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One route from brain to heart is via the sympathetic nervous system, and another is via the vagus nerves of the parasympathetic nervous system.
In the sympathetic pathway, signals from the brainstem travel down the spinal cord and exit via nerves from the cervical to upper thoracic level. Signals travel a two-neuron pathway through a cardiac plexus in the thorax and cardiac nerves to the heart. These nerve fibers end on the sinoatrial (SA) and atrioventricular (AV) nodes of the heart and the muscle (myocardium) of the atria and ventricles. They increase the speed and force of the heartbeat.
The right and left vagus nerves arise from the medulla oblongata of the brainstem, pass through the cardiac plexus, synapse with short neurons in the heart itself, and then those neurons terminate in the SA node (right vagus nerve) and AV node (left vagus); they do not supply the myocardium. They slow the heart rate and antagonize the sympathetic effects.
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