Cardiac Muscle Conduction
The heart can generate and conduct electrical impulses without direct stimulation from the nervous system. This property is called autorhythmicity. These impulses coordinate atrial and ventricular contraction so that the heart pumps blood efficiently.
Sinoatrial Node
The impulse normally begins in the sinoatrial node (SA node), located in the right atrial wall near the opening of the superior vena cava. Because it generates impulses faster than the other conducting tissues, it sets the normal heart rhythm and is called the natural pacemaker. Its usual rate is about 60–100 impulses per minute.
The impulse spreads through both atria, producing atrial depolarization and contraction, and travels towards the AV node through the internodal pathways.
Atrioventricular Node
The atrioventricular node (AV node) lies in the lower interatrial septum near the tricuspid valve. Conduction through it is slow, producing a brief AV nodal delay. This allows the atria to finish contracting and the ventricles to fill before ventricular contraction begins.
Bundle of His and Bundle Branches
The impulse then enters the atrioventricular bundle, or bundle of His. The fibrous tissue separating the atria and ventricles does not normally conduct electrical impulses, making the bundle of His their normal electrical connection. It passes through the interventricular septum and divides into the right and left bundle branches.
Purkinje Fibres
The bundle branches conduct the impulse towards the apex and divide into Purkinje fibres. These fibres rapidly distribute the impulse throughout the ventricular muscle. Ventricular activation begins near the apex and spreads upwards, pushing blood efficiently towards the pulmonary trunk and aorta.
SA node → Atrial muscle → AV node → Bundle of His → Right and left bundle branches → Purkinje fibres → Ventricular muscle
Conduction Between Cardiac Muscle Cells
Cardiac muscle cells are joined by intercalated discs. Their gap junctions allow electrical current to pass rapidly from one cell to the next. The atrial cells therefore contract together as one functional unit, while the ventricular cells contract together as another. This coordinated arrangement is called a functional syncytium.
Other Pacemaker Sites
If the SA node fails, other parts of the conduction system can generate impulses. The AV node usually discharges at about 40–60 impulses per minute, while the Purkinje system produces about 15–40 impulses per minute. These slower sites are called latent or subsidiary pacemakers.
Autonomic Regulation
Sympathetic stimulation
Increases heart rate and conduction velocity.
Parasympathetic stimulation
Reduces heart rate and slows conduction through the AV node.
Heart block may occur when conduction from the atria to the ventricles is impaired.
Ectopic pacemakers can produce abnormal rhythms called arrhythmias.
Artificial pacemakers may be required when the natural conduction system cannot maintain an adequate heart rate.
Present the conducting structures in their correct sequence. Remember that the SA node sets the rhythm, the AV node delays the impulse, and the Purkinje fibres rapidly activate the ventricles.