SECTION 6.4
Conduction system and heart rate
Explain how the conduction system sets the heartbeat and how nervous and chemical factors change the rate.
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The heart beats without being told to. A small group of specialised muscle cells in the right atrium generates an electrical impulse on its own, and that impulse spreads through a conduction pathway to make the chambers contract in the right order. This property is called autorhythmicity, and it is why a heart can continue beating even when its nerve supply is cut. The impulse begins in the sinoatrial node, the natural pacemaker, pauses briefly at the atrioventricular node so the atria can finish filling, and then travels rapidly through the ventricles. Although the heart sets its own basic rhythm, the autonomic nervous system and chemical factors adjust the rate continuously, speeding it during exercise and slowing it at rest. This section explains the pathway and the control of heart rate.
Key Concepts & Topics
The heart sets its own rhythm
Cardiac muscle has a property that skeletal muscle does not: some of its cells can generate an electrical impulse without any nerve input. This is autorhythmicity, and the cells that do it are called pacemaker cells. They slowly leak ions across their membranes until the charge reaches a threshold, at which point they fire an impulse and then begin the process again. Because this happens automatically, the heart continues to beat even if all its nerves are cut, which can be shown in laboratory preparations. The rate at which pacemaker cells fire is not fixed, however; nerves, hormones, temperature and electrolytes all shift it. Autorhythmicity sets a baseline, and the rest of the body adjusts that baseline to match current needs.
For example: A heart removed from an animal and kept supplied with oxygen and nutrients will continue to beat on its own, because its rhythm does not depend on nerves.
The conduction pathway
The impulse follows a fixed route. It begins in the sinoatrial (SA) node, a small mass of pacemaker cells in the wall of the right atrium near the opening of the superior vena cava; this node is the normal pacemaker because it fires fastest. The impulse spreads through the atria, causing atrial systole, and also travels through internodal pathways to the atrioventricular (AV) node, located at the junction of the atria and ventricles. The AV node delays the impulse briefly, which lets the atria finish emptying before the ventricles contract. The impulse then passes into the bundle of His, which divides into right and left bundle branches running down the interventricular septum, and finally into the Purkinje fibres, which spread through the ventricular walls. Because Purkinje fibres conduct quickly, the ventricles contract almost simultaneously from the apex upwards, which is efficient for ejecting blood.
For example: On an ECG, atrial depolarisation appears as the P wave and ventricular depolarisation as the QRS complex, reflecting the spread of the impulse.
Why the AV node delays
The delay at the AV node is short, about a tenth of a second, but it matters a great deal. During that pause, atrial contraction finishes and the ventricles are still relaxed, so they can receive the last portion of blood before contracting. Without the delay, the atria and ventricles would contract almost together, and ventricular filling would be incomplete, reducing the volume ejected. The AV node also acts as a gatekeeper: it limits how many impulses reach the ventricles, which protects them from very fast atrial rhythms such as atrial fibrillation. In that condition the atria may beat several hundred times a minute, but the AV node blocks most impulses, so the ventricles beat at a manageable rate. This is why diseases of the AV node can cause a slow pulse and why some drugs slow the ventricular rate by acting on it.
For example: In a complete AV block, the atria and ventricles beat independently, and a slower backup pacemaker in the ventricles may take over.
Autonomic control of heart rate
Although the heart beats on its own, the autonomic nervous system adjusts the rate continuously. Sympathetic fibres release norepinephrine, which increases the rate and the force of contraction; this is the response to exercise, stress or danger. Parasympathetic fibres, mainly the vagus nerve, release acetylcholine, which slows the rate. At rest, the vagus nerve exerts a steady slowing influence, so the resting heart rate is lower than the SA node’s intrinsic rate; cutting the vagus would raise the rate. The two systems usually act in opposition, and the balance between them sets the current rate. Changes in rate are described as chronotropic effects, and changes in the force of contraction as inotropic effects. A rate that is too slow is bradycardia and one that is too fast is tachycardia.
For example: During exercise, sympathetic activity rises and vagal activity falls, so the heart speeds up; during sleep, the reverse happens.
Chemical and clinical influences
Heart rate responds to many chemical and physical factors. Epinephrine from the adrenal glands raises the rate during stress, and thyroid hormones raise it over longer periods, which is why an overactive thyroid causes tachycardia. Elevated body temperature raises the rate, while cold lowers it. Electrolyte balance matters greatly: abnormal potassium, calcium or sodium levels can disturb conduction and produce dangerous rhythms. Many drugs act on the conduction system, either speeding it up or slowing it down, and some are used deliberately to control rate in atrial fibrillation or to strengthen contraction in heart failure. Finally, the conduction system can fail or be bypassed: if the SA node stops working, a slower pacemaker lower in the pathway may take over as an escape rhythm, and an artificial pacemaker may be implanted to restore a reliable rate.
For example: A patient with an overactive thyroid may have a persistently fast pulse, while one with a high potassium level may develop dangerous rhythm changes.
Key facts to remember
- Cardiac pacemaker cells are autorhythmic: they generate impulses without nerve input.
- The impulse runs SA node → atria and internodal pathways → AV node → bundle of His → bundle branches → Purkinje fibres → ventricular myocardium.
- The AV node delays the impulse so the atria can finish filling, and it limits how many impulses reach the ventricles.
- Sympathetic activity raises heart rate and contractility; parasympathetic (vagal) activity slows the rate.
- Hormones, temperature, electrolytes and drugs also change rate; if the SA node fails, a slower escape rhythm may take over.
Example · Follow the impulse. It begins in the SA node and spreads across the atria, causing them to contract. It reaches the AV node and pauses briefly, letting the atria empty. It then travels down the bundle of His and the bundle branches on either side of the septum, and finally through the Purkinje fibres, so the ventricles contract almost together from the apex upwards. Sympathetic signals during exercise speed the SA node up, while vagal signals at rest slow it down. The pathway sets the order; the autonomic nervous system sets the pace.
Read full notes
Read one topic at a time. These notes explain the mechanisms and relationships behind the key facts.
The heart sets its own rhythm
Autorhythmicity is one of the most distinctive properties of cardiac muscle. Skeletal muscle contracts only when a nerve tells it to, so a paralysed limb is silent. Cardiac muscle can generate its own rhythm, which means the heart keeps beating even if its nerves are damaged. This is possible because pacemaker cells have an unstable resting membrane potential: instead of staying steady, the charge slowly drifts until it reaches threshold. The rate of drift determines the intrinsic rate, and it is fastest in the SA node, which is why that node normally leads. Other cells can also fire, but more slowly, and they act as backup pacemakers. This arrangement gives the heart both independence and a safety margin.
The conduction pathway
The pathway exists to make the chambers contract in the right order and at the right time. The SA node fires first and the impulse spreads through the atria, so atrial contraction precedes ventricular contraction. The impulse then reaches the AV node, which slows it, and finally enters the ventricular conduction system, which spreads it rapidly so the ventricles contract as a coordinated unit. The Purkinje fibres are the key to this coordination: they conduct far faster than ordinary cardiac muscle, so the whole ventricular wall is activated almost simultaneously. If conduction is blocked at one point, the chambers it serves may contract late or independently, which reduces pumping efficiency and can be seen on an ECG.
Why the AV node delays
The AV delay is a design feature, not a defect. It ensures that atrial contraction is complete before ventricular contraction begins, so the ventricles are as full as possible when they eject. It also protects the ventricles from fast atrial rhythms. In atrial fibrillation, the atria may fire hundreds of times per minute, but the AV node conducts only a fraction of those impulses, so the ventricles beat at a rate compatible with life. Treatments for fast atrial rhythms often target the AV node for this reason. When the AV node fails to conduct at all, the ventricles depend on a slower escape rhythm, which may be too slow to meet the body’s needs and may require an artificial pacemaker.
Autonomic control of heart rate
The autonomic nervous system is the main external adjuster of heart rate. Sympathetic nerves increase both the rate and the force of contraction, preparing the heart for activity; parasympathetic vagal fibres do the opposite, slowing the rate and reducing the force slightly. At rest, vagal tone dominates, so the resting rate is lower than the intrinsic rate of the SA node. During exercise, vagal activity falls first and sympathetic activity rises, so the rate climbs quickly. This balance is continuous and automatic, which is why heart rate responds within seconds to standing, emotion, pain or exertion. Because the effectors are the pacemaker cells and the myocardium, changes in rate are chronotropic and changes in force are inotropic.
Chemical and clinical influences
Heart rate is a sensitive indicator because so many factors feed into the conduction system. Hormones such as epinephrine and thyroid hormone raise it; fever raises it; cold lowers it; and electrolytes can disturb it or even provoke dangerous rhythms. Many drugs deliberately target conduction, either to slow an excessively fast rhythm or to strengthen contraction. The system can also fail structurally: the SA node may become diseased, the AV node may block, or an abnormal focus may generate impulses from the wrong place, producing an ectopic rhythm. Symptoms depend on whether the resulting rate is too fast, too slow or irregular, because each changes how much blood the heart can pump. Recognising the conduction pathway is therefore the basis for interpreting an ECG and for understanding rate-control treatment.
Explore the diagram
Start with the whole view, then select a numbered structure. Compare the additional illustrations to connect what a structure looks like with what it does.
The conducting pathway
Reduced motion · use the slider to explore each state.
Usual pacemaker near the superior vena cava in the right atrium.
Educational sequence · slowed and simplified, not real-time physiology
Reading the electrical cycle
Reduced motion · use the slider to explore each state.
Atrial depolarisation precedes atrial contraction.
Educational schematic · selected major pathways, not to scale. Motion shows a slowed trace or comparison, not real-time physiology or disease progression.
Connect the illustrations to the key ideas
SA NODE · autorhythmic pacemaker in the right atrium
It fires fastest and therefore sets the normal rhythm.
CONDUCTION PATHWAY · AV node → bundle of His → branches → Purkinje fibres
The AV delay lets the atria empty; Purkinje fibres make the ventricles contract together.
RATE CONTROL · sympathetic speeds; parasympathetic slows
The balance of autonomic activity sets the current rate around the intrinsic rhythm.
Core Concepts Defined
- Autorhythmicity
- The ability of cardiac pacemaker cells to generate impulses without nerve input.
- Pacemaker cell
- A cardiac cell that spontaneously generates an electrical impulse.
- Sinoatrial (SA) node
- The normal pacemaker of the heart, in the right atrial wall.
- Atrioventricular (AV) node
- The junction that delays the impulse and limits how many reach the ventricles.
- Bundle of His
- The conducting bundle that carries the impulse from the AV node to the bundle branches.
- Bundle branches
- Right and left branches running down the interventricular septum.
- Purkinje fibres
- Fast-conducting fibres that spread the impulse through the ventricular walls.
- Internodal pathways
- Conducting routes that carry the impulse through the atria to the AV node.
- Vagus nerve
- The main parasympathetic nerve to the heart, which slows the rate.
- Acetylcholine
- The neurotransmitter released by parasympathetic fibres, which slows heart rate.
- Norepinephrine
- The neurotransmitter released by sympathetic fibres, which raises rate and contractility.
- Chronotropic effect
- A change in heart rate.
- Inotropic effect
- A change in the force of contraction.
- Bradycardia
- An abnormally slow heart rate.
- Tachycardia
- An abnormally fast heart rate.
- Escape rhythm
- A slower rhythm generated by a backup pacemaker when the SA node fails.
Go deeper · worked example
How the impulse sets the beat
- Start at the pacemaker. Pacemaker cells in the SA node leak ions until they reach threshold and fire an impulse on their own, without nerve input.
- Spread through the atria. The impulse travels through atrial muscle and internodal pathways, causing atrial systole and delivering blood to the ventricles.
- Pause at the AV node. The impulse is delayed briefly, so the atria finish emptying before the ventricles contract; the AV node also limits how many impulses pass through.
- Race through the ventricles. The impulse passes down the bundle of His and bundle branches and then through the Purkinje fibres, so the ventricles contract almost together from the apex upwards.
- Adjust the rate. Sympathetic signals during exercise speed the SA node up and strengthen contraction, while vagal signals at rest slow it down; the balance sets the current rate.
Explain it yourself · Explain why the ventricles still contract when the SA node fails.
Reveal a model explanation
Autorhythmicity is not limited to the SA node. Other cells in the conduction pathway, particularly in the AV node and the Purkinje system, can also generate impulses spontaneously; they simply do so more slowly. When the SA node fails, one of these slower pacemakers takes over and produces an escape rhythm. The heart therefore continues to beat, but at a slower rate, which may cause dizziness or fainting. If the rate is too slow or unreliable, an artificial pacemaker can be implanted to restore a normal rhythm.
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OpenStax, Anatomy and Physiology 2e, 19.2 Cardiac Muscle and Electrical Activity; Tortora & Derrickson, Principles of Anatomy and Physiology, 12e, ch. 20. Author review only; medical and French-language expert review pending.
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