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Thursday, September 8, 2016

The Nervous System and its Role in the Cardiac Cycle

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The heart is a magnificent creation that has the ability to mostly govern itself, under normal circumstances. The cardiac conduction system and SA Node are usually the primary, autonomous controllers, of the cardiac cycle. But sometimes the heart needs some input from outside sources. The nervous system plays very important roles in assisting with the management the cardiac cycle. The two primary components of the nervous system involved in the cardiac cycle are the parasympathetic and sympathetic portions of the nervous system (Shier, Butler, & Lewis, 2016).

The parasympathetic nerve fibers transmit signals, with the help of the medulla oblongata and vagus nerves. When these signals reach their destination, acetylcholine is released which ultimately, decreases the heart rate. The parasympathetic actions primarily are involved with decreasing the heart rate but can play a dual role and initiate both heart rate increase and decrease, depending on the situation. The vagus nerves play a role within the parasympathetic system to assist with increasing heart rate. The parasympathetic system initiates actions that are much faster than sympathetic system actions because they operate in a wider range of heart rate variability frequencies. The parasympathetic system can operate in low frequency and high frequency modes, where the sympathetic system is restricted to the low frequency variability band (Stauss, 2003).

Generally opposite of the actions of the parasympathetic system, the sympathetic system’s primary function assists with increasing the heart rate. It does this by increasing the rate of slow diastolic depolarization and by initiating the release of norepinephrine. These actions result in an increased rate and force of heart contractions. Research has shown that the sympathetic system primarily operates in the low frequency and very low frequency variability bands. Further research implicates the sympathetic system as a role player in instances of heart failure and sudden cardiac death. The sympathetic system enhances the autonomous actions of the heart while the parasympathetic system slows things down, or inhibits automatic actions of the heart (Sztajzel, 2004).

The parasympathetic system and sympathetic system need a check and balance system to maintain the proper balance between activities that increase and decrease heart rate. This is done through baroreceptor reflexes and the cardiac control center located within the medulla oblongata in the brain. Blood pressure is controlled this way from baroreceptors that are sensitive to the stretching of major blood vessels such as the aorta and carotid arteries. Signals are sent to either the parasympathetic system or sympathetic system to cause the desired response in heart rate which affects blood pressure. Research has indicated that these systems are slightly different between men and women. Women appear to have a slightly altered sympathetic system with minor differences that try to fight off hypertension. The nervous system plays a critical role in assisting the cardiac conduction system with the cardiac cycle. Any number of variables can alter or interfere with this complex system, resulting in a variety of medical issues, such as congestive heart failure or sudden cardiac death. The complexity of the nervous system and cardiac conduction system and their interactions, are truly amazing, as they join forces to keep the cardiac cycle operating at optimal levels (Hinojosa-Laborde, Chapa, Lange, & Haywood, 1999).

References:

Shier, D., Butler, J., & Lewis, R. (2016). Hole’s human anatomy and physiology (14th ed.). New York, NY: McGraw-Hill.

Stauss, H. (2003). Heart Rate Variability. American Journal of Physiology. Retrieved from http://ajpregu.physiology.org/content/285/5/R927.short

Sztajzel, J. (2004). Heart Rate Variability: Noninvasive Electrocardiographic Method to Measure the Autonomic Nervous System. Swiss Medical Weekly. Retrieved from http://www.firstbeattechnologies.com/userData/firstbeat/download/Heart-rate-variability_a-noninvasive-electrocardiographic-_2004.pdf

Hinojosa-Laborde, C., Chapa, I., Lange, D., & Haywood, D. (1999). Gender Differences in Sympathetic Nervous System Regulation. Clinical and Experimental Pharmacology and Physiology. Retrieved from http://onlinelibrary.wiley.com/doi/10.1046/j.1440-1681.1999.02995.x/full

Photo Credit: https://www.pinterest.com/pin/318840848591062869/

Eric Dempsey
MS, ISSA Master Trainer
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