The conduction system refers to how the heart contracts. Here we explain how electrical impulses ensure it beats regularly including specific parts of the heart involved such as the Bundle of His, Synoartial node as well as exam-type questions and quizzes.
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.
When skeletal muscles contract they require nervous stimulation from the central nervous system. For example, when performing flexion at the elbow joint, a nervous impulse is sent from the central nervous system which stimulates the biceps muscle causing flexion at the elbow joint. Unlike skeletal muscles, the heart does not require stimulation from the central nervous system.
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), often referred to as the heart’s pacemaker. When the SA node initiates an electrical impulse this starts the conduction system of the heart.
What is the SA node?
The SA node is a small mass of specialised muscle tissue found in the right atrial wall. The SA node sets the hearts rhythm and when an electrical impulse is initiated it sends a wave of excitation through the atria, spreading through the atria like a Mexican wave, causing atrial systole. Systole refers to the contraction phase of the heart.
The impulse eventually reaches the atrioventricular node (AV node), 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 to allow time for the atria to fully contract and fill the ventricles before the ventricles can then contract.
The impulse is then passed from the AV node down the septum, the muscular wall that separates the right and left sides of the heart. 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 initiate 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
Explain the conduction system of the heart (4 marks)
(one mark for the following)
- The heart is myogenic, it generates its own electrical impulses.
- The sinoatrial node, the heart’s pacemaker, initiates the heartbeat through sending an electrical impulse.
- The impulse causes a wave of excitation spreading through the atria.
- This causes atrial systole.
- The impulse reaches the atrioventricular node with electrically connects the right atria and right ventricle.
- The impulse is passed onto the bundle of his which conducts the impulse through the septum.
- The impulse eventually reaches the Purkinje fibres, which transmit the impulse to the ventricles.
- This causes ventricular systole.
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 the 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.