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Postganglionic neurons are a type of neuron found in the autonomic nervous system, which controls involuntary bodily functions such as heart rate, digestion, and breathing. These neurons are located in autonomic ganglia, which are clusters of nerve cell bodies outside the central nervous system. Postganglionic neurons receive signals from preganglionic neurons, which originate in the brainstem or spinal cord, and then transmit these signals to effectors, such as smooth muscles, cardiac muscles, or glands.

The autonomic nervous system is divided into two main branches: the sympathetic and parasympathetic systems. Postganglionic neurons in the sympathetic system typically release the neurotransmitter norepinephrine, which acts on adrenergic receptors to produce effects such as increased heart rate and dilated pupils. In contrast, postganglionic neurons in the parasympathetic system usually release acetylcholine, which acts on muscarinic receptors to produce effects such as slowed heart rate and increased digestion.

Postganglionic neurons play a crucial role in the body's response to stress and other external stimuli. For example, during the "fight or flight" response, the sympathetic nervous system activates postganglionic neurons to prepare the body for action. Conversely, during periods of rest and digestion, the parasympathetic nervous system activates postganglionic neurons to promote relaxation and nutrient absorption. The balance between these two systems is essential for maintaining homeostasis in the body.