Trystan Casey
The electrical system of the heart is critical to its functionality. The heart generates its own electrical signal, which is produced by a tiny structure known as the sinus node, located in the upper portion of the right atrium. The electrical impulse travels from the sinus node to the atrioventricular (AV) node, where it is slowed down for a fraction of a second, and then continues down the conduction pathway into the ventricles. This electrical signal controls the electrical impulses that cause the heart to beat and their conduction, which organises the beating of the heart. The strength of these electrical signals can vary, and in some cases of heart block, they may be weakened or even completely blocked. These variations in heart rhythm are called arrhythmias and can be treated with an automated external defibrillator (AED).
| Characteristics | Values |
|---|---|
| Normal heart rate at rest | 60-100 beats per minute |
| Heart rate during exercise | Significantly higher than 60 beats per minute |
| ECG voltage sensitivity | 0.1mv/mm |
| P-R interval | 0.12-0.2 seconds |
| QRS complex duration | Less than or equal to 0.1 seconds |
| Q-T interval | Less than or equal to 0.44 seconds |
| SA node size | 15mm x 4mm |
| AV node size | 5mm x 5mm |
| Conduction disorder severity | First, second, and third-degree heart block |
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What You'll Learn
The sinus node, or SA node, acts as the heart's pacemaker
The heart's electrical system is critical to its function. It controls the electrical impulses that cause the heart to beat, thereby organising its beating. The sinus node, or SA node, acts as the heart's pacemaker.
The SA node is a cluster of myocytes with pacemaker activity. These cells can spontaneously generate an electrical impulse, which is then transmitted by perinodal cells, or transitional (T) cells, to the right atrium and then through the rest of the heart's electrical conduction system. This results in myocardial contraction and blood distribution to the rest of the body. The sinus node continuously generates electrical impulses, setting a healthy heart's normal rhythm and rate. The rate of action potentials produced, and therefore the heart rate, is influenced by the nerves that supply it.
The SA node is located in the upper portion of the right atrium, which is one of four chambers and related valves in the heart. The right atrium is located at the top of the heart, with two ventricles at the bottom. The SA node is approximately 15 mm long, 3 mm wide, and 1 mm thick, and is located directly below and to the side of the superior vena cava.
The SA node initiates an action potential that results in an electrical impulse travelling through the heart's electrical conduction system, causing myocardial contraction. The electrical impulse originates in the sinus node and spreads across the right atrium and the left atrium, causing both atria to contract. This is referred to as atrial depolarisation, an action that pushes blood into the right and left ventricles, the bottom two chambers of the heart. The sinus node signal also controls electrical conduction as it "spreads" across the heart, causing the cells of the heart muscle to contract in the correct sequence, and ensuring regular, efficient, and coordinated heartbeats.
The heart rate can adjust higher or lower to meet the body's needs. For example, during exercise, when the body requires more oxygen, signals from the body cause the heart rate to increase significantly to deliver more blood and oxygen to the body.
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Electrical signals spread across the heart's chambers
The human heart is a pump made up of muscle tissue. Like all muscles, the heart requires a source of energy and oxygen to function. The heart's pumping action is controlled by an electrical conduction system that coordinates the contraction of the various chambers of the heart. The cardiac electrical system is essential to cardiac function, controlling the heart rate and the contraction of cardiac muscle.
The electrical system of the heart controls the electrical impulses that cause the heart to beat and their conduction, which organises the beating of the heart. The movement of electrical signals across the heart is what is traced on an electrocardiogram (EKG). The EKG also allows irregularities in the heart's electrical system and any related symptoms and medical conditions to be assessed.
The electrical impulse originates in the sinus node, also known as the sinoatrial (SA) node. This is a small mass of specialised tissue located in the right upper chamber (atrium) of the heart. The sinus node generates an electrical stimulus regularly, 60 to 100 times per minute under normal conditions. The SA node is the heart's natural pacemaker, and the rate at which these cells send out electrical signals determines the rate at which the entire heart beats.
From the sinus node, the electrical impulse spreads across the right atrium and the left atrium (the top two chambers of the heart), causing both atria to contract. This is referred to as atrial depolarisation, which pushes blood into the right and left ventricles (the bottom two chambers of the heart). As the electrical impulse passes through the atria, it generates the "P" wave on the EKG.
The atrioventricular (AV) node, located near the central area of the heart, delays the SA node's electrical signal by a fraction of a second. This delay ensures that the atria are empty before the contraction stops. The bundle of His is a branch of nerve cells that extends from the AV node. This bundle receives the electrical signal from the AV node and carries it to the Purkinje fibres, which send electrical signals to the heart's right and left ventricles. When the electrical signal reaches the ventricles, they contract, and blood flows from the right ventricle to the pulmonary arteries and from the left ventricle to the aorta.
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Electrocardiograms (EKGs) can trace electrical signals
The heart's electrical system controls the electrical impulses that cause the heart to beat and their conduction, which organises the beating of the heart. The electrical signal is produced by a tiny structure known as the sinus node, located in the upper portion of the right atrium. The sinus node signal controls electrical conduction as it "spreads" across the heart, causing the cells of the heart muscle to contract in the correct sequence, and ensuring regular, efficient, and coordinated heartbeats.
The EKG moves at a speed of 25 mm/sec. Time is plotted on the x-axis and voltage on the y-axis. On the x-axis, 1 second is divided into five large squares, each representing 0.2 seconds. Each large square is further divided into five small squares of 0.04 seconds each. The EKG machine is calibrated so that an increase of voltage by one mVolt should move the stylus by 1 cm.
The EKG waveform represents atrial depolarisation, ventricular depolarisation, and ventricular repolarisation. The P-R Interval is taken from the start of the P wave to the start of the QRS complex, which represents atrial depolarisation. The Q-T interval is taken from the start of the QRS complex to the end of the T wave and represents the time taken to depolarise and repolarise the ventricles. The S-T segment is the period between the end of the QRS complex and the start of the T wave, during which all cells are normally depolarised.
The EKG allows irregularities in the heart's electrical system and related symptoms and medical conditions to be assessed.
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Heart block can disrupt electrical signals
The electrical system of the heart is critical to its functioning, controlling the electrical impulses that cause the heart to beat. The movement of electrical signals across the heart can be traced on an electrocardiogram (EKG). The EKG also allows for the assessment of irregularities in the heart's electrical system and related medical conditions.
The heart generates its own electrical signal, which is produced by a tiny structure known as the sinus node, located in the upper portion of the right atrium. The sinus node is often referred to as the heart's "natural pacemaker" as it controls electrical conduction, causing the cells of the heart muscle to contract in the correct sequence, ensuring regular, efficient, and coordinated heartbeats.
Heart block is a conduction disorder in which the heart's electrical signals are disrupted as they travel from the atria (upper chambers) to the ventricles (lower chambers). This interference prevents the atria from communicating with the ventricles, resulting in irregular or missed heartbeats. In most cases, the electrical signals are weakened but do not stop completely. Heart block can range from mild to severe, with three distinct degrees of severity.
First-degree heart block is the least severe form, characterised by a slowing of electrical signals as they move through the AV node, resulting in a slower heart rate. This type of heart block may not cause any noticeable symptoms and often does not require treatment. Second-degree heart block is more severe, with some electrical impulses reaching the ventricles while others are blocked. This can cause the heart to beat slowly and irregularly, and symptoms may include chest pain and fainting. The third-degree heart block is the most severe, with a complete blockage of electrical signals from the atria to the ventricles. This significantly impacts the heart's ability to pump blood, leading to a slow pulse or even the absence of a pulse.
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Catecholamines can cause the heart to beat faster
The human heart's electrical system is critical to its functioning. The heart generates its own electrical signal, which is produced by a tiny structure known as the sinus node, located in the upper portion of the right atrium. The electrical impulse originates in the sinus node and spreads across the right and left atria, causing them to contract and push blood into the ventricles. This process is known as atrial depolarization. The electrical signal then passes through the atrioventricular node (AV node), a layer of fibrous tissue that separates the atria from the ventricles. This electrical conduction is controlled by the sinus node, which ensures regular, efficient, and coordinated heartbeats, earning it the name of the heart's "natural pacemaker."
Catecholamines are neurotransmitters that play a crucial role in the body's response to stress and are responsible for the fight-or-flight response. When an individual experiences stress, the adrenal glands release catecholamines such as adrenaline and noradrenaline into the bloodstream. This release increases the heart rate and blood pressure, ensuring that more oxygen-rich blood is delivered to the body. Unlike the instantaneous and short-lived signal sent by the sympathetic nervous system, catecholamines like norepinephrine can increase the heart rate for several minutes or more.
High levels of catecholamines have been linked to various pathologies and conditions. For example, excessively high catecholamine loads can cause myocardial damage in otherwise healthy hearts. Catecholamines have also been implicated in stress-induced heart disease. Under stressful conditions, high concentrations of catecholamines can become oxidized, forming aminolutins and generating oxyradicals. These oxidation products can induce abnormalities in calcium handling, leading to coronary spasms, arrhythmias, and cardiac dysfunction. Additionally, catecholamines have been associated with worsening prognoses in patients with heart failure, as well as increased sensitivity in patients with cardiomyopathy.
While catecholamines typically function to increase heart rate and facilitate the body's response to stress, abnormal catecholamine levels can be indicative of underlying health issues. For example, neuroendocrine tumors, such as pheochromocytomas, can produce high levels of catecholamines. Neuroblastomas, the most common cancer in infants, can also cause increased catecholamine levels. Doctors may test an individual's catecholamine levels through blood or urine tests to check for the presence of these tumors or to assess for potential health concerns associated with abnormal catecholamine levels.
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Frequently asked questions
The electrical signal in the heart is what causes the heart's chambers to contract and pump out blood. The signal is generated by a tiny structure known as the sinus node, located in the upper portion of the right atrium.
The electrical signals in the heart are strong enough to cause the heart's chambers to contract and pump out blood. The strength of the electrical signals can be measured by a test called an electrocardiogram (EKG).
The strength of the electrical signals in the heart can be affected by various factors, including age, stress, and physical activity. For example, during exercise, the body requires more oxygen, and signals from the body cause the heart rate to increase to deliver more blood and oxygen to the body.
