11.3: The Parasympathetic Division of the ANS

Learning Objectives

By the end of this section, you will be able to:

• Name the components that generate the parasympathetic responses of the autonomic nervous system
• Explain the bodily responses associated with the parasympathetic divisions
• Describe the signaling molecules and receptor proteins involved in communication within the parasympathetic division

Parasympathetic Division of the Autonomic Nervous System

The parasympathetic system can also be referred to as the craniosacral system (or outflow) because the preganglionic neurons are located in nuclei of the brain stem and the lateral horn of the sacral spinal cord.

The connections, or “circuits,” of the parasympathetic division are similar to the general layout of the sympathetic division with a few specific differences. The preganglionic fibers from the cranial region travel in cranial nerves, whereas preganglionic fibers from the sacral region travel in spinal nerves. The targets of these fibers are terminal ganglia, which are located near—or even within—the target effector. These ganglia are often referred to as intramural ganglia when they are found within the walls of the target organ. The postganglionic fiber projects from the terminal ganglia a short distance to the target effector, or to the specific target tissue within the organ. Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibers are long and the postganglionic fibers are short because the ganglia are close to—and sometimes within—the target effectors.

A preganglionic neuron of the parasympathetic division will release acetylcholine onto the postganglionic neuron. The postganglionic neuron will then respond by also releasing acetylcholine to the target organ. The target organ will bind acetylcholine through muscarinic receptors on its cell membrane. The binding of acetylcholine to the muscarinic receptors of the target organ will then trigger the parasympathetic response in that organ.

Figure \(\PageIndex{1}\) Parasympathetic Division of the ANS. (Image and caption credit: "Autonomic Nervous System" by OpenStax is licensed under CC BY 4.0) / Cropped from original

Parasympathetic preganglionic fibers primarily influence the heart, bronchi, and esophagus in the thoracic cavity and the stomach, liver, pancreas, gall bladder, and small intestine of the abdominal cavity. The postganglionic fibers from the ganglia activated by the vagus nerve are often incorporated into the structure of the organ, such as the mesenteric plexus of the digestive tract organs and the intramural ganglia. The responses by the parasympathetic division will often oppose those of the sympathetic division. These responses can be seen in Figure \(\PageIndex{2}\) below.

Figure \(\PageIndex{2}\) Parasympathetic Innervation: Neurons from brain-stem nuclei, or from the lateral horn of the sacral spinal cord, extend their preganglionic nerve fibers (shown in red) to terminal ganglia near the effector organs or to intramural ganglia within the walls of the effector organs. Postganglionic axons (shown in black) from these ganglionic neurons then project the short distance to target effectors (Image and caption credit: "ANS - Parasympathetic Division Innervation" by Jennifer Lange is licensed under CC BY-NC-SA 4.0. Image incorporates originals from Functional Neuroanatomy, KnowledgeWorks, Patrick J. Lynch, and Ron Slagter.)

The neurotransmitters used by the parasympathetic vs. sympathetic systems can be compared in the following table.

Table \(\PageIndex{1}\):Autonomic System Signaling Molecules

	Sympathetic	Parasympathetic
Preganglionic	Acetylcholine → nicotinic receptor	Acetylcholine → nicotinic receptor
Postganglionic	Norepinephrine → adrenergic receptors Acetylcholine → muscarinic receptor (associated with sweat glands)	Acetylcholine → muscarinic receptor