 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 pacemaker potentials. The parasympathetic cholinergic fibres of vagus nerve that is 10th cranial nerve inner weight, 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 pacemaker 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 pacemaker 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 and from atrioventricular nodes the acetylcholine slows the excitation of ventricles. This way acetylcholine has three types of acetylcholine, action potential length, number two, atria to ventricular node excitation transfer and number three, atrioventricular node say ventricles that excitation key transmission in tino processes co-slow kati hai as a result it slows the heart rate. Dear students, now we shall discuss the effect of catecholamines on pacemaker potentials. Sino-atrial node, atria, atrioventricular node or ventricles in tamamke under adrenergic nerve fibres innervate kartei hai. Hidu adrenergic nerve fibres hotei hai, they release norepinephrine, which is a catecholamine. Norepinephrine increases the rate of contraction of myocardium. Ye heart rate ko increase kartei hai. It increases heart rate by increasing the conductance of sodium and calcium ions. The sodium and calcium ions accelerate pacemaker 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 tomotropic effect.