The heart's cardiac conduction system can be intrinsic and extrinsic. I am not going to be discussing the cardiovascular structures in detail here, only unless necessary. If you want to learn about the structures of the heart, you can check the cardiovascular posts on my blog to learn more. I will be looking at the electrophysiology of the cardiovascular system.
The heart has the ability to conduct both intrinsic and extrinsic. The heart can depolarize itself without the help of the nervous system being an extrinsic innervation of the heart rate although this is done also. The heart actually can depolarize itself, sending action potentials to other parts of the heart such as the myocardium of the heart, and it is known as Automaticity. The myocardium of the heart has two sides with one having the nodal cells which are non-contractile cells that generate action potentials. These node cells include the SinoAtrial (SA) node, the Attial ventricular node, the Atrial ventricular bundle, the bundle branches where one goes to the right and the other to the left, and the purkinje fibers. The other has contractile cells which have contractile proteins such as actin, myosin, tropomyosin, troponin, and sarcoplasmic reticulum. This side of the myocardium creates the force in the heart.
The nodal cells such as the SA node can be found in the superior region of the right atrium, below the superior vena cava. The SinoAtrial (SA) node is responsible for setting the sinus rhythm. The pace is set to 60 to 80 beats per minute without extrinsic effects. The SinoAtrial (SA) node is connected to the left atrium through the Bachmann's bundle, and electrical potentials are sent through it to depolarize the left atrium. The SA also supplies different parts of the right atria and they make up the internodal pathway which then connects to the Atrioventricular Node (AV node) which runs from the right atrium through the intraventricular septum to the bundle of his where there is a delay of 0.1 seconds to transmit the electrical potentials to the intraventricular septum so it would allow for atrial contraction before ventricular contraction. The AV node has a smaller diameter fiber and less gap junction reducing ion flow from cell to cell. The bundle of his receives action potentials from the AV node, and it sends them to the Right Bundle branch and the Left Bundle branch after which it goes to the Purkinje fibers.
To understand how the action potential works in the heart, it is important to look at the Nodal cells and the contractile cells specifically. Based on cellular connections, the nodal cells connect themselves via gap junctions and they transfer ions to the contractile cells allowing for depolarization. The Nodal Cells are made of up funny sodium channels which are leaky causing the slow flow of sodium into the nodal cells. The Nodal cell membrane potential is not stable but then, it has a potential of around -60mv. As the sodium gets into the cell, the potential starts to become positive and then the T-type calcium channels open when the charge gets to about -55mv allowing calcium to come into the cells causing the cell to become more positive. AT -40mv another channel known as an L-type calcium channel opens causing the increase of calcium in the cells leading to a positive cell and causing the potential to get to +40mv leading to depolarization of the cell. The positive charges (Cations) will cause the gap junctions made of Connexin proteins to connect and the cations move from the nodal cell to the contractile cell. The cells are kept together via Desmosomes. A combination of gap junctions and desmosomes to hold cells are known as intercalated discs
Positive cells then contractile cells which are at a resting membrane potential of -85 to -90 mv. When the positive ion gets into the cells, they cause the potential to increase to threshold potential where the voltage-gated sodium channels open allowing sodium to flow in very fast and leading to an increase in the positivity of the cell. The positive charge goes around the Sarcolemma of the cell. At about +10mv, other channels open such as the calcium calcium channels, and the potassium channel also opens causing the potassium in the cell to leak out leading to a drop in the potential of the cell to 0mv. At 0mv, the calcium channels open again causing an increase in the positive ion in the cell but do not forget that potassium ions are leaving the cells. The cell plateau for 250ms as a result of the stable positive output and input. This is the second phase of the heart's cardiac conduction. Calcium ions can also flow into the contractile cell through the T-tubules and go to the sarcoplasmic reticulum. The calcium bind to ryanodine receptor type 2 and opens the channel to cause the release of calcium ion into the sarcoplasm. The calcium binds to a special protein known as troponins which include actin, tropomyosin, and calcium. Troponin T impedes the myosin from actin but when calcium is present the tropomyosin is removed and the myosin interacts with the actin which leads to contraction and allows the heart to pump. ,
Image Reference
Image 1 || Wikimedia Commons || Cardiac ConductionN