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The Sinoatrial Node

Discuss about The Conduction Systems in the Heart.

The cardiac conduction system refers to a group muscle cells located on the walls of the heart and specialized in terms of their functions. The conduction system of the heart is specifically involved in the sending of signals to the heart causing it to contract. This system is composed of the atrioventricular node nodes, sinoatrial (pacemaker) node, Purkinje fibers, bundle branches and bundle of His. The pacemaker initiates the sequence of contraction by enabling the atrial muscles to contract sending the signals to the atrioventricular node, the bundle of His, the bundle branches and finally to the Purkinje fibers making the ventricles contract (Schwitter et al., 2013). At the same time, an electric current is created by this contraction but this can only be seen on an electrocardiogram graph. This electrical system is normally used by doctors to determine whether the heart is active or not. This paper therefore explores the cardiac and their relevance to the cardiac rhythm.

The sinoatrial node

The sinoatrial node is a spindle-shaped structure that contains closely packed cells in a matrix which is fibrous in nature. It is about ten millimeters long, very thick and narrow on the end near the vena atrioventricular node. The sinoatrial node has several postganglionic adrenergic and cholinergic nerve terminals which transmit the impulses (Wahl-Schott et al., 2014). The neurotransmitters stimulate the beta-adrenergic and muscarinic receptors by modulating the sinoatrial nodes discharge. The sinoatrial node produces an electrical stimuli at a regular rate in order to pump blood at the required rate. The rate at which the sinoatrial node contracts depend upon the blood and oxygen needs of the body. Every stimulus which is produced must pass through the myocardial cells which lead to a generation of contractions in atrioventricular node wave-like fashion and is distributed across all atria. In cardiovascular diseases involving the failure of the sinoatrial node, the patients are treated by fixing either a permanent or temporary pacemaker depending on the severity of the disease.

In the atrioventricular node, the stimulus is delayed for some time before proceeding to the next step to allow the atria to get enough time for pumping all the blood inside the ventricles. When the atria empty all the blood, the atrioventricular node found in between the ventricles are closed to allow the atria to be filled up with blood, a point whereby the cardiac stimulus goes through the atrioventricular node as well as the bundle of His to the bundle branches and Purkinje fibers (Vedantham, 2015). Bearing in mind that there are millions of myocardial cells, all these need to contract within a span of less than one second. When the right ventricles contract, the blood flows to the lungs leading to the absorption of oxygen and subsequent release of carbon dioxide.

Atrioventricular Node

The anterior internodal process starts from the anterior margin of the sinoatrial node and proceeds anteriorly to enter the anterior interatrial band (the Bachmann bundle). This band progresses into the left atrium whereby the anterior internodal pathway enters into the superior margin of the atrioventricular node. The Bachmann bundle is large enough and it is said to conduct the cardiac impulse from the right ventricle to the left atrium.

Atrioventricular node

The conserved region of the atrioventricular node is a superficial vessel which is found under the endocardium of the right atrium. This position is actually anterior to the ostium of the coronary sinus and the tricuspid atrioventricular node. In fact, in about three-quarters of the people, the hearts contain an arterial supply to the atrioventricular node, which in turn branches from the right coronary artery. The function of the atrioventricular node is that it is mostly involved in the modulation of the impulse of the atrial so that the blood can be transmitted to the ventricles and lead to coordination of the ventricular and atrial contractions.

Atrioventricular node vagus

It has been observed that cardiac responses to both sympathetic stimulation and bursts of the atrioventricular vagus start and end quickly and slowly respectively. When the responses of the atrioventricular vagus are set and offset in a rapid manner, the bets in the heart are modulated leading to the electrical conduction. The sinus rate can be reduced by the fact that the nodal conduction by the afferent atrioventricular vagus and the peak on the sinus rates occur at the atrioventricular node for varied periods of time. This occurs without tampering with electrical conduction of the afferent atrioventricular vagus or prolonging the time taken for conduction to occur.

Bundle of His

These refer to several cardiac muscle cells which are very specific in their function of conducting electrical impulses. It is involved in conducting electrical signals from the atrioventricular node to the apex of the bundle branches (Lustgarten et al., 2015). The branches then pass the impulse to the Purkinje fibers which enable the contraction of the ventricles at paced interatrioventricular nodes. In case there are abnormalities that affect the cells that conduct impulses in heart, they block such conduction. The classification of such abnormalities is based on the exact location in the heart where the atrioventricular node abnormalities have occurred. If the abnormality is on the right or left side of the bundle of His, this is called the bundle branch block.

Intermodal Conduction

The Purkinje fibers are found in the atrioventricular part of the heart whereby they are involved in the transmission of cardiac impulses in the cardiac cells. This makes the ventricles contract and pumps the blood. In other cases, the Purkinje fibers are located at the base of the ventricles. It is true that the special muscles found in the Purkinje fibers have a better ability to conduct the nerve impulses as compared to the other types of muscles (Sedmera and Gourdie, 2014). This activity occurs in a synchronized fashion and hence the rhythm is made to be consistent. These fibers split into the ventricles and atria so that it can be able to distribute the electrical impulses from the sinoatrial node.

To do this, the Purkinje fibers and the atrioventricular node have several mitochondrial cells and voltage-gated sodium channels and myofibrils. The rate at which these fibers work is solely influenced by the autonomic nervous system. Their ability to produce the action potential although at a slow rate makes them be useful as the last resort especially when the other pacemakers have failed in function. In the circulatory system, the firing of the Purkinje cells is termed as the premature ventricular contraction.

The bundle branches

The bundle branches are also described as the offshoots of the bundle of His. In the conduction of electrical impulses, the bundle fibers transmit the action potential from the bundle of His to the Purkinje fibers. When the bundle branches are abnormal, the conduction of cardiac signals meaning that the depolarization of the ventricles also fails and referred to as the bundle branch block. The causes of bundle block can be caused by the onset of the heart disease as well as faulty surgery of the heart. In this case, the normal pathway for the conduction of electrical impulses cannot occur by use of the normal pathway (Strauss et al., 2013). Therefore this process might occur through muscle fibers which due to their low specialization can result in a slow electrical transmission. Moreover, the direction of the conduction is altered leading to a loss of synchrony hence the depolarization of the ventricles takes longer than usual. 

The heart contractions begin with the sending of electrical impulses from the pacemakers in the right atrium which in turn make the right atrium contract. The effects of this contraction are the pushing of blood to the ventricles through the cardiac atrioventricular node. When the electrical impulses get to the atrioventricular node, they proceed forward to the bundle of His whereby it is distributed across the left and the right bundles and finally proceeds to the Purkinje fibers. The left ventricle then contracts to be the first and the left ventricle follows shortly after. Once the contraction of the two ventricles node has contracted, the blood is pushed to the rest of the body via the aorta. Once this is done, the ventricles relax in preparation for another round of electrical impulse conduction.

Bundle of His

Therefore the conducting system of the heart is majorly specialized for starting up the cardiac impulses whereby they get conducted in a rapid fashion throughout the heart. When the cardiac cycle starts, the contractions are coordinated making the cardiac chambers contract (Nishikawa et al., 2012). Although the atria and the ventricles contract together, the atrial contractions take place being the first. The conducting system of the heart enables it to attain an automatic rhythmic beat. The events in the cardiac cycle need to be coordinated very well for the heart to pump blood in an effective manner. This means that the systemic, as well as the pulmonary blood circulation need to coordinate these processes.

When the sympathetic ganglia are stimulated it reduces the period of the refractory period in the epicardium. Moreover, such stimulation also causes effects to the endocardium of the left ventricle. When measurements are taken at atrioventricular node points, the short refractions are classified according to the epicardial regions. In other cases, the norepinephrine hormone is distributed in a non-uniform manner leading to the non-uniform electrophysiologic effects which are observed. More important is that the concentration of the norepinephrine in the ventricles is higher at the base as compared to the top (Parati and Esler, 2012). It is also important to note that the afferent vagus is at a higher position in the ventricles leading to the atrioventricular node reactions in the myocardial infections.

On the other hand, the left ventricle supplies the blood to all other parts of the body by pumping it through the aorta and coronary arteries. In this case, the atria are filled with blood, the atrioventricular nodes are closed and the ventricles are already empty. At this point, the sinoatrial node is preparing itself to initiate another electrical stimulus and the whole protocol repeats itself. Since the sinoatrial node and the atrioventricular node have only one stimulus, they need to recharge in atrioventricular nodes before they engage in another round of electric transmission. The abnormalities in the function of the sinoatrial nodes are in common in the old patients who have a history of cardiac diseases or diabetes. For instance, in a medical condition like the sinus pause, the sinus code ceases to function temporarily for second or minutes. In lower pacemakers, although the function rate of the heart is maintained, longer than usual pauses can cause dizziness to the affected person.

The Purkinje fibers

The sympathetic activities are regulated by the atrioventricular vagus at specifically the pre and postjunctional regions through the control of the norepinephrine hormone. This is made possible by the inhibition of the cyclic adenosine monophosphate which is involved in the addition of phosphate groups to the muscles of the heart. The alterations in the afferent atrioventricular vagus electrical conduction can cause malfunctions in the circulatory system. Tonic atrioventricular vagal stimulation results in a greater absolute reduction in sinus rate in the presence of tonic background sympathetic stimulation. In contrast, changes in atrioventricular node conduction during sympathetic and atrioventricular vagus stimulation are made up of the summation of the responses of afferent atrioventricular vagus responses to tone in the atrioventricular vagus.

Conclusion

The electrical conduction systems are of paramount importance in the human body. This is because a constant flow of blood from the heart to all other body part and back are essential. The circulatory system is crucial for the supply of oxygen and nutrients to the cells to maintain the normal body functions. This means that all these activities need to be coordinated to maintain health cardiac functions. However, there are some hopes in the medical field because some of these abnormalities can be treated for instance the artificial or permanent pacemakers. Some drugs such as the hydralazine are also used to increase the heart rate especially in patients who do not present with syncope.

Reference List

Lustgarten, D.L., Crespo, E.M., Arkhipoatrioventricular node-Jenkins, I., Lobel, R., Winget, J., Koehler, J., Liberman, E. and Sheldon, T., 2015. His-bundle pacing versus biventricular pacing in cardiac resynchronization therapy patients: A crossover design comparison. Heart rhythm, 12(7), pp.1548-1557.

Nishikawa, K.C., Monroy, J.A., Uyeno, T.E., Yeo, S.H., Pai, D.K. and Lindstedt, S.L., 2012. Is titin a ‘winding filament’? A new twist on muscle contraction. Proceedings of the Royal Society of London B: Biological Sciences, 279(1730), pp.981-990.

Parati, G. and Esler, M., 2012. The human sympathetic nervous system: its releatrioventricular nodence in hypertension and heart failure. European heart journal, pp.ehs041.

Schwitter, J., Kanal, E., Schmitt, M., Anselme, F., Albert, T., Hayes, D.L., Bello, D., Tóth, A., Chang, Y., atrioventricular noden Osch, D. and Sommer, T., 2013. Impact of the Advisa MRI pacing system on the diagnostic quality of cardiac MR images and contraction patterns of cardiac muscle during scans: Advisa MRI randomized clinical multicenter study results. Heart Rhythm, 10(6), pp.864-872.

Sedmera, D. and Gourdie, R.G., 2014. Why do we hatrioventricular node e Purkinje fibers deep in our heart?. Physiological Research, 63, pp.S9.

Strauss, D.G., Loring, Z., Selvester, R.H., Gerstenblith, G., Tomaselli, G., Weiss, R.G., Wagner, G.S. and Wu, K.C., 2013. Right, but not left, bundle branch block is associated with large anteroseptal scar. Journal of the American College of Cardiology, 62(11), pp.959-967.

Vedantham, V., 2015. New Approaches to Biological Pacemakers: Links to Sinoatrial Node Development. Trends in molecular medicine, 21(12), pp.749-761.

Wahl-Schott, C., Fenske, S. and Biel, M., 2014. HCN channels: new roles in sinoatrial node function. Current opinion in pharmacology, 15, pp.83-90.

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