Question

The cardiac impulse is initiated and conducted further up to the ventricle. The correct sequence of conduction of impulse is
A. SA Node, AV Node, Purkinje fibre, AV Bundle
B. SA Node, Purkinje fibre, AV Node, AV Bundle
C. SA Node, AV Node, AV Bundle, Purkinje fibre
D. SA Node, Purkinje fibre, AV Bundle, AV Node

Answer 1

The beating of the heart occurs due to the rhythmic contraction and relaxation of heart muscles. It is a cyclic process that occurs in four steps, which involves; generation of the pacemaker, impulse conduction, bundle contraction, and impulse conduction.

Complete answer: Cardiac impulses cause the heart to relax and contract. This contraction and relaxation occur in a rhythmic pattern. The contraction is followed by relaxation of heart muscles so that the blood is pumped throughout the body. The SA or the sinoatrial node which is also known as the pacemaker of the heart undergoes contraction. This generates nerve impulse that travels throughout the heart wall. This results in the contraction of atria. The SA Node is present in the upper wall of the right atrium and is composed of nodal tissue. The nodal tissue has characteristics of both nervous tissue and muscle. The second step or the AV node impulse conduction is characterized by atria contraction leading to emptying of its content into ventricles before the ventricle contraction. This is caused by the arrival of impulse from the SA Node. The impulse from the SA node reaches the AV node and is delayed for about a tenth of a second. This delay is responsible for atria to contract. The AV node is present on the right side of the partition that divides the atria just at the bottom of the right atrium. The third step is the AV bundle impulse conduction. The impulses generated in the second step are sent down to the atrioventricular bundle. The impulses are carried down to the centre of the heart to the right and left ventricles. The fourth step is the Purkinje fibre impulse conduction. The atrioventricular bundles at the base of the heart divide into purkinje fibres. The signal reaches these fibres and triggers the muscle fibres in the ventricle to contract. This causes the right ventricle to send blood to the lungs through the pulmonary artery. On the other hand, the left ventricle sends blood to the aorta. Thus, the correct sequence of the cardiac impulse is SA Node, AV Node, AV Bundle, Purkinje fibre.
So, the correct answer is option C.

Note: The cardiac cycle is solely dependent on cardiac conduction. It is a sequence of events that occurs during the heartbeat. In the diastole phase of the cardiac cycle, the ventricles and atria relax causing blood to flow into ventricles and atria respectively. On the other hand, in the systole phase, the blood is sent to the rest of the body through the ventricular contraction.