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The heart's activity is dependent on the electrical impulses from the sinoatrial (SA) node, and the atrioventricular (AV) node, which form the intrinsic conduction system of the heart. The SA and AV nodes act as a pacemaker for the heart, and determine the rate at which the heart beats, even without signals from the larger nervous system of the human body. The SA and AV nodes initiate the electrical impulses that cause contraction within the atria and ventricles of the heart.
The SA node is a bundle of nerve cells located on the outer layer of the right atria. The SA node nerve cells are specialized in that they undergo spontaneous depolarization and generation of action potentials, without stimulation from the rest of the nervous system. The SA node nerve impulses travel through the atria and cause muscle cell depolarization and contraction of the atria directly. The SA node stimulates the right atria directly, and stimulates the left atria through the Bachmann's bundle. The SA node impulses also travel to the AV node, which stimulates ventricular contraction.
The SA node generates its own action potentials, but may be influenced by the autonomic nervous system. Without autonomic nervous stimulation, the SA node will set the heart rate itself, acting as the primary pacemaker for the heart. The SA node fires to set a heart rate in a range of 60-100 beats per minute (bpm), a normal range that varies from person to person.
The AV node is a bundle of conducting tissue (not formally classified as nerve tissue) located at the junction between the atria and ventricles of the heart. The AV node receives action potentials from the SA node, and transmits them through the bundle of His, the left and right bundle branches, and purkinje fibers, which cause depolarization of ventricular muscle cells, leading to ventricular contraction. The AV node slows the neural impulse from the SA node by a slight amount, which causes a delay between depolarization of the atria and the ventricles.
The normal firing rate in the AV node is lower compared to the SA node because it slows the rate of neural impulses. Without autonomic nervous stimulation, it sets the rate of ventricular contraction at 40-60 bpm. Certain types of autonomic nervous stimulation will alter the rate of firing in the AV node. Sympathetic nervous
stimulation will still increase heart rate, while parasympathetic
nervous stimulation will decrease heart rate, by acting on AV node.
Source: Boundless. “Electrical Events.” Boundless Anatomy and Physiology. Boundless, 08 Aug. 2016. Retrieved 25 Aug. 2016 from https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/cardiovascular-system-the-heart-18/physiology-of-the-heart-175/electrical-events-876-9185/