Postganglionic signaling to effector organs in the sympathetic system involves which type of mediators?

• A. Biogenic amines
• B. Acetylcholine
• C. GABA
• D. Glutamate

Answer: (A)

In the sympathetic system, signaling to most effector organs after a sympathetic preganglionic neuron fires is carried by norepinephrine, which is a biogenic amine. This group of transmitters includes catecholamines like norepinephrine and epinephrine, and they act on adrenergic receptors on target tissues. The general rule is sympathetic postganglionic neurons release norepinephrine to modulate heart rate, smooth muscle, and gland activity, making biogenic amines the right classification. There is a notable exception for sweat glands, where acetylcholine is used, but that doesn’t change the overall tendency for the majority of sympathetic efferents. GABA and glutamate are primarily CNS neurotransmitters, not typical peripheral autonomic transmitters, and acetylcholine is the main transmitter for preganglionic sympathetic fibers (and parasympathetic postganglionic fibers), rather than the postganglionic sympathetic mediators to most effectors.