The heart receives nerve signals from the vagus nerve and from nerves emanating from the sympathetic trunk. These actions influence, but do not control, the heart rate. Sympathetic nerves also contribute to the force of heart contraction. [30] These signals, delivered by these nerves, originate from two heart centers in the political bulb or medulla oblongata. The vagus nerve transmits signals from the parasympathetic autonomic nervous system and decreases the heart rate, while the sympathetic trunk nerves increase this rate.[9] These nerves form a network of nerves located above the heart called the cardiac plexus.[9][29]
The vagus nerve is a long nerve that emerges from the brainstem and provides parasympathetic innervation to a large number of organs in the chest and abdomen, including the heart.[31] While the sympathetic nerves of the heart emerge from the sympathetic trunk of the thoracic nodes from the first to the fourth, and then migrate to the sinoatrial and atrioventricular nodes, as they innervate the atria and ventricles. The ventricles are innervated by sympathetic innervation more than by parasympathetic innervation. Sympathetic stimulation leads to the release of the neurotransmitter norepinephrine (also called noradrenaline) at the neuromuscular junction of the cardiac nerves, which leads to a decrease in the repolarization period, thus accelerating the rate of depolarization and contraction, which together leads to an increase in the heart rate. This stimulation opens chemical or gated channels for calcium and sodium ions, allowing an influx of positively charged ions.[9] Norepinephrine binds to beta-1 receptors.[9]
