Parasympathetic Nervous System: Functions And Path

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, alter us and cause 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 energy expenditure, calming us down, relaxing our body systems and returning to a normal state. 

This process, which is carried out unconsciously and involuntarily at the physiological level, as in the moment of activation, 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 body, starting from  the brain stem and following the  spinal cord

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

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 heartbeat or respiratory rhythm.

Main 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 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 performance of digestion and 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 are opposed to 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 (producing opposite effects).

Neuroanatomical situation 

Although the sympathetic nervous system has a large number of nerve innervations at very different heights of the spinal cord, 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 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-pituitary, paraventricular-pituitary and tubero-pituitary nerves stands out), midbrain (we find the ciliary ganglion, from which they are born nerve connections that produce the movement and adjustment of the eye to the light, being able to contract the iris thanks to it) and rhomboencephalus (a large number of cranial nerves are located in it). In this region of the parasympathetic nervous system , the presence and participation of many very important nerve fibers stand out.

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 participated in this system, carrying information that allows the generation of saliva and mucosa in the mouth and tears in the eyes.

2. Sacral region

In the lower 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 the intracranial level. In the sacrum we find ganglia that innervate the urogenital system, which is logical considering the section of the spinal cord in which it is located.

Reactions in the 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 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 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 hint of a possible threat in time and to be able to give way to a prompt reaction.

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

Cardiac system

The parasympathetic system causes a response in the heart that is opposite to that of the sympathetic system. Since it is about reducing energy expenditure and restoring the internal balance of the body, the heart rate and blood pressure slow down, the blood flowing more slowly through the body.

Respiratory system

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

Digestive system

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

In addition to recovering the normal state, this causes the body to be able to recover the energy reserves that it has lost, which is something fundamental. 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 as it requires a certain level of energy to carry it out, 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 essential for the balance of the body. In this regard, the parasympathetic system innervates both the bladder and the anal sphincter, contracting the first and relaxing the second.

Genital system

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

Bibliographic references:

  • Kandel, ER; Schwartz, JH & Jessell, TM (2001). Principles of neuroscience. Fourth edition. McGraw-Hill Interamericana. Madrid.
  • Guyton, AC & Hall, J. (2006). Treaty of Medical Physiology. Elsevier; 11th edition.

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