Autorhythmic fibers are specialized cardiac cells that spontaneously fire and signal contractile fibers to contract. Rather than the steady negative membrane potential common to most cells, autorhythmic fibers have a pacemaker potential. The membrane potential gradually becomes less negative. When the membrane potential reaches threshold, an action potential fires. This constantly rising pacemaker potential causes threshold to be released and an action potential to fire over and over again.
The SA (sinoatrial) node sets the heart rate. It sends electrical signals through the atria. Nonconductive fibrous tissue separates the atria and ventricles so that the electrical signal must pass through the atrioventricular (AV) node and into the AV bundle of His. These conducting fibers pass through the interventricular septum to the Purkinje fibers that conduct the action potential from the apex of the heart toward its base, squeezing blood up and out.
Heart rate is controlled by parasympathetic and sympathetic innervation and circulating hormones. The parasympathetic vagus nerve slows the heart rate. Its receptors are cholinergic (acetylcholine is the neurotransmitter). Increased heart rate up to about 100 bpm is mostly due to decreased vagal tone. Sympathetic innervation and circulating catecholamines (epinephrine) increase heart rate. In both cases, changes in heart rate are due to altered pacemaker potentials.
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