Question

Which of the following shows the correct sequence of conduction of action potential in the heart?
A) SA node to Bundle of His to AV node to Right and left branches of bundle of His to Purkinje fibre
B) SA node to AV node to Purkinje fibre to bundle of his to Right and left branches of bundle of His
C) SA node to AV node to bundle of his to Right and left branches of bundle of His to Purkinje fibre
D) AV node to SA node to bundle of his to Right and left branches of bundle of His to Purkinje fibre

Answer 1

Hint: Action potential defined as an explosion of electrical activity which is created by a depolarizing current. This means that a stimulus causes the resting potential to move towards \[0{\text{ }}mV\]. When the depolarization reaches around \[ - 55{\text{ }}mV\] a neuron will fire an action potential, this is called the threshold.

Complete answer:
Action potential comprises four phases which are hyperpolarization, depolarization, overshoot, and repolarization. An action potential propagates along the cell membrane of an axon till it reaches the terminal button.

Sinoatrial which is commonly known as SA node normally generates the action potential, which means the electrical impulse initiates contraction. The SA node then excites the right atrium, which then travels through Bachmann’s bundle to excite the left atrium.

The impulse travels through internodal pathways in the right atrium to the atrioventricular or AV node.
From the AV node, the impulse then travels through the bundle of His and down the bundle branches, fibres which are specialized for rapid transmission of electrical impulses, on either side of the interventricular septum.

The right bundle branch depolarizes the right ventricle. The left bundle branch depolarizes the left ventricle and interventricular septum. Both the bundle branches terminate in Purkinje fibres, which are millions of small fibres projecting throughout the myocardium.

A) In option A, the action potential does not go to the bundle of his from the SA node but instead it goes to the AV node. Therefore, it is incorrect.
B) Option B is incorrect because Purkinje fibres are the last step in the conduction of action potential and not the third step.
C) Option C is the correct sequence of order sequence of conduction of action potential in the heart
D) Action potential starts from the SA node and not from the AV node, therefore option D is incorrect.

The diagram below depicts the conduction of action potential in the heart

Therefore the correct answer is option ‘C’.

Note: An organized rhythmic contraction of the heart requires adequate propagation of electrical impulses along the conduction pathway. The impulses in the His-Purkinje system travel in such a way that papillary muscle contraction occurs before that of the ventricles, thereby preventing the regurgitation of blood flow through the AV valves.