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The parasympathetic nervous system is a subdivision of the autonomic nervous system, which is
the subdivision of the peripheral nervous system that controls automatic processes in
the body like digestion, heart rate, and respiration.
The parasympathetic nervous system is typically associated with energy conservation and processes
like digestion and elimination of waste products from the body.
Because of these functions, the parasympathetic nervous system is sometimes referred to as
the “rest and digest” system, although this term is an oversimplification that does
not accurately describe the full range of activities of the parasympathetic nervous
system.
The nerves that make up the parasympathetic nervous system originate in the brainstem
and the sacral spinal cord.
Like most other neurons of the autonomic nervous system, they do not travel directly from the
brainstem or spinal cord to their targets but instead extend first to clusters of neurons
known as ganglia.
The parasympathetic ganglia are typically found near or in the wall of the organs they
supply.
The neurons that travel from the brainstem and spinal cord to the parasympathetic ganglia
are called preganglionic neurons, and they synapse with neurons in the ganglia called
postganglionic neurons, which then extend to the targets of the parasympathetic nervous
system.
The preganglionic and postganglionic neurons of the parasympathetic nervous system primarily
release the neurotransmitter acetylcholine.
The preganglionic neurons that originate in the brainstem arise from the cranial nerve
nuclei for the oculomotor, facial, glossopharyngeal, and vagus nerves.
These preganglionic neurons travel with the cranial nerves to provide parasympathetic
innervation to the head and neck, but the fibers that travel with the vagus nerve also
supply the internal organs of the thorax and abdomen.
The preganglionic neurons that originate in the sacral spinal cord come together to form
the pelvic nerves, which supply the organs of the pelvis.