INNERVATION OF THE HEART
Parasympathetic fibers primarily innervate the atria and conducting tissues. Acetylcholine acts on spe-cific cardiac muscarinic receptors (M2) to produce negative chronotropic, dromotropic, and inotro-pic effects. In contrast, sympathetic fibers are more widely distributed throughout the heart. Cardiac sympathetic fibers originate in the thoracic spi-nal cord (T1–T4) and travel to the heart initially through the cervical (stellate) ganglia and from the ganglia as the cardiac nerves. Norepinephrine release causes positive chronotropic, dromotropic, and inotropic effects primarily through activation of β1-adrenergic receptors. β2-Adrenergic receptors arenormally fewer in number and are found primarily in the atria; activation increases heart rate and, to a lesser extent, contractility.
Cardiac autonomic innervation has an apparent sidedness, because the right sympathetic and rightvagus nerves primarily affect the SA node, whereas the left sympathetic and vagus nerves principally affect the AV node. Vagal effects frequently have a very rapid onset and resolution, whereas sym-pathetic influences generally have a more gradual onset and dissipation. Sinus arrhythmia is a cyclic variation in heart rate that corresponds to respira-tion (increasing with inspiration and decreasing during expiration); it is due to cyclic changes in vagal tone.