The transformed muscle of the heart wall is called myogenic or connective tissue. Myogenic tissue of heart is-
(i) SAN or Pacemaker: Between the superior vena cava and the right atrium is a group of specialized cardiac muscles. It is called SAN. Its length is 10-15 mm, width is 3 mm and thickness is 1 mm. The SAN gives rise to cardiac contractile stimulation or action potentials. Hence the SA node is called primary motion generator. The SAN is called a pacemaker because it acts as a stimulus for excitatory waves and waves. It makes the heart beat 100 beats per minute. Even when the heart is separated from the body, it continues to contract. If its effectiveness decreases, fatigue and shortness of breath occur. This is called ischemia. Martin Flack (1907) discovered it.
(ii) AVN: AVN is located in the right atrioventricular septum of the heart. It is composed of cardiac muscle. This is called a protected pacemaker. Because if the SAN fails to generate an electrical signal, it generates it. It receives impulses from the SAN and transmits them to the bundle of His. It takes 0.03 seconds for the impulse from the SAN to reach the AVN. But the AVN has a delay of 0.09 seconds in the arrival of the stimulus. This is called AV Nodal Delay. It then takes another 0.04 seconds for the AVN to travel from the thalamus to the ventricle muscle. That is, the impulse from the AVN takes a total of (0.03+0.09+0.04) 0.16 seconds to reach the ventricle muscle.
(iii) Bundle of His: Behind the AVN lies the bundle of His. It is composed of multidimensional small fascicles. Of its two branches, one extends to the wall of the left ventricle and the other to the wall of the right ventricle. Swiss cardiologist Wilhelm His (1893) discovered it.
(vi) Purkinje fibers: The ends of the bundles of His become fine fibers and form a meshwork in the walls of the ventricles. These are called Purkinje fibers. It is made up of specialized cardiac muscle and conducts the heartbeat quickly and efficiently. Its speed is 40-60 times per minute. It causes contraction of the ventricle wall. John Evangelist Perkinzie (1839) discovered it.
