The heart is a remarkable organ. It works through contracting and relaxing its muscles without tiring or stopping for life sustainability. Contracting of the heart muscles ensures the pumping of blood near and far the body of the organism. However, rhythmic hence relaxing and contracting with every relaxation acting as an intermediary of the next contraction. Contracting of the heart muscles is a result of depolarization. Vermij, Abriel, and Kucera (2019) define depolarization as changes within a cell where the cell becomes negatively charged due to a shift in electrical charge distribution. The parts responsible for generating electric currents in the heart are the sinoatrial (SA) node and the Atrioventricular (AV) node. The SA node, also known as the sinus node, is located on the right atrium. It contains a cluster of muscles responsible for the conduction of electric current generated throughout the heart muscles. The AV node, on the other hand, connects the electrical system of the atria and ventricular. These electrical cells contain imperative properties which ensure the smooth process of depolarization. These properties include; automaticity, excitability, and conductivity. Automaticity is the ability to generate and distribute the electric impulse spontaneously, while excitability connotes that the cell can respond to electric impulses. Conductivity is the ability to conduct impulses from one cell to another.
Before depolarization occurs, the cardiac cells are at rest hence polarized, which means no electrical activity is in process. Therefore, a resting potential must be achieved or established before depolarization. Sodium ions and potassium ions are heavily involved in establishing both resting potential and depolarization. In establishing the resting potential, more positively charged sodium ions are pumped in the exterior of the cell, while less positively charged potassium ions are pumped to the cell's interior. In establishing the resting potential, an environment is created outside the cell favoring depolarization since more sodium ions are outside than the inside of the cell. The concentration of each cation cannot be reversed due to the concentration gradient created since the voltage–gated membrane is kept closed by the resting potential established. Besides, most cells contain negatively charged ions within them, ensuring the cell's interior maintains a negative charge. However, the concentration of potassium-positive charge is in high concentration in the interior.
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Depolarization is possible after establishing resting potential. Due to the automaticity property of the specialized cardiac cells, the SA and AV nodes, electrical impulses are generated. The electrical impulses lead to a change in voltage hence creating a cardiac action potential. The voltage-gated membranes initially closed during resting potential are opened, leading to the rushing in of the sodium ions adding the positive charge to the cell's interior, therefore, changing the membrane potential from negative to positive. When the cell's interior is positively charged, depolarization is complete, and the proteins known as ion channels are closed again. The movement of ions across the protein channels is the contraction that occurs as a wave through the heart muscles (Christensen, 2019).
The SA has its action potential; hence depolarization and contraction occur spontaneously after generating an electric impulse. The impulses cause direct muscle contraction of the right atria and travel to the left atria via the Bachmann's bundle. The impulses then travel to the AV, which leads to the contraction of the ventricles. While the SA sets its heart rate at 60-100 beat per minute (bpm), the AV set its heart rate at 40-60 bpm depending on an individual. These disparities indicate that the AV slows the rate of impulses from the SA. Slowing down the rate of depolarization in the AV is essential and advantageous to the heart's functioning. The benefit is due to delays caused between the contraction of the atria and that of ventricles. Therefore, allowing the atria to depolarize and contract first and then ventricles after that. The turn-taking of atria and ventricles contraction ensures blood flows in the expected direction (Lumen, 2020).
References
Christensen, S. (2019). Anatomy, physiology, and electrophysiology . Home:: Andrews University. https://www.andrews.edu/~schriste/Course_Notes/Anatomy__Physiology__and_Elect/anatomy__physiology__and_elect.html#:
Lumen. (2020). Physiology of the heart | Boundless anatomy and physiology . Lumen Learning – Simple Book Production. https://courses.lumenlearning.com/boundless-ap/chapter/physiology-of-the-heart/#:
Vermij, S. H., Abriel, H., & Kucera, J. P. (2019). Modeling depolarization delay, sodium currents, and electrical potentials in cardiac transverse tubules. Frontiers in physiology , 10 , 1487.
