The cardiac conduction system refers to how electrical impulses enable the heart to beat.
The heart is unique in that it never tires and is able to generate its own electrical impulses without the need for stimulation from the central nervous system. On average the heart beats approximately 70 beats per minute.
Skeletal muscles need stimulation from a nerve to contract. This comes from the central nervous system. For example, to stand on your toes (plantar flexion), the central nervous system sends a nervous impulse to the muscles. This stimulates the calf muscles making them contract. As a result, your ankle moves. The heart does not require central nervous system stimulation to contract.
The heart is said to be myogenic. This means it is able to generate its own impulses. These impulses begin at the sinoatrial node (SA node) which is often referred to as the heart’s pacemaker. When the SA node initiates an electrical impulse this starts the heart’s conduction system.
The SA node is a small mass of specialised muscle tissue in the right atrial wall. It sets the heart’s rhythm. When it initiates an electrical impulse it sends a wave of excitation through the atria. This spreads through the atria like a Mexican wave, causing atrial systole. Systole refers to the contraction phase of the heart.
Eventually, the impulse reaches the atrioventricular node (AV node). This is another specialised mass of tissue that electrically connects the right atrium and right ventricle. There is a slight delay as the impulse passes through the AV node. This allows time for the atria to fully contract and fill the ventricles before the ventricles contract.
The electrical impulse then passes from the AV node down the septum. The septum is the muscular wall that separates the right and left sides of the heart.
Bundle of His
The Bundle of His, specialised bundles of nerve tissue, conduct the impulse through the septum to the tip of the ventricles. The impulse is then passed onto smaller branches that spread through the wall of the ventricles. The impulse eventually reaches the Purkinje fibres which conduct the impulse to the ventricles causing ventricular systole.
Key terms & definitions:
- Sinoatrial node – small mass of specialised fibres found in the right atrium which initiates the heartbeat
- Atrial systole – the contraction phase of the atria
- Atrioventricular node – small mass of fibres found in the atrioventricular septum that electrically connects the right atrium and right ventricle
- Bundle of his – specialised bundle of nerve tissue located in the septum which passes the electrical impulse through the septum
- Purkinje fibre – specialised nerve fibres found in the ventricles which cause ventricular systole
- Ventricular systole – the contraction phase of the ventricles
The following are examples of exam-style questions on the cardiac conduction system:
Explain the conduction system of the heart (4 marks)
You get one mark for mentioning the following
- Heart is myogenic and generates its own electrical impulses
- Sinoatrial node, the heart’s pacemaker, initiates the heartbeat through electrical impulse
- Electrical impulse causes a wave of excitation spread through the atria
- This causes atrial systole.
- Impulse reaches the atrioventricular node which electrically connects the right atria and right ventricle
- The impulse passes onto the Bundle of His which conducts the impulse through the septum
Impulse eventually reaches the Purkinje fibres, which transmit the impulse to the ventricles causing ventricular systole
Question: Explain the roles of the sinoatrial node and the atrioventricular node in controlled heart rate (4 marks)
(One mark for any of the following)
- The sinoatrial node is the heart’s pacemaker and initiates/starts the heartbeat.
- The impulse sends a wave of excitation through the atria.
- The impulse causes atrial systole.
- The atrioventricular node electrically connects the right atrium and right ventricle./ The atrioventricular node conducts the impulse from the atria down the septum of the heart.
- The impulse is passed down the bundle of His to the Purkinje fibres.
- This causes ventricular systole.
- Explain how a rise in carbon dioxide may cause an increase in heart rate (4 marks)
- (one mark for any of the following)
- During exercise, a rise in carbon dioxide occurs as a result of exercise.
- The rise in carbon dioxide has detected chemoreceptors.
- The chemoreceptors send a nerve impulse to the medulla oblongata.
- The medulla oblongata stimulates the sympathetic nervous system.
- The sympathetic nervous system initiates the release of adrenaline and noradrenaline.
- Adrenaline and noradrenaline act upon the sinoatrial node increasing the rate in which it fires impulses.