Acetylcholine and the catecholamines ( nor-epinephrine and epinephrine) are the major neurotransmitters in the autonomic nervous system. These are both classified as excitatory. neurotransmitters but the effect of the same neurotransmotter may vary at different target organ dependent on the class ( or sub-class) or combination of types of receptors to which the neurotransmitter binds. Thus acetylcholine binding causes excitation in most cases, but causes inhibition of cardiac muscle activity There are different kinds of acetylcholine, or cholinergic receptors(AChR) and there are even more types of nor-epinephrine and epinephrine (adrenergic) receptors. Cholinergic receptorr classes Cholinergigic receptors are usually divided into two large classes , namely, nicotinic receptors and muscarinic recptors. Nicotinic receptors exist post-synaptically in all autonomic nervous system ganglia (and at neuromuscular junctions). Binding to these receptors causes excitatory activity at postsynaptic autonomic ganglion cells( and at somatic muscle fibers). Muscarinic receptors is the other large division of AChRs. These receptors are located post-synaptically at parasympathetic neuromuscular junctions. Stimulation of these receptors may increase or decrease effector activity. Muscarinc receptors are also located postsynaptically at the neuroeffector junctions of the sympathetic fibers to sweat glands. Neurotransmitter binding to these receptors results in increased sweating.
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The effect o neurotransmitter binding to a target cannot be predicted with confidence except one knows not only the chemical nature of the transmitter or transmitters, but also the nature of the receptors ( or combination of classes of receptors) to which the neurotransmitter(s) are binding. In the autonomic nervous system, the two main neurotransmitters are acetylcholine and norepinephrine. These neurotransmitters bind to several types and sub-types of receptors located on target organs: the nature of the activity produced by the binding depends both on the nature of the neurotransmitter and the subclass and combination, or blend of sub-classes of the receptors. Cholinergic recceptors(AChRs) are divided into nicotinic and muscarinic receptors. The nicotinic type is named after its binding affinity to nicotine from the tobacco plant ( Nicotiania tabacum) and the muscarinic typis named after the binding affinity for a chemical derived from the mushroom species Amantia muscara. Muscarinic receptors are further subdivided into three subclasses: the M1 (CNS) type. Binding usually results in increased activity; the M2 receptors are found in heart muscle, heart pacemaker, and eye. Binding causes decrease in activity, for example ,decrease in heart rate and force of heart contraction. The M3 type , found in glands and smooth muscles ; binding results in actions like pupil constriction in the eye, bronchiolar dilation in lungs, and increased secretion by salivary glands. Nicotinic receptor subclasses are Nn and Nm receptors. Nn subtype is found oi autonomic ganglia and the adrenal medula. Binding increases the amount of neorotransmitter released.The Nm subtype is found at neuromuscular junctions. There are also subdivisions of adrenergic receptors: the alpha subtype mediates excitation in smooth muscles, but the beta subtype of adrenergic receptors mediates inhibition. The alpha receptors are futher subdivided into alpha-1(1a,1b. 1d ....) and alpha-2(2a,2b, 2c....) .Alpha -1 receptors and alpha -2 receptors affect cardiovascular activity. Binding to alpha-1 receptors causes vasoconstriction, while binding to alpha-2 receptors produces vascular muscle relaxation and a lowering of blood pressure. There are three sub-types of adrenergic beta receptors: they are beta-1, beta-2 and beta -3.. Beta-1 receptors are to be found in heart, kidneys and adipose tissue. Binding to these receptors increases heart rate, and force of heartbeat and stimulates secretion of renin from the kidneys. Beta-2 receptors are found in the lungs, sympathetic system targets, and blood vessels to heart and to the liver. Binding of neurotransmitter(NE) produces mainly inhibitorry effects--dilation of blood vessels and of bronchioles, relaxation of smooth muscles of GI, and urinary tracts and of the uterus. Beta -3 receptors are found in adipose tissue. Neurotransmitter binding to these receptors results in lypolysis in fat cells.