PARASYMPATHETIC NERVOUS SYSTEM: PARTS, FUNCTIONS, NEURONS - NEUROPSYCHOLOGY - 2025

The parasympathetic nervous system is one of the two main parts of the autonomic nervous system, which in turn is part of the peripheral nervous system. It is the counterpart of the sympathetic nervous system, and most of its functions are the opposite of those of this other set of nerves.

The parasympathetic nervous system is in charge of regulating some of the unconscious actions of the body; specifically those that have to do with rest, relaxation and repair of the body. Thus, its functions are often called "rest and digest" and "feed and reproduce," while those of the sympathetic nervous system are known as "fight and flight."

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The nerves that are part of this set start from the central nervous system. Some of the cranial nerves, a special type of nerve, are also categorized within the parasympathetic nervous system. Due to its position in the body, this system is often said to have a craniosacral direction.

Some of the most important functions of the parasympathetic nervous system are to digest the food consumed, to expel waste from the body through urination and excretion, to produce salivation in the presence of food, or to provoke sexual arousal in the presence of a potential partner.

Parts

The nerves of the parasympathetic nervous system are part of the autonomic and visceral branches of the peripheral nervous system. Normally, they are divided into three areas: cranial nerves, vagus nerve, and splanchnic pelvic efferent preganglionic cell bodies.

There are several more classifications for dividing the components of the parasympathetic nervous system, but this is the most common. Next we will see what each of these parts consists of.

Cranial nerves

The cranial nerves are those that go directly to the brain through the skull, without having to go through the spinal cord as most do. There are twelve cranial nerves; but those involved in the parasympathetic nervous system are III, VII, and IX.

All these cranial nerves originate in certain nuclei in the central nervous system, and synapse with one of the four parasympathetic ganglia: the ciliary, the pterygopalatine, the otic, or the submandibular.

From these four ganglia, the parasympathetic cranial nerves continue their journey to their target tissues via the trigeminal branches (eg, the maxillary or mandibular nerve).

Vagus nerve

The vagus nerve behaves slightly differently from the cranial nerves, in that it has no direct connection with these typical ganglia of the parasympathetic nervous system. Rather, most of its fibers target a number of nodes in other parts of the body.

Most of these nodes are found either in some organs in the chest area (such as the esophagus, lungs and heart), or in the abdominal area (pancreas, stomach, kidneys, small intestine and large intestine). This is where most of its functions are concentrated.

Splanchnic pelvic nerves

The cell bodies of these nerves are located in the lateral gray horn of the spinal cord, at the height between the T12 and L1 vertebrae. Its axons exit the spinal column as nerves from the S2 - S4 zone, through the sacral foramina.

These axons then continue their way through the central nervous system to form synapses in an autonomic ganglion. The parasympathetic ganglia where these axons arrive will be close to the organ of innervation.

This is somewhat different from what occurs in the central nervous system, where synapses between efferent pre and post ganglionic nerves generally occur far from the target organ.

Features

The parasympathetic nervous system is also known as "rest and digest" or "feed and reproduce." These nicknames are due to the fact that it is responsible for regulating all the functions that have to do with rest, relaxation and the activities that occur during them.

In the moments in which we are relaxed or performing any of the functions related to the parasympathetic system, it mainly releases a neurotransmitter known as acetylcholine. This affects the nicotinic and muscarinic receptors, producing different effects in the body.

Some of the most important are increasing blood flow in the digestive system, reducing oxygen intake, stimulating salivary secretion, producing sexual arousal, falling asleep and maintaining sleep, and generally causing a state of relaxation throughout the body..

Increased blood flow in the digestive system

One of the main functions of the parasympathetic nervous system is to activate and promote digestion. The main way in which it does this is by increasing the flow of blood that reaches the organs that form it, by dilating the blood vessels that reach them.

By doing this, the digestive organs begin to produce a series of secretions that prepare the body for the digestion of food. This can only occur in a relaxed state, whereby the action of the sympathetic nervous system interrupts the process.

Decreased oxygen intake

When we are in "fight or flight" mode, it greatly increases the amount of oxygen that our bloodstream carries to the muscles in preparation for a conflict. To do this, the bronchi have to dilate and absorb more of this component from the air.

Once we enter a relaxed state, by contrast, the parasympathetic nervous system reverses this effect. The bronchi return to their natural state, decreasing the amount of oxygen in the blood, and preparing the body to go to rest.

Stimulation of salivary secretion

At the same time that the digestive organs are activated, the parasympathetic nervous system also encourages the action of the salivary glands. This makes the mouth prepare to eat and chew food, so it would also be a process related to nutrition.

Sexual arousal

The sexual response is a different process from the rest of those that occur in the body, in the sense that it requires the action of the sympathetic and parasympathetic nervous systems to occur. However, even in this case, each of the two subsystems has a specific and different function from the other.

In the case of the parasympathetic nervous system, its role is to relax the body and increase blood flow to the genital area. This causes an increase in the sensitivity of this area, in addition to the subjective feeling of excitement. In men, it causes the erection of the penis, and in women the lubrication of the vagina.

The sympathetic nervous system, by contrast, comes into play only at the moment of orgasm. He is the main one in charge of this phenomenon; When the body is activated by this system before arousal is reached, it simply cannot occur.

Sleep and rest

Various studies seem to indicate that the activity of the parasympathetic nervous system is related to a greater ease in falling asleep, as well as to maintaining it longer and more deeply.

The exact relationship between this subsystem and rest is not yet known. Some theories propose that the relaxation caused by your activity is key to being able to sleep; if it is in a state of alert, our brain simply would not allow us to fall asleep or to maintain it for a long time.

State of relaxation

As a summary of the functions of the parasympathetic nervous system, we could say that it is related to all those functions that the body carries out at rest and in the absence of any threat. Its main role, therefore, is to replenish energy and repair all components of the body.

Types of neurons

As in the sympathetic nervous system, efferent signals from the parasympathetic nerves are carried from the central nervous system to their destinations by a system of two neurons.

The first is known as the "presynaptic or preganglionic neuron." Its cell body is located in the central nervous system, and its axon normally extends to synapse with the dendrites of a "postganglionic neuron" (the second type) somewhere in the body.

The axons of presynaptic neurons are normally long, extending from the central nervous system to a ganglion either within or near the target organ. As a result, the fibers of postsynaptic neurons tend to be much shorter.

Receivers

The main neurotransmitter used by the parasympathetic nervous system is acetylcholine, although some peptides are also used occasionally.

For these substances to have an effect on the body, it is necessary that they activate a series of receptors located in the ganglia connected with their neurons.

In the human body, these receptors are of two types: muscarinic (of which we can find five variants, each with a specific function), and nicotinic. Of the latter we can find two versions, one related to skeletal muscles, and the other in various neural systems.

Diseases

The autonomic nervous system, of which the parasympathetic is a part, can suffer from a wide variety of problems. Because it is a fundamental component of our body, the symptoms caused by these pathologies are very wide. Some of the most common are the following:

- Dizziness and fainting when the person stands up.

- Problems varying the heart rate with exercise (exercise intolerance).

- Excess or lack of sweating, which makes it difficult to regulate body temperature.

- Trouble urinating, incontinence, or difficulty completely emptying the bladder.

- Various sexual dysfunctions. In men, erectile dysfunction, or lack of excitement, may appear; and in women, vaginal dryness and vaginismus. Both genders can suffer from anorgasmia (inability to reach orgasm).

- Vision problems, such as blurred images, or difficulty of the pupils to react properly to changes in light.

- Muscle weakness or lack of strength.

All these symptoms can appear in greater or lesser intensity, and be caused by a large number of causes. Next we will look at some of the most common diseases that can affect the parasympathetic nervous system, or the autonomic system altogether.

Postural orthostatic tachycardia syndrome

This syndrome affects several million people around the world to a greater or lesser degree. It is a problem whereby, when going from a sitting or lying position to a vertical one, the heart rate is severely altered.

The tachycardias produced by this syndrome can cause all kinds of symptoms, from dizziness to fainting; and sometimes they appear even when standing for a long time or trying to raise the arms above the head. Its causes are not very clear, but luckily it can be treated.

Neurocardiogenic syncope

It is a problem related to the vagus nerve, which causes blackouts and fainting in those who suffer from it. These are caused by a decrease in blood flow to the brain, which may be due to staying in the same posture for a long time, stressful emotions, or dehydration.

Individuals with this problem often experience nausea, cold sweats, excessive tiredness, and general malaise before and after an episode.

Multiple system atrophy

Multiple system atrophy is a neurodegenerative disease characterized by a combination of symptoms that affect both the autonomic nervous system and conscious movement. Its main effects are the progressive loss of functions and capacities, and the death of different nerve cells in the brain and spinal cord.

Some of the first symptoms experienced by people with this disease are fainting, problems with heart rhythm, erectile dysfunction, and loss of bladder control. As for movement symptoms, they can include tremors, stiffness, loss of muscle coordination, and trouble walking and speaking.

Unfortunately, it is a disease for which there is no known cure, and in its later stages it can confine a person to bed or even cause death due to respiratory or heart failure. It is possibly the most serious parasympathetic nervous system dysfunction.

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