Which neurotransmitters are primarily released by postganglionic fibers of the sympathetic nervous system?

The postganglionic fibers of the sympathetic nervous system primarily release epinephrine and norepinephrine. These neurotransmitters play a crucial role in the body's "fight or flight" response, which prepares the body to react to stressful situations. Norepinephrine acts primarily at adrenergic receptors on target tissues, resulting in various physiological changes such as increased heart rate, dilation of airways, and heightened alertness. Additionally, epinephrine, released from the adrenal medulla into the bloodstream, reinforces and prolongs the effects of norepinephrine.

In contrast, other neurotransmitters mentioned in the options, like serotonin, dopamine, acetylcholine, GABA, histamine, and substance P, serve different functions in various neurotransmitter systems. While acetylcholine is released by preganglionic fibers in the autonomic nervous system and at the neuromuscular junctions, it is not the primary neurotransmitter for postganglionic sympathetic fibers.