The sinoatrial node produces a regular series of impulses and is called the "pacemaker" of the heart. The sinoatrial node spontaneously produces an impulse for contraction of the atrial myocardium, depolarizes the atrioventricular node, and sends an impulse through the bundle fibers to the ventricular myocardium. In addition to the pacemaker activity of the sinoatrial node, the heart is also under autonomic, or involuntary, control.
The autonomic nervous system is separated into the sympathetic and parasympathetic nervous systems. These two systems send neurons to the same target, but convey opposite effects. In emergency situations, sympathetic nerves travel to the heart and innervate the sinoatrial and atrioventricular nodes to increase the rate and force of contraction. In resting situations, parasympathetic nerves innervate the sinoatrial and atrio-ventricular nodes to slow the heart rate, reduce the force of contraction, and constrict the coronary arteries, thus saving energy.
Both the sympathetic and parasympathetic nerves are composed of a two-neuron pathway. These two neurons meet, or synapse, somewhere in the middle and form a structure called a ganglion ("swelling"). Neurons of the sympathetic nervous system emerge from the spinal cord. They emerge from all eight of the cervical segments and the first five of the thoracic spinal cord segments. These neurons travel laterally just centimeters from the spinal cord before they synapse. All of the neurons to the heart are believed to synapse in only two places: the middle cervical ganglion and the cervicothoracic (fused inferior cervical/first thoracic or stellate "star-shaped") ganglion. Multitudes of fibers then emanate from these ganglia and run to the heart as sympathetic cardiac nerves.
Parasympathetic neurons emerge directly from the brain as part of the vagus nerve or cranial nerve X. The vagus nerve and its branches form the parasympathetic part of the cardiac nerves running toward the heart.
Sympathetic and parasympathetic cardiac nerves interweave. In addition, nerves of the right and left side overlap; altogether, this huge group of common innervation forms the cardiac plexuses. The dorsal cardiac plexus is located posterior to the arch of the aorta near the bifurcation of the trachea. The ventral plexus is located anterior to the aorta. Nerves from the cardiac plexuses extend to the atria and ventricles, the sinoatrial node, the atrioventricular node, the coronary arteries, and the great vessels. It is generally believed that there is sympathetic and parasympathetic innervation of the myocardium that forms a network from the atria to the ventricles. For more details about the role of the autonomic nervous system in the physiological control of the heart, refer to Chapter 10.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.