Ans. is 'c' i.e., Modified cardiac muscle Conducting system of hea The conducting system is made of specialized myocardium (cardiac muscle), that is capable for initation and conduction of cardiac impulse. It has following pas : ? 1) SA node :- SA node is located in the upper pa of crista terminalis at the junction of SVC and the right atrium. It is the pacemaker of the hea and generates impulse at a rate of 70-100/min. SA node is sniffled by nodal aery, a branch of RCA in 65% cases and a branch of circumflex branch of LCA in 35% cases. SA node is supplied by right vagus/parasympathetic (inhibitory) and right Sympathetic (excitatory) system as it develops from structures on the right side of embryo. 2) AV node :- It lies in the right atrial floor near the interatrial septum in the 'triangle of koch'. It is supplied by AV nodal aery, a branch of RCA. AV node develops from left side of hea, thus is supplied by left vagus and left sympathetic fibers. 3) Atrioventricular bundle or bundle of His :- It arises from AV node and crosses the AV ring (annulus fibrosus). In the muscular septum it divides into right and left branches. It has a dual blood supply from AV nodal aery (branch of RCA) and anterior descending (interventricular) branch of LCA. 4) Right bundle branch (RBB) and left bundle branch (LBB) :- These bundle branches consist of modifeid muscle fibers (Purkinje fibers). Both RBB and LBB are supplied by LCA, except a small pa of the LBB which is supplied by RCA.