Parasympathetic nervous system

Definitions & Key takeaways

The parasympathetic nervous system is one of two divisions of the autonomic nervous system (ANS), the other being the sympathetic nervous system. The ANS controls the body's automatic or unconscious functions, such as heart rate, digestion, and breathing.

The activity of the parasympathetic nervous system can be summarized as �rest and digest' because it slows the heart rate and keeps the body's energy use as low as possible to stimulate organs like the gastrointestinal tract and the bladder.

In the parasympathetic nervous system, preganglionic cholinergic neurons release acetylcholine to activate postganglionic cholinergic neurons, which then release acetylcholine to target cells. Thus, their nerve fibers are referred to as cholinergic fibers.

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Introduction0:00–2:15

The nervous system is divided into the central nervous system, so the brain and spinal cord, and the peripheral nervous system, which includes all the nerves that connect the central nervous system to the muscles and organs.
The peripheral nervous system can be divided into the somatic nervous system, which controls voluntary movement of our skeletal muscles, and the autonomic nervous system, which is further divided into the sympathetic and the parasympathetic, and controls the involuntary movement of the smooth muscles and glands of our organs.
The sympathetic and parasympathetic nervous systems have opposite effects on the body. The sympathetic nervous system controls functions like increasing the heart rate and blood pressure, as well as slowing digestion.
All of this maximizes blood flow to the muscles and brain, and can help you either run away from a threat or fight it, which is why it’s also called the fight-or-flight response.
The parasympathetic nervous system instead slows the heart rate and stimulates digestion - the effects can be summarized as 'rest and digest'.
Now, neurons are the main cells of the nervous system. They’re composed of a cell body, which contains all the cell’s organelles, and nerve fibers, which are projections that extend out from the neuron cell body.
These are either dendrites that receive signals from other neurons, or axons that send signals along to other neurons. Where two neurons come together is called a synapse, and that’s where one end of an axon sends neurotransmitters to the dendrites or directly to the cell body of the next neuron in the series.
Now the autonomic nervous system - so both the sympathetic and parasympathetic nervous system - is made up of a relay that includes two neurons.
And when there’s a group of neuron cell bodies located next to each other in the central nervous system, the whole thing is called a nucleus, while a group of neuron cell bodies located outside the central nervous system is called a ganglion.
We’ll focus on just the parasympathetic nervous system. Signals for the autonomic nervous system start in the hypothalamus, which is a portion of the brain located at the base of the brain.

Parasympathetic fibers and ganglia2:15–4:57

Some hypothalamic neurons have relatively short axons that synapse with preganglionic neurons in the brainstem nuclei. The preganglionic neurons make up cranial nerve III which is the oculomotor nerve, cranial nerve VII which is the facial nerve, cranial nerve IX which is the glossopharyngeal nerve, and cranial nerve X which is the vagus nerve.
The rest of the hypothalamic neurons have really long axons - up to 1.4 meters or 4.5 feet - that synapse with preganglionic neuron cell bodies all the way down at the second, third, and fourth sacral spinal cord segments, which is S2, S3, and S4.
From there, the signal travels down the preganglionic neurons and exits the brainstem or spinal cord, and goes to the parasympathetic ganglia, which are collections of postganglionic neuron cell bodies.
The postganglionic neurons exit the parasympathetic ganglion and extend all of the way to the target cell. The vagus nerve and the nerves that arise from the sacral spinal cord segments S2, S3, and S4 - also called the pelvic splanchnic nerves, have their parasympathetic ganglia located directly in their target organs.
More specifically, the vagus nerve synapses at postganglionic ganglia on the esophagus, heart, lungs, liver, and most of the gastrointestinal tract, up to the transverse colon.
The pelvic splanchnic nerves synapse at postganglionic ganglia on the reproductive organs and the bladder. In contrast, the oculomotor, facial, and glossopharyngeal nerves synapse in cranial ganglia that aren’t directly in the target organ, but are nearby.
The oculomotor nerve synapses with postganglionic neurons of the ciliary ganglia located right behind the eye, and it innervates the pupils.
The facial nerve synapses with postganglionic neurons of two cranial ganglia, the pterygopalatine ganglion which is located in the pterygopalatine fossa behind the maxilla on each side, as well as with the submandibular ganglion which is located above the submandibular salivary glands.
The facial nerve innervates the sublingual and submandibular salivary glands, the lacrimal glands, and glands in the nasal cavity.
The glossopharyngeal nerve synapses with postganglionic neurons in the otic ganglia, which is located in the infratemporal fossa situated below and medial to the zygomatic arch, and it innervates the parotid salivary gland.
Now let’s zoom into the synapses of the parasympathetic nervous system. The preganglionic and postganglionic neurons release neurotransmitters, which are the tiny molecules that nerve cells use to communicate with one another.

Parasympathetic synapses4:57–6:10

They both release the neurotransmitter acetylcholine, and that’s why they are called cholinergic neurons. Acetylcholine released by preganglionic neurons binds to nicotinic receptors on the cell membrane of postganglionic neuron cell bodies.
Nicotinic receptors are ion channels that open when acetylcholine binds to them, and they allow positive ions like sodium and potassium to cross the cell membrane, activating the postganglionic neurons.
Postganglionic neurons are also called cholinergic neurons because like the preganglionic neurons, they also release acetylcholine.
This time, however, the acetylcholine binds to muscarinic receptors on the cells of target organs. Muscarinic receptors are G-protein-coupled receptors, meaning that when acetylcholine binds, they activate intracellular proteins, called G proteins.
These G proteins ultimately enable cells to change in a number of ways, and that’s how the parasympathetic nervous system creates change at the cellular level.
‘Rest and digest’ describes how the parasympathetic nervous system keeps body energy use as low as possible, and how it stimulates activities like digestion, defecation, more commonly called pooping, and urination.

Parasympathetic Effects6:10–7:19

It acts in the heart, slowing the heart rate and reducing the cardiac output. In the gastrointestinal tract, it increases its motility to stimulate digestion and defecation.
In salivary glands, it increases saliva secretion to aid in digestion. In the liver, it triggers glucose storage to reduce blood glucose levels, since after a meal they will be really high.
In the lungs, it causes bronchoconstriction, since in a relaxing situation our cells do not consume as much oxygen. In the pupils, it triggers miosis or constriction of the pupils to improve close vision.
In the bladder, it causes constriction of the bladder muscle, called the detrusor muscle, which stimulates urination. Finally, in females, it stimulates secretions from the genital tract, and in males, it triggers penile erection.
These organs are controlled by the sympathetic nervous system as well, with opposite effects. Alright, as a quick recap, the parasympathetic nervous system is one of the divisions of the autonomic nervous system.

Review7:19–7:50

The entire system is based on a preganglionic cholinergic neuron that releases acetylcholine to activate a postganglionic cholinergic neuron that then releases acetylcholine to a target cell.
The activity of the parasympathetic nervous system can be summarized as ‘rest and digest’ because it slows heart rate and keeps body energy use as low as possible to stimulate organs like the gastrointestinal tract and the bladder.