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The parasympathetic nervous system: functions and travel

The parasympathetic nervous system: functions and travel

January 17, 2022

There are multiple stimuli that trigger our alarm reactions. Stress, potential threats alter us and provoke an activation of the organism. This activation involves the consumption of a large amount of energy. But nevertheless, past the moment when it is necessary to be alert, it is necessary to stop this energy expenditure reassuring us , relaxing our bodily systems and returning to a normal state.

This process, which at the moment of activation is carried out unconsciously and involuntarily at the physiological level, is performed by the parasympathetic nervous system .

A subdivision of the autonomic nervous system

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


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

Along 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, like the beating of the heart or the breathing rhythm.

Main functions of the parasympathetic nervous system

The main function of the parasympathetic nervous system is that of generate a state of rest that allows the body to save or recover energy , causing a relaxation of the body and recovering its state after the presence of activating stimuli. In this sense, apart from inducing relaxation, it also participates in the realization 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 in general perform actions that oppose each other . In this way while the sympathetic prepares for action and generally causes an acceleration of the organism and its metabolism, the parasympathetic causes 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 opposite effects to this).

Neuroanatomical situation

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


Thus, typically you can find two divisions, cranial and sacral .

1. Cranial region

Within this region we can find connections with different regions, both at the hypothalamus level (in which the presence of the supraoptic-pituitary, paraventricular-pituitary and tubero-pituitary nerves), mesencephalon (we find the ciliary ganglion, from which they are born) stands out. nervous connections that produce the movement and adjustment of the eye to light, being able to contract the iris thanks to him) and rhombus (there are a lot of cranial nerves). In this region of the parasympathetic nervous system highlight the presence and participation of many nerve fibers of great importance .

For example, through the vagus nerve the system reaches the heart, the lungs and the digestive tract , provoking different actions. In addition, the glossopharyngeal nerve can also be found in this area, managing swallowing. 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. Sacred region

In the lower part of the spinal cord we find the sacral vertebrae, being in adults fused in 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 the intracranial level . In the sacrum we find ganglia that innervate the urogenital system, which is logical considering the stretch of the medulla in which it is located.

Reactions in different systems 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 performs.To solve this problem, we proceed to indicate how it affects the multiple systems it supplies.

Visual system

In situations of danger the human being dilates the pupil since it is necessary to be able to perceive the more the better in order to detect and discriminate threatening stimuli. This is done to detect any glimpse of possible threat in time and be able to give way to an early reaction.

But nevertheless, in a state of rest it is not necessary to capture so much light . The parasympathetic system is responsible for contracting the pupil, decreasing the light that enters the visual system and is projected on the retina.

Cardiac system

The parasympathetic system provokes in the heart an opposite response to that of the sympathetic system. Since it is about decreasing energy expenditure and recovering the internal balance of the body, slows heart rate and blood pressure , blood flowing more slowly through the body.

Respiratory system

In the respiratory system the parasympathetic it acts producing bronchoconstriction , that is, allowing its contraction and relaxation. It participates in its normal rhythm and allows the respiratory system to reduce the oxygen input in situations in which it has previously required an increase in it. This causes the energy obtained and used by the body to be within normal limits.

Digestive system

Although the energetic expenditure that the body makes when doing the digestion is elevated, reason for which it is stopped in situations of tension in which all the available energy is needed, it is normal situations in which the body relaxes its operation 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 fundamental. Thus, the parasympathetic system stimulates the movement of the digestive tract and the release of digestive enzymes. In the mouth, stimulates the production of saliva.

Excretory system

In situations of danger excretion poses a risk by requiring a certain level of energy to carry it out, in addition to the risk posed by both the excretory process and the actual excretion itself (it can serve to locate the subject by smell or heat). However, the expulsion of waste is essential for the balance of the body. In this aspect the parasympathetic system innervates both vegija and anal sphincter, contracting the first and relaxing the second .

Genital system

The parasympathetic also has an important connection with human sexuality. And is that the body in a state of rest allows sexual arousal , causing the erection (both of the penis and the clitoris).

Bibliographic 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). Treaty of Medical Physiology. Elsevier; 11th edition.

Parasympathetic Nervous System: Crash Course A&P #15 (January 2022).


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