How is noradrenaline and acetylcholine involved in the regulation of heart?
Noradrenaline (also known as norepinephrine) and acetylcholine are two important neurotransmitters that play crucial roles in the regulation of heart function. They exert opposing effects on the heart rate and contractility, working in a balanced manner to maintain normal cardiac activity. Here's an overview of their involvement:
Noradrenaline (Sympathetic Activation):
1. Positive Chronotropic Effect: Noradrenaline increases the heart rate by acting on beta-1 adrenergic receptors in the heart. It directly stimulates the sinoatrial node (SA node), which is the natural pacemaker of the heart, leading to an increased rate of electrical impulse generation.
2. Positive Inotropic Effect: Noradrenaline also enhances the force of heart contractions. It increases myocardial contractility by stimulating beta-1 adrenergic receptors on the heart muscle cells. This results in stronger and more forceful heartbeats.
3. Vasoconstriction: Noradrenaline causes vasoconstriction, or narrowing, of blood vessels in various organs, including the heart. This increased peripheral resistance elevates blood pressure, which in turn can indirectly affect heart function.
Acetylcholine (Parasympathetic Activation):
1. Negative Chronotropic Effect: Acetylcholine slows down the heart rate by acting on muscarinic cholinergic receptors in the SA node. This inhibitory effect decreases the rate of electrical impulse generation, leading to a slower heart rate.
2. Negative Inotropic Effect: Acetylcholine also reduces the force of heart contractions. It negatively influences contractility by stimulating muscarinic cholinergic receptors on the heart muscle cells, resulting in weaker and less forceful heartbeats.
3. Vasodilation: Acetylcholine induces vasodilation, or widening, of blood vessels in various organs, including the heart. This decreased peripheral resistance lowers blood pressure, which in turn can indirectly affect heart function.
The autonomic nervous system, composed of the sympathetic and parasympathetic divisions, controls the release of noradrenaline and acetylcholine, respectively. These neurotransmitters work in a complementary manner to maintain a delicate balance in heart rate and contractility.
During times of stress, physical activity, or increased demand for oxygen and nutrients, the sympathetic system activates the release of noradrenaline, which increases the heart rate and contractility to meet the body's needs. On the other hand, during periods of rest and relaxation, the parasympathetic system promotes the release of acetylcholine, which slows down the heart rate and reduces contractility, allowing the body to conserve energy.
Dysregulation of noradrenaline and acetylcholine signaling can lead to various heart conditions, such as arrhythmias, heart failure, and hypertension. Therefore, understanding their roles in cardiac regulation is crucial for developing effective therapeutic interventions in cardiovascular medicine.