Parasympathetic Nervous System

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The autonomic nervous system is divided into sympathetic and parasympathetic systems. In the sympathetic system, the soma of the preganglionic neurons is usually located in the spine while in the parasympathetic system the soma is usually in the brainstem or sacral, at the bottom of the spine. In both systems, the preganglionic neuron releases the neurotransmitter acetylcholine into the synapse. Postganglionic neurons of the sympathetic system have somas in a sympathetic ganglion, located next to the spinal cord. Postganglionic neurons of the parasympathetic system have somas in ganglions near the target organ. Postganglionic neurons of the sympathetic system release norepinephrine into the synapse, in which the fight or flight response is activated; while postganglionic neurons of the parasympathetic system release acetylcholine or nitric oxide, in which the rest and digest response is activated.
In the autonomic nervous system, a preganglionic neuron of the CNS synapses with a postganglionic neuron of the PNS. The postganglionic neuron, in turn, acts on a target organ. Autonomic responses are mediated by the sympathetic and the parasympathetic systems, which are antagonistic to one another. The sympathetic system activates the “fight or flight” response, while the parasympathetic system activates the “rest and digest” response. Source: OpenStax Biology 2e

OpenStax Biology 2e

Parasympathetic nervous system allows an animal to “rest and digest.” One way to remember this is to think that during a restful situation like a picnic, the parasympathetic nervous system is in control (“picnic” and “parasympathetic” both start with “p”). Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (toward the bottom) spinal cord. The axons of the preganglionic neurons release acetylcholine on the postganglionic neurons, which are generally located very near the target organs. Most postganglionic neurons release acetylcholine onto target organs, although some release nitric oxide.

The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated (the common adrenaline dump you feel after a ‘fight-or-flight’ event). Effects of acetylcholine release on target organs include slowing of heart rate, lowered blood pressure, and stimulation of digestion.

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e

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