Regulation of Cardiac Activity

Even though the human heartbeat is myogenic and self-generated, its rate and strength are constantly adjusted by three cooperating control systems: the nervous system, the endocrine system, and local paracrine signals tied to a tissue's own metabolic activity, all acting together to change arteriole diameter and blood flow as the body's needs shift. Neural control comes through the autonomic nervous system, which has two opposing arms. The sympathetic arm releases nor-epinephrine from its neurons and epinephrine from the adrenal medulla; both hormones bind beta-adrenergic receptors on the heart and speed the heart rate up. The parasympathetic arm, carried mainly by the vagus nerve (which supplies the atria and specifically the SA and AV nodes), releases acetylcholine, which binds muscarinic receptors and slows the heart rate down. Hormonal control adds a further layer: vasopressin and angiotensin II (both linked to kidney function) cause vasoconstriction, narrowing vessels and raising resistance, while natriuretic peptide has the opposite effect, promoting vasodilation. To …