Final answer:
The activation of postganglionic sympathetic neurons is triggered by the release of acetylcholine from preganglionic fibers, which then release norepinephrine to stimulate target organs.
Step-by-step explanation:
The stimulation of almost all postganglionic sympathetic neurons is a result of the neurotransmitter acetylcholine (ACh) being released from preganglionic fibers. These fibers release ACh onto nicotinic receptors on the postganglionic neurons, which are ligand-gated ion channels. When ACh binds to these receptors, it results in the depolarization of the ganglia and the activation of the postganglionic neurons.
Subsequently, these activated neurons release norepinephrine onto target organs, inducing a sympathetic response that prepares the body for 'fight or flight' situations. This mechanism is critical for the divergent effects of the sympathetic nervous system during states of stress.