Cholinergic and Adrenergic Nerves

Epinephrine (adrenaline), and the closely related norepinephrine (noradrenaline), are the chief neurotransmitters at peripheral sympathetic or adrenergic terminations, whereas acetylcholine is generally associated with parasympathetic, or cholinergic effects. However, in reality, acetylcholine is an important neurotransmitter at synapses in both sympathetic and parasympathetic pathways. Dale’s terms were initially applied only to postganglionic fibers; acetylcholine, in fact, is the chief neurotransmitter at synapses between preganglionic fibers and ganglionic neurons of both the sympathetic and parasympathetic systems.

The illustration shows the sites at which acetylcholine (C) and norepinephrine (A) are the chief neurotransmitters. Other chemical substances, such as adenosine triphosphate (ATP), gamma-aminobutyric acid (GABA), a polypeptide called substance P, histamine, glutamic acid, and prostaglandins have also been implicated as neurotransmitters.

Sympathetic or adrenergic efferent nerve fibers usually elicit active reactions in effector structures, such as smooth (unstriated) muscle or glands, which are the reverse of the diminished activity produced by parasympathetic, or cholinergic, fibers. Thus stimulation of the sympathetic and parasympathetic cardiac nerves produces cardiac acceleration and deceleration. However, these effects are not universal. For example, activity of the alimentary adrenergic nerves produces slowing of gastrointestinal motility; conversely, activity in the cholinergic supply results in acceleration of gastric and intestinal movements. Similar reactions occur in other structures. Thus in the urinary tract, the sympathetic nerves produce relaxation of the bladder wall, and the parasympathetic nerves cause contraction, so the former have been aptly described as “filling” and the latter as “emptying” nerves.

Sweat glands are classified as apocrine or eccrine glands. The apocrine glands open into the lumen of the sweat glands in the axilla, perineum, and periareolar region and are innervated by adrenergic fibers and probably respond to humoral epinephrine. The eccrine sweat glands open directly into the skin and are innervated by sympathetic postganglionic fibers that are cholinergic. Eccrine glands are one of the most important skin appendages and play a vital role in temperature regulation.

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