There are multiple stimuli that trigger our alarm reactions. Stress, potential threats alter us and cause an activation of the organism. Such activation involves the consumption of a great deal of energy. However, once the moment has passed when it is necessary to be alert, it is necessary to stop spending energy by calming down , relaxing our body systems and returning to a normal state.

This process, which, just like the moment of activation, is carried out on a physiological level in an unconscious and involuntary way, is performed 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 brain stem and following the spinal cord.

In this circuit we find that the neurons do not directly connect brain and target organ, there being intermediate connections in the autonomous nodes. Communication between neurons, both at a pre- and post-node level, is based on acetylcholine transmission.

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

Major functions of the parasympathetic nervous system

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

In this way we can consider the parasympathetic system the inverse reflection of the sympathetic system, because both systems in general perform actions that are opposed to each other . In this way, while the sympathetic system prepares for action and generally provokes an acceleration of the organism and its metabolism, the parasympathetic system provokes reactions that prepare for the saving and recovery of energy, 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 itself (producing effects opposite to this).

Neuroanatomical status

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

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

1. Cranial region

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

For example, through the vagus nerve the system reaches the heart, lungs and digestive tract , causing different actions. In addition, the glossopharyngeal nerve can also be found in this area, managing swallowing. The facial nerves also participate 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, being in adults fused into a single bone structure. In this region, we can find one of the few connections of the parasympathetic nervous system that are not found at an intracranial level . In the sacrum we find nodes that innervate the urogenital system, which is logical considering the stretch of the marrow 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 difficult 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 situations of danger the human being dilates the pupil since it is necessary to be able to perceive the more better in order to be able to detect and discriminate threatening stimuli. This is done in order to detect any hint of possible threat in time to make way for a prompt reaction.

However, in the idle state it is not necessary for so much light to be captured . The parasympathetic system is responsible for contracting the pupil, diminishing the light that enters the visual system and is projected onto the retina.

Heart system

The parasympathetic system causes the heart to respond in the opposite way to the sympathetic system. Since the aim is to reduce energy expenditure and restore the body’s internal balance, the heart’s rhythm and blood pressure are slowed down, and blood flows through the body more slowly.

Respiratory system

In the respiratory system the parasympathetic acts by producing bronchoconstriction , that is, allowing it to contract and relax. It participates in its normal rhythm and allows the respiratory system to reduce the entry of oxygen in situations where it has previously been necessary to increase it. This causes the energy obtained and used by the body to be within the normal range.

Digestive system

Although the energy expenditure made by the body when digesting is high, which is why it is stopped in situations of tension in which all the available energy is required, it is normal situations in which the body relaxes and resumes its functioning thanks to the parasympathetic system .

In addition to returning to a normal state, this causes the body to be able to recover the energy reserves it has lost, so this 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 hazardous 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 may serve to locate the subject by smell or heat). However, the expulsion of waste is fundamental to the body’s balance. In this aspect the parasympathetic system innervates both the bladder and the anal sphincter, contracting the former and relaxing the latter .

Genital system

The parasympathetic also has an important link to human sexuality. The fact is that when the organism is in a state of rest it allows sexual excitement , causing an erection (of both the penis and the clitoris).

Bibliographic references:

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