Final answer:
The atrioventricular (AV) node delays the electrical impulse for muscle contraction, ensuring that the atria have time to contract and fill the ventricles with blood before the ventricles themselves contract.
Step-by-step explanation:
The structure that delays the electrical impulse for muscle contraction, giving the atria time to contract completely and fill the ventricles with blood is called the atrioventricular (AV) node.
The AV node is a critical component of the cardiac conduction system, responsible for slowing down the electrical signal from the sinoatrial (SA) node before it reaches the ventricles.
This delay is essential to ensure that the atria have time to contract and propel blood into the ventricles, contributing to efficient pumping of the heart.
As the atrial muscles contract, the blood is pumped into the ventricles through the open atrioventricular valves. This phase is known as atrial systole and lasts about 100 ms.
Subsequently, the ventricles contract (ventricular systole), pumping blood out of the heart. The pause provided by the AV node is crucial for the coordinated sequence of heart contractions, which is reflected by the waves of the ECG.
The electrical impulse travels from the SA node to the AV node, which then transmits the signal to the bundle of His and further down to the Purkinje fibers, leading to ventricular contraction.