In question the word " command" used command is come from SA node ( origin of impluse)so on this basis answer is SA node

Ventricular contraction is a complex process that involves the coordinated activation of various components of the heart's electrical conduction system. This system is responsible for generating and conducting electrical signals that control the rhythmic contraction of the heart muscles. Let's discuss the role of each component in ventricular contraction:

1) Sinoatrial (SA) Node:
The SA node, often referred to as the "natural pacemaker" of the heart, is located in the upper-right atrium. It initiates the electrical impulses that regulate the heartbeat. The SA node generates an action potential, causing the atria to contract and pump blood into the ventricles. This contraction is known as atrial systole.

2) Atrioventricular (AV) Node:
The AV node is located between the atria and the ventricles. It receives the electrical signals from the SA node and delays their transmission for a brief period. This delay allows the atria to contract and complete their pumping action before the ventricles are activated. After the delay, the AV node sends the electrical signals to the ventricles through the bundle of His.

3) Purkinje Fibres:
Purkinje fibers are specialized cardiac muscle fibers that rapidly conduct the electrical signals to the ventricular muscles. They are spread throughout the walls of the ventricles and ensure synchronized ventricular contraction. Once the electrical signals reach the Purkinje fibers, they rapidly propagate through the ventricles, causing them to contract forcefully. This contraction is known as ventricular systole.

4) Papillary Muscles:
The papillary muscles are small, cone-shaped muscles located within the ventricles. They are attached to the cusps of the atrioventricular valves (mitral and tricuspid valves) by chordae tendineae. The main function of the papillary muscles is to prevent the valves from prolapsing during ventricular contraction. They contract just before the ventricles contract, tightening the chordae tendineae and ensuring proper closure of the valves to prevent backflow of blood.

Overall, the sequence of events in ventricular contraction is as follows:
- The SA node initiates an electrical impulse, leading to atrial systole.
- The electrical signals are delayed at the AV node, allowing the atria to contract fully.
- The signals then travel through the bundle of His and spread rapidly through the Purkinje fibers.
- The Purkinje fibers distribute the electrical signals throughout the ventricles, causing them to contract forcefully.
- The papillary muscles contract, tightening the chordae tendineae and preventing valve prolapse.

This coordinated sequence of events ensures efficient blood flow through the heart, allowing it to pump oxygenated blood to the body and deoxygenated blood to the lungs.