The intrinsic ability of cardiac muscle to generate action potentials (APs) rhythmically is achieved
by the normal pace maker, the sino-atrial node (S-A node). The S-A node is part of the special
excitatory and conductive system of cardiac muscle. With the help of a diagram, describe this
conductive system how the AP spreads throughout the muscle.
The conductive system of cardiac muscle consists of several specialized structures, including the sino-atrial (S-A) node, the atrioventricular (A-V) node, the bundle of His, and the Purkinje fibers.
The S-A node is located in the right atrium near the opening of the superior vena cava. It serves as the primary pacemaker of the heart, initiating the electrical signal that triggers each heartbeat. The S-A node generates action potentials (APs) rhythmically and spontaneously.
Here is a description of how the AP spreads throughout the muscle, supported by a diagram:
1. Sino-Atrial (S-A) node: The S-A node initiates the electrical signal. It consists of a cluster of specialized cardiac muscle cells that have a faster rate of depolarization than the surrounding cells. These cells act as the natural pacemaker of the heart due to their ability to spontaneously generate APs.
2. AP initiation and spread: The AP initiated at the S-A node spreads through the atria, causing contraction and subsequently pumping blood into the ventricles. The AP then reaches the atrioventricular (A-V) node.
3. Atrioventricular (A-V) node: The A-V node is located at the junction between the atria and ventricles, near the septum. It acts as an electrical gateway, slowing down the electrical signal before it reaches the ventricles. This delay allows the atria to contract fully and fill the ventricles with blood.
4. Bundle of His: After passing through the A-V node, the AP travels rapidly down the bundle of His, a specialized conduction pathway composed of Purkinje cells. The bundle of His divides into the left and right bundle branches, which further divide into smaller branches and extend throughout the ventricles.
5. Purkinje fibers: The small branches of the bundle of His are known as Purkinje fibers. These specialized muscle fibers rapidly conduct the electrical signal to reach every muscle cell in the ventricles.
6. Ventricular contraction: Once the AP reaches the individual cardiac muscle cells in the ventricles via the Purkinje fibers, it triggers their depolarization and subsequent contraction. The coordinated contraction of the ventricular muscle cells effectively pumps blood out of the heart.
This diagrammatic representation of the conductive system illustrates how the action potential is initiated in the S-A node and spreads through the atria, A-V node, bundle of His, and Purkinje fibers to cause synchronized ventricular contraction.