Autonomic Neurotransmitters

AUTONOMIC NEUROTRANSMITTERS

Two PNS neurotransmitters, acetylcholine and norepi-nephrine, have particular clinical importance. Both are synthesized and stored primarily in the nerve terminals until released by a nerve impulse. It should be noted, to avoid confusion, that in the United States the transmit-ter in the sympathetic nervous system is referred to as norepinephrine and the major adrenal medullary hor-mone is referred to as epinephrine. In Europe and most of the world these two substances are called noradrena-line and adrenaline, respectively.

Neurotransmission in the PNS occurs at three major sites: (1) preganglionic synapses in both parasympa-thetic and sympathetic ganglia, (2) parasympathetic and sympathetic postganglionic neuroeffector junctions, and all somatic motor end plates on skeletal muscle. Acetylcholine is the transmitter released at all of these sites except for the majority of sympathetic neuroeffec-tor junctions. Neurons that release acetylcholine are called cholinergic neurons.

Norepinephrine is the transmitter released at most sympathetic postganglionic neuroeffector junctions. Neurons that release this substance are called adrener-gic or noradrenergic neurons. Not all sympathetic post-ganglionic neurons are noradrenergic. The sympathetic postganglionic neurons that innervate the sweat glands and some of the blood vessels in skeletal muscle are cholinergic; that is, they release acetylcholine rather than norepinephrine, even though anatomically they are sympathetic neurons (Fig. 9.1).

Drugs that mimic the actions of acetylcholine are termed cholinomimetic, and those that mimic epineph-rine and/or norepinephrine are adrenomimetic. The cholinomimetic drugs are also called parasympatho-mimetic drugs. The adrenomimetic drugs are often called sympathomimetic.

The receptors with which acetylcholine and other cholinomimetic drugs interact are called cholinorecep-tors, while the receptors with which norepinephrine, epinephrine, or other adrenomimetic drugs combine are called adrenoceptors. It is common both in textbooks and the scientific literature to see these receptors re-ferred to as cholinergic or adrenergic receptors. This is improper usage of the terms cholinergic and adrenergic, since these terms should be applied only to nerves.

Drugs that antagonize the actions of acetylcholine are known as cholinoreceptor antagonists; those that an-tagonize norepinephrine are known as adrenoceptor an-tagonists.

A number of other substances are released by sym-pathetic and parasympathetic neurons, often the same neurons that release norepinephrine or acetylcholine. These substances include adenosine triphosphate (ATP), neuropeptide Y, and substance P.