Postganglionic neurons in the parasympathetic division of the autonomic nervous system release at their synapses.

In the parasympathetic division of the autonomic nervous system, postganglionic neurons primarily release acetylcholine (ACh) at their synapses. This neurotransmitter is responsible for transmitting signals to target organs, leading to various involuntary responses such as decreased heart rate, increased digestive activities, and relaxation of muscles. The release of acetylcholine plays a crucial role in the 'rest and digest' functions associated with the parasympathetic system, contrasting the fight or flight responses elicited by the sympathetic division. Acetylcholine acts on muscarinic receptors in the target tissues, resulting in the modulation of organ functions.