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Chapter 5: Neurotransmission

A 42-year-old man has just been prescribed a new drug. After several doses he notices dry mouth, dry eyes, and a rapid heart rate. This is most likely due to an inhibition of which of the following neurotransmitter:

a. Norepinephrine

b. Serotonin

c. Glutamate

d. Acetylcholine

e. Epinephrine

Answer is d. ACh is the predominant neurotransmitter of postganglionic parasympathetic nerves. A cholinergic antagonist acts on muscarinic receptors to produce the side effects noted in this patient (see Figures 5-1 and 5-2, and Table 5-1).


Figure 5-1. The autonomic nervous system. Schematic representation of the autonomic nerves and effector organs based on chemical mediation of nerve impulses. Blue, cholinergic; grey, adrenergic; dotted blue, visceral afferent; solid lines, preganglionic; broken lines, postganglionic. In the rectangle at the right are shown the finer details of the ramifications of adrenergic fibers at any 1 segment of the spinal cord, the path of the visceral afferent nerves, the cholinergic nature of somatic motor nerves to skeletal muscle, and the presumed cholinergic nature of the vasodilator fibers in the dorsal roots of the spinal nerves. The asterisk (*) indicates that it is not known whether these vasodilator fibers are motor or sensory or where their cell bodies are situated.


Figure 5-2. Schematic representation of the somatic motor nerves and the efferent nerves of the autonomic nervous system. The principal neurotransmitters, acetylcholine (ACh) and norepinephrine (NE), are shown in grey. The receptors for these transmitters, nicotinic (N) and muscarinic (M) cholinergic receptors, α and β adrenergic receptors are shown in grey rectangles to the right. The somatic nerves innervate skeletal muscle directly without a ganglionic relay. The autonomic nerves innervate smooth muscles, cardiac tissue, and glands. Both parasympathetic and sympathetic systems have ganglia where ACh is the transmitter of the preganglionic fibers; ACh acts on nicotinic receptors on the postganglionic nerves. ACh is also the neurotransmitter at cells of the adrenal medulla, where it acts on nicotinic ACh receptors to cause release of the catecholamines epinephrine (Epi) and NE into the circulation. Epi represents ~80% of the released catecholamines. ACh is the predominant neurotransmitter of postganglionic parasympathetic nerves and acts on muscarinic receptors. NE is the principal neurotransmitter of postganglionic sympathetic nerves, acting on α or β adrenergic receptors. Note that somatic nerves form a specialized synaptic junction, termed the motor end plate. Autonomic nerves form a more diffuse pattern with multiple synaptic sites. The ganglia in the parasympathetic system are near or within the organ being innervated with generally a one-to-one relationship between pre- and postganglionic fibers. In the sympathetic system, the ganglia are generally ...

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