In the heart there is Atrial muscle and Ventricular muscle which are separated from each other by the fibrous AV Rings containing Valves. ATRIAL MUSCLE: thin walled. There are two sheets, superficial and deep sheet. Superficial sheet is common over both atria. Deep sheet is separate for each atrium. Muscle fibers in the deep sheet are at right angle to the muscle fibers in the superficial sheet. FUNCTIONS OF THE ATRIUM: 1. Receive venous blood from large veins. So atria act as reservoir. 2. Conduct the blood into the ventricles. 3. Atrial contraction is responsible for last 25 % of ventricular filling. 4. In the right atrium there is SA Node(Pace maker) and AV node. 5. In the wall of the atria, there are low pressure stretch receptors and these are involved in various reflexes like brain bridge reflex and left atrial reflex. 6. Atria also produce a hormone i.e. Atrial Natriuretic Hormone. Whenever NaCl increases in ECF, it causes release of ANH which causes natriuresis. VENTRICULAR MUSCLE: Much thicker than atrial muscle. Thickness of right ventricle wall is 3-4 mm and thickness of left ventricle is 8 – 12 mm. 1.Involuntary 2.Has cross striations 3.Each cardiac muscle fiber consists of a number of cardiac cells, united at ends in series. Where as in skeletal muscle each muscle fiber is individual cell. 4.Cardiac muscle cells are branching and interdigitate. 5.Single central nucleus in each cell. 6. Atrial muscle and ventricular muscle act as separate functional syncytium and impulses from atria are conducted to ventricles through the AV Node and AV Bundle. 7. Sarcoplasmic system is present. In skeletal muscle triad is at the junction of A and I bands. In cardiac muscle T Tubules are much large and thus in cardiac muscle if we take a section it may form a diad or a triad. And these diads and triads are present at the level of Z Disks. 8.Between adjacent cardiac cells there are side to side and end to end connections and these are the intercellular junctions. These junctions are Gap Junctions. Or intercalated discs 9.When one part of myocardium is excited the whole muscle is excited. 10.Whole myocardium obeys all or none law as a whole. 11.No spike potential but action potential with plateau. 12.Has got long refractory period. Absolute refractory period in ventricular muscle is 250 – 300 milli sec. In atrial muscle Absolute refractory period is 150 milli sec Because of long refractory period cardiac muscle cannot be tetanized.