Parasympathetic nervous system: functions and pathway

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, past the moment when it is necessary to be alert, it is necessary to stop this energy expenditure by reassuring ourselves, relaxing our body systems and returning to a normal state.

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

A subdivision of the autonomic nervous system

When speaking of the parasympathetic system we are referring to a nervous system or circuit that innervates the different systems of the body, starting from Brain stem and following the spinal cord.

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

With him 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 rhythm of respiration.

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 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 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.

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

Neuroanatomic situation

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, 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 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 nerve connections are born 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 pairs cranial). In this region of the parasympathetic nervous system highlight the presence and participation of many very important nerve fibers.

For example, through vagus nerve the system reaches the heart, lungs and digestive tract, causing different performances. 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 in the parasympathetic nervous system that are not found intracranially. In the sacrum we find ganglia that innervate the urogenital system, which is logical considering the section of the 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 performance that 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 be able 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 so 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 opposite to that of the sympathetic system. Since it is about reducing energy expenditure and recovering the internal balance of the body, heart rate and blood pressure slow, 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 where 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 when digesting is high, which is why it is stopped in stressful situations in which all available energy is required, these are normal situations in which the body is relax its function 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 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 since 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 be used to locate the subject by smell or hot). However, the expulsion of waste is essential for the balance of the body. In this respect 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 it is that being the organism in a state of rest allows sexual arousal, causing 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.