on calcium channels rather than on Sodium channels. Rate of impulse generation in decreasing order- SA node> AV node (40 tp 60/min)> Bundle of His (40/min)> Purkinjee fibers (24/min).


physiology said…
very good description

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