Molly Ayala
The heart is a vital organ that pumps oxygenated blood around the body through a process of contraction and relaxation. This process is facilitated by electrical excitation, which is initiated by the SA node. The SA node is the primary pacemaker of the heart, with a normal firing rate of 60 to 100 times per minute. When stimulated, the SA node causes the cell membrane to become receptive to sodium ions, which then rush into the cell, changing its electrical charge and triggering contraction. This process, known as depolarization, is an important aspect of the heart's electrical conduction system and plays a crucial role in maintaining the heart's rhythm.
| Characteristics | Values |
|---|---|
| Process | Electrical excitation |
| Initiated by | SA node |
| Action potential | A rapid sequence of changes in the membrane potential, resulting in an electrical impulse |
| Action potential travels through | Heart's electrical conduction system |
| Action potential causes | Myocardial contraction followed by relaxation |
| Types of cells in the heart | Cardiomyocytes and pacemaker cells |
| Pacemaker cells located in | SA and atrioventricular (AV) nodes, bundle of His and Purkinje fibres |
| Pacemaker cells possess | Automaticity |
| Depolarization phase occurs when | Cardiac ion channels open, allowing positively charged ions (Na+ and Ca2+) to enter the cell |
| Influx of positive ions | Changes the cell's electrical charge, causing it to become more positive |
| Repolarization phase begins when | Potassium channels open, allowing K+ to leave the cell |
| Outflow of positive ions | Restores the cell's resting membrane potential and prepares it for another depolarization |
| Hyperpolarization occurs when | Excess potassium leaves the cell during repolarization, making the inside more negative than its resting state |
| Atrial depolarization | P wave |
| Ventricular depolarization | QRS wave |
| Ventricular repolarization | T wave |
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What You'll Learn
The role of the SA node
The SA node, or sinoatrial node, is a group of specialized cardiac muscle cells known as pacemaker cells. These cells are located in the upper back wall of the right atrium, near the opening of the superior vena cava. The SA node's primary function is to act as the heart's natural pacemaker, creating an excitation signal or action potential that initiates each heartbeat.
The SA node produces electrical impulses known as cardiac action potentials. These action potentials are rapid changes in membrane potential, resulting from the movement of positively charged ions like sodium (Na+) and calcium (Ca2+) into the cell. This influx of positive ions increases the cell's electrical charge, causing it to become more positive in a process known as depolarization.
The SA node's ability to generate these action potentials is influenced by the autonomic nervous system, which includes the sympathetic and parasympathetic nervous systems. During physical activity or stressful situations, the sympathetic nervous system increases the SA node's firing rate, leading to a higher heart rate. Conversely, the parasympathetic nervous system slows down the SA node's firing rate, resulting in a decreased heart rate during rest or digestion.
From the SA node, the depolarization current spreads through the right atrium via gap junctions and passes to the left atrium through Bachmann's bundle. The impulse then reaches the atrioventricular (AV) node, located in the right atrium near the central area of the heart. The AV node introduces a brief delay in the electrical signal, ensuring that the atria empty their blood into the ventricles before contraction occurs.
After passing through the AV node, the depolarization wave continues through the bundle of His, located in the interventricular septum, and eventually reaches the Purkinje fibers in the ventricles. This entire process constitutes the heart's electrical conduction system, which coordinates the contraction and relaxation of the heart, allowing it to pump oxygenated blood throughout the body.
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The role of calcium ions
Calcium ions play a crucial role in the electrical depolarization of the heart, which is a vital process for the heart's rhythmic contraction and relaxation. This process ensures that oxygenated blood is pumped around the body efficiently.
During the Plateau phase of depolarization, voltage-gated L-type calcium channels open, allowing an influx of calcium ions (Ca2+) into the cardiac cells. This influx of positive ions further increases the cell's positive charge, contributing to the depolarization process. The calcium ions also balance the outflow of potassium ions (K+), creating a plateau in the electrochemical potential at around +50mV. This plateau is an essential component of the Effective Refractory Period, during which the heart's cells are readying for the next contraction.
The calcium ions play a dual role in this phase: they stimulate the release of additional calcium from the sarcoplasmic reticulum, and they activate chloride channels, allowing chloride ions (Cl-) to enter the cell. This increased calcium concentration within the cell enhances the activity of sodium-calcium exchangers, which is crucial for maintaining the delicate balance of ions within the cell.
Moreover, calcium ions are essential for muscle contraction. Approximately 20% of the calcium required for contraction is supplied by the influx of Ca2+ during the Plateau phase, while the remaining calcium is released from the sarcoplasmic reticulum. Calcium ions combine with the regulatory protein troponin, removing the inhibition that prevents the formation of cross-bridges between myosin and actin molecules. This mechanism enables the power stroke of contraction, allowing the heart to pump blood effectively.
The influx of calcium ions through the slow calcium channels contributes to the prolongation of the Plateau phase and the Absolute Refractory Period. This prolongation is vital for the proper functioning of the cardiac muscle, ensuring that the heart contracts in a coordinated and synchronized manner.
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The role of potassium ions
Potassium ions also play a role in maintaining the balance of electrical charges within the heart cells. During the Plateau phase, the influx of calcium ions (Ca2+) creates a positive electrochemical potential of around +50mV. The efflux of potassium ions during this phase helps to balance the calcium influx, creating a plateau and preventing a further increase in positive potential. This balance is essential for maintaining the stability and regularity of the heart's electrical activity.
Additionally, potassium ions are involved in the hyperpolarization phase, which occurs when excess potassium ions leave the cell during repolarization. This excess outflow of potassium ions makes the inside of the cell more negative than its resting state. Hyperpolarization can also influence the threshold for the next depolarization event, potentially affecting the timing and regularity of heart contractions.
In summary, potassium ions are essential for the repolarization, hyperpolarization, and overall electrical stability of the heart. They help the heart cells reset, recharge, and prepare for the next contraction. The movement of potassium ions during these phases contributes to the ECG waveform, specifically the T wave, which represents ventricular repolarization. By understanding the role of potassium ions, healthcare professionals can gain insights into the heart's electrical activity and identify potential cardiac issues.
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The impact of polarisation
Polarisation refers to the resting state of heart cells, where they are negatively charged. In this state, the cells are ready to receive electrical signals to initiate contraction. The process of depolarisation involves the influx of positively charged ions, primarily sodium (Na+) and calcium (Ca2+), into the cardiac cells. This rapid shift in ion concentration changes the electrical charge of the cells, making them more positive. Depolarisation is triggered by the SA node, the primary pacemaker of the heart, which has a normal firing rate of 60 to 100 times per minute. The electrical impulse then spreads through the right atrium and passes to the left atrium via Bachmann's bundle.
The SA node plays a crucial role in initiating the action potential, which is a rapid sequence of changes in membrane potential resulting in an electrical impulse. This impulse travels through the heart's electrical conduction system, causing myocardial contraction followed by relaxation in a coordinated manner. The atria and ventricles, composed of specialised heart cells, receive these electrical signals, leading to their contraction. The atria begin contracting around 100 ms after the start of the P wave, which represents atrial depolarisation on an ECG reading.
Following depolarisation, repolarisation occurs when potassium (K+) channels open, allowing K+ to leave the cell. This outflow of positive ions restores the cell's resting membrane potential, returning it to a negative charge and preparing it for the next beat. Repolarisation is vital for the heart's "reset" stage, ensuring it is ready for the next contraction. During this phase, the heart cells recharge, and the ventricles relax and refill with blood.
Any disruption to the process of depolarisation or repolarisation can have significant consequences for the heart's rhythm, potentially leading to arrhythmias or cardiac arrest. Therefore, the impact of polarisation, and its interplay with depolarisation and repolarisation, is essential for maintaining the heart's normal functioning and ensuring the efficient pumping of blood throughout the body.
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The conduction system
The SA node, located in the right atrium, is the primary pacemaker of the heart. It has a normal firing rate of 60 to 100 times per minute and initiates the action potential under normal conditions. The action potential is the rapid sequence of changes in the membrane potential, resulting in an electrical impulse. This impulse then spreads through the right atrium via gap junctions and passes to the left atrium via Bachmann's bundle.
From the SA node, the electrical impulse passes to the AV node, located in the right atrium at the interatrial septum, through the internodal fibres. The AV node acts as a secondary pacemaker with a normal firing rate of 40 to 60 times per minute. The atria and ventricles are electrically isolated, and impulses can only pass between them via the AV node. This conduction delay ensures that ventricular contraction occurs after the atria empty blood into the ventricles.
From the AV node, the depolarisation wave passes through the bundle of His, located in the interventricular septum. The bundle of His splits into the left and right bundle branches, which conduct the impulses down both sides of the septum, causing the septum to depolarise from left to right. This results in the Q wave of the QRS complex on an ECG.
The Purkinje fibres, a special system of muscle fibres in the ventricles, then bring depolarisation to all parts of the ventricles almost simultaneously, causing ventricular contraction. This results in the R and S waves of the QRS complex. The QRS complex represents ventricular depolarisation and is a strong electrical signal due to the mass of ventricular muscle.
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Frequently asked questions
Electrical depolarization is when the heart cells fire off an electrical signal to contract. This occurs when the cardiac ion channels open, allowing positively charged ions (Na+ and Ca2+) to enter the cell.
The primary pacemaker of the heart, the SA node, initiates an action potential as an electrical impulse. This impulse then travels down through the heart's electrical conduction system, causing depolarization and contraction of the heart muscle.
The ECG reading, or electrocardiogram, is an indirect indicator of heart muscle contraction. It measures the sum of all the action potentials from the heart detected on the body's surface. The PQRST waveform is created by the sequence of atrial and ventricular depolarization and repolarization.
Depolarization leads to a decrease in negative charge and causes the heart to contract. Repolarization leads to resting, relaxing, and refilling with blood, and prepares the heart for the next beat.
