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Anatomy & Physiology - Sympathetic or not....

The Sympathetic and Parasympathetic Nerve Affect

           In the previous section on cellular properties, the process by which a cardiac cell can initiate and regulate the process of depolarization and repolarization was discussed. While cardiac cells have the capability of initiating this process without any outside influence, the function of the cells may be influenced by input from the Autonomic Nervous System (ANS).1 The Autonomic Nervous System exerts an influence on the contraction of smooth and cardiac muscles as well as providing a regulatory influence on heart rate, blood pressure, respiratory rate and several other body functions.1

          There are two primary components of the Autonomic Nervous System, the Sympathetic and Parasympathetic Nervous Systems. The Sympathetic portion of the ANS exerts a stimulative effect on cardiac cells while the Parasympathetic influence acts in an opposing manner, inhibiting activity within cardiac cells.1 The balance between these two aspects of the ANS provides an influence on cardiac function that helps the heart rate and contractility of cardiac cells increase or decrease as needed.

          For example, the Sympathetic nervous system triggers the "fight or flight" influence when a danger presents itself.3 The Parasympathetic nervous system provides a calming influence that acts opposite of the sympathetic nervous system. While these two aspects of the ANS work in opposing manners, they are complimentary to each other, providing a regulation that keeps the heart from responding too aggressively or in a manner that is insufficient for the given situation.2

          The interactions of these two systems usually occur in a complimentary manner, though there are occasions where an imbalance of the sympathetic and parasympathetic responses can cause difficulties. Two conditions that may be affected by an out of balance response from the Autonomic Nervous System are Inappropriate Sinus Tachycardia and Neuro-Cardiogenic Syncope.

          Inappropriate Sinus Tach, or IST, is believed to be due to an either an as yet unidentified disorder of the Sinus Node or an imbalance in the regulatory affects of the ANS. This is referred to as Autonomic Dysfunction. Studies suggest that a decrease in vagal tone may be responsible for IST as patient's do tend to show some influences of parasympathetic activity at night when the resting heart rate decreases, but not to levels demonstrated in most patients.3  The result of this disorder is that patients with IST show a higher resting heart rate and an increased response to sympathetic input. Even small amounts of excitement result in a rapid and dramatic rise in heart rate.

          While IST appears to be due to an insufficient response of the parasympathetic response, Neuro-Cardiogenic Syncope is believed to be the result of an overly aggressive influence of the parasympathetic nervous system. Whenever the sympathetic nervous system exerts and influence, the parasympathetic responds to prevent an overly aggressive result. In simple terms, when the heart rate increases due to stimulation that triggers the sympathetic nervous system to respond, the parasympathetic response kicks in to act as a calming influence that prevents the initial response from causing the heart rate to increase too much. In patients with NCS, the response of the parasympathetic system is massive resulting in a decrease in heart rate and blood pressure instead of the increase called for by the sympathetic nervous system. The result is that bradycardia and low blood pressure occur when stimulation occurs. This is the exact opposite of what should happen.

          The interaction of the two branches of the Autonomic Nervous System are complex and varied. This section only address a small component of how the sympathetic and parasympathetic nervous systems interact. It is important to understand that these two influences usually act in concert to help regulate cardiac function and thereby gain a better understanding of what occurs when they do not work together.


(1) Information found in Human Anatomy / Kent M. VanDeGraaff - ISBN 0-697-04743-1
(2) Information found on web site Organization of the Nervous System by John W. Kimball / Online Biology
(3) Information found in Clinical Cardiac Electrophysiology - Techniques and Interpretations / Mark E. Josephson, MD - ISBN 978-0-7817-7739-1

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