 Dear students, in this topic, we shall discuss the role of acetylcholine and catechol amines on excitation. Dear students, first we shall study the effect of acetylcholine on paste maker potentials. The parasympathetic cholinergic fibres of vagus nerve that is 10th cranial nerve. Innovate the sinoatrial and atrioventricular nodes. From these cholinergic fibres, acetylcholine is released. Acetylcholine slows the heart rate. This action of acetylcholine is known as negative chronotropic effect. Acetylcholine produces this effect by increasing the potassium ion conductance of paste maker cells. The flow of potassium ions keeps the membrane potential near the potassium equilibrium potential for a longer time. That is why the actions of acetylcholine slow paste maker depolarization. So the interval between the action potentials is increased. Acetylcholine also reduces the velocity of conduction of excitation from atria to the atrioventricular node. This way acetylcholine has three types of action potential length. Number two, atria to ventricular node excitation transfer. And number three, atrioventricular node excites the ventricles. As a result, it slows the heart rate. Dear students, now we shall discuss the effect of catecholamines on paste maker potentials. Sino-atrial node, atria, atrioventricular node, or ventricles. Adrenergic nerve fibers, they release norepinephrine, which is a catecholamine. Norepinephrine increases the rate of contraction of myocardium. It increases heart rate by increasing the conductance of sodium and calcium ions. The sodium and calcium ions accelerate paste maker depolarization. Norepinephrine also increases the force of contraction of myocardium. This effect is known as positive ionotropic effect. It also increases the speed of conduction of wave of excitation over the heart. This effect of norepinephrine is known as positive dromotropic effect.