Final answer:
The Mitral Valve is not a part of the heart's electrical conduction system; it is a structural component that regulates blood flow. The atrioventricular node has the slowest firing rate in the heart's electrical conduction system, as it holds the action potential for around 100 milliseconds.
Step-by-step explanation:
The component that is not considered a major part of the heart's electrical conduction system is D. Mitral Valve. The mitral valve is a structural component of the heart, specifically an atrioventricular valve that regulates blood flow between the left atrium and the left ventricle, but it is not involved in the electrical conduction system, which includes the sinoatrial (SA) node, atrioventricular (AV) node, bundle of His (atrioventricular bundle), right and left bundle branches, and the Purkinje fibers. These components are responsible for the generation and transmission of electrical impulses that control the heart's rhythm.
Regarding which component of the heart conduction system has the slowest rate of firing, the atrioventricular node (AV node) holds the action potential for approximately 100 milliseconds, creating a delay that ensures the atria contract and empty their blood into the ventricles before the ventricles begin to contract. This makes the AV node the component with the slowest firing rate in the conduction system.