Parasympathetic nervous system: functions and course

This part of the autonomic nervous system allows us to return to a state of rest after stress.

There are multiple stimuli that trigger our alarm reactions. Stress, potential threats upset us and provoke an activation of the organism. This activation involves the consumption of a large amount of energy. However, after the moment in which it is necessary to be alert, it is necessary to stop this energetic expenditure by calming down, relaxing our body systems and relaxing our body systems.relaxing our body systems and returning to a normal state.

This process, which as in the moment of activation is carried out at the physiological level in an unconscious and involuntary way, is performed by the parasympathetic nervous system, is carried out by the parasympathetic nervous system.

A subdivision of the autonomic nervous system

When we speak of the parasympathetic system we are referring to a nervous system or circuit that innervates the different systems of the organism, starting from the brainstem and following the spinal cord.starting from the brainstem and following the spinal cord.

In this circuit we find that neurons do not directly connect brain and target organ, having intermediate connections in the autonomic ganglia. Communication between neurons, both pre- and postganglionic, is based on acetylcholine transmission.

Together with the sympathetic nervous system and the enteric system, the parasympathetic is one of the divisions of the autonomic or neurovegetative nervous system, which governs and controls the unconscious and involuntary processes essential for the maintenance of life, such as the beating of the heart or the respiratory rhythm.

Main functions of the parasympathetic nervous system

The main function of the parasympathetic nervous system is to generating a state of rest that allows the organism to save or recover energy, causing a relaxation of the body and recovering its state after the presence of activating stimuli.The body relaxes and recovers its state after the presence of activating stimuli. In this sense, apart from inducing relaxation, it also participates in the performance of digestion and in the reproductive response.

In this way we can consider the parasympathetic system the inverse reflex of the sympathetic system, because both systems generally perform actions that oppose each other.. Thus, while the sympathetic system prepares for action and in general provokes an acceleration of the organism and its metabolism, the parasympathetic system provokes reactions that prepare for energy saving and recovery, slowing down the system.

In short, the parasympathetic nervous system performs a series of automatic functions whose existence makes sense from the joint action with the sympathetic nervous system, with which it complements (producing opposite effects to the sympathetic nervous system).

Neuroanatomical situation

Although the sympathetic nervous system has a large number of nerve innervations at very different heights in the medulla, in the case of the parasympathetic nervous system, this distribution is more concentrated, being able to be located especially in the medulla.It can be located especially in specific intracranial locations and in the sacral region of the spinal cord.

Thus, typically two divisions, cranial and sacral can be found.

1. Cranial region

Within this region we can find connections with different regions, both at the level of the hypothalamus (where the presence of the supraoptic-hypophyseal, paraventricular-hypophyseal and tubero-hypophyseal nerves stand out), midbrain (we find the ciliary ganglion, from which nerve connections arise that produce the movement and adjustment of the eye to the light, being able to contract the iris thanks to it) and rhomboencephalon (a large number of cranial nerves are located in it). In this region of the parasympathetic nervous system the presence and participation of many nerve fibers of great importance stand out..

For example, through the vagus nerve, the parasympathetic nervous system reaches the heart, lungs and gastrointestinal tract, provoking different actions.and the gastrointestinal tract, causing different actions. In addition, the glossopharyngeal nerve can also be found in this area, managing swallowing. The facial nerves also participated in this system, carrying information that allows the generation of saliva and mucous in the mouth and tears in the eyes.

2. Sacral region

In the lowest part of the spinal cord we find the sacral vertebrae, which in adults are fused into a single bony structure. In this region, we can find one of the few connections of the parasympathetic nervous system that are not found at the intracranial level.. In the sacrum we find ganglia that innervate the urogenital system, which is logical considering the section of the medulla in which it is located.

Reactions in the different innervated systems

The fact that the main nuclei of the parasympathetic system are located in parts of the brain (with the exception of those located in the sacral medulla) makes it more complicated to imagine the type of action it carries out. To solve this problem, we proceed to indicate how it affects the multiple systems it innervates.

Visual system

In dangerous situations, the human being dilates the pupil because it is necessary to be able to perceive the more the better in order to be able to in order to be able to detect and discriminate threatening stimuli. This is done in order to detect any hint of a possible threat in time and to be able to react promptly.

However, In the resting state, however, not as much light needs to be captured.. The parasympathetic system is responsible for contracting the pupil, reducing the light that enters the visual system and is projected onto the retina.

Cardiac system

The parasympathetic system provokes in the heart a response opposite to that of the sympathetic system. Since the aim is to reduce energy expenditure and restore the body's internal balance, the heart rate slows down, the heart rate and Blood Pressure are slowed down.The blood flows more slowly through the organism.

Respiratory system

In the respiratory system the parasympathetic acts by producing bronchoconstrictioni.e., allowing its contraction and relaxation. It participates in its normal rhythm and allows the respiratory system to reduce oxygen intake in situations in which an increase in oxygen has been previously required. This causes the energy obtained and used by the body to be within the normal range.

Digestive system

Although the energy expenditure of the body during digestion is high, which is why it is stopped in situations of tension in which all available energy is required, in normal situations in which the body relaxes, its functioning is resumed thanks to the digestive system. its functioning is resumed thanks to the parasympathetic system..

In addition to recovering the normal state, this causes the body to be able to recover the energy reserves it has lost, which is essential. Thus, the parasympathetic system stimulates the movement of the digestive tract and the release of digestive enzymes. In the mouth, it stimulates the production of saliva.

Excretory system

In dangerous situations, excretion poses a risk by requiring a certain level of energy to perform it, in addition to the risk posed by both the excretory process and the excretion itself (it can serve to locate the subject by smell or heat). However, the expulsion of waste is fundamental for the balance of the body. In this respect the parasympathetic system innervates both bladder and anal sphincter, contracting the former and relaxing the latter..

Genital system

The parasympathetic also has an important link with human sexuality. And the fact is that the organism being in a state of rest allows sexual arousalcausing erection (both of the penis and the clitoris).

Bibliographical references:

• Kandel, E.R.; Schwartz, J.H. & Jessell, T.M. (2001). Principles of neuroscience. Fourth edition. McGraw-Hill Interamericana. Madrid.
• Guyton, A. C. & Hall, J. (2006). Treatise on Medical Physiology. Elsevier; 11th edition.