Sympathetic nervous system: anatomy, functions and course.

One of the most important branches of the autonomic nervous system for activating us in the face of danger.

When something frightens and alarms us, our body reacts by provoking different alterations in the organism. Our breathing and heart speed up, our mouth dries up, our muscles receive more blood flow, our pupils dilate and we contract our sphincters.

These are actions that we do unconsciously, which prepare us for action.These are actions we take unconsciously, which prepare us for action if necessary. These responses are controlled by the autonomic nervous system, and within this by what is known as the sympathetic system.

One of the divisions of the autonomic nervous system

The sympathetic nervous system is one of the branches of the autonomic nervous system.This is the element that controls visceral reactions and reflexes. This autonomic system is made up of both the sympathetic system and two other divisions, the parasympathetic system and the enteric system.

On the other hand, the sympathetic system is made up of a chain of ganglia originating in the originating in the medulla oblongata, connecting to the spinal cord and the organs they innervate. Thus, we generally find preganglionic and postganglionic neurons.

Preganglionic neurons are those that connect the spinal cord and the ganglionic ganglion.The preganglionic neurons are those that connect the spinal cord and ganglion, generally functioning on the basis of the neurotransmitter known as acetylcholine. As for the postganglionic neurons, which connect the ganglion and the target organ, in the sympathetic system the action is produced by the emission of noradrenaline.

Main functions of the sympathetic nervous system

While the parasympathetic system is responsible for energy-saving processes in the body and the enteric system focuses on the normal management of the digestive tract, the sympathetic system has as its main function to prepare the organism to respond quickly to external stimulation, provoking processes that involve the consumption of energy by the organism.The sympathetic system's main function is to prepare the organism to respond quickly to external stimulation, triggering processes that involve the consumption of a large amount of energy in order to ensure survival.

Thus, the sympathetic system triggers a series of energetic physiological reactions that enable survival.The most important of its functions is to enable the fight-flight reaction. These reactions will be subsequently combated by the parasympathetic system, homeostatic balance that maintains the organism in an optimal state of functioning according to external stimulation. according to external stimulation.

In summary, it can be considered that the main functions of the sympathetic system lie in the acceleration of the organism's functions and the preparation for action in the face of possible threats.. It also contributes to the existence of homeostasis by regulating and preventing excessive action of the parasympathetic system (which could cause a too slow heart rate, for example).

However, it may be interesting to see what kind of reactions trigger the activation of this system, reactions that will be discussed in the next section.

When the sympathetic system is activated: reactions it provokes

The main function of the sympathetic system is to activate the organism to facilitate reaction to stimuli. To do this, it activates a series of physiological reactions that prepare us to respond. It should be borne in mind that this activation of the sympathetic system facilitates the fight or flight from threatening events.but its activation does not occur only in this type of situation.

This system acts regularly to maintain body homeostasis and participates in multiple processes that require physiological activation. Let us see below some of the reactions it provokes.

1. Ocular reflex

The sympathetic system produces at the ocular level mydriasis or pupillary dilationThis fact allows a greater visual capacity that can allow to see better possible dangers. It is an automatic and unconscious process since it is used constantly regardless of the relevance of the target.

2. Action on the cardiovascular system

The heart rate increases with the activation of the sympathetic system, producing an increase in the rate at which oxygen and nutrients are sent through the blood. This increase is directed to the muscles, preparing them for action. and causing resources to be devoted to keeping the motor aspects of the organism in motion.

In addition, Blood Pressure is regulated and increased, so that blood flows more quickly through the vascular system and reaches the various organs sooner. This, of course, helps the organs to respond quickly to the needs of the moment, which in turn causes other parts of the body to do the same in order to adapt to this rhythm. In this way a balance is maintained even though conditions have changed at the command of the sympathetic nervous system.

3. Adrenaline, noradrenaline and glucose secretion

The sympathetic system also triggers the release of adrenaline and noradrenaline into the blood via the kidneys, in order to increase physical and psychological activation. The release of glucose into the blood from the liver is also increased.

4. Pulmonary dilatation

In response to the action of the sympathetic system, the lungs begin a process of bronchodilatation. begin a process of bronchodilation in order to capture a higher level of oxygen and optimize the oxygen supply system.

5. Decreased performance of the gastrointestinal system

The digestive process consumes a large amount of energy by itself. In order to be able to conserve this energy, the parasympathetic system greatly reduces and slows down the activity of the digestive tract and the glands that secrete energy. and the glands that secrete digestive enzymes. At the oral level, it also stops the production of saliva, which is why our mouth often dries up in stressful situations.

6. Stops excretion

In the face of possible danger, excretion can lead to a situation of vulnerability incompatible with survival. The sympathetic nervous system causes the sphincters to contract, making it difficult. Urinating or defecating are often delayed processes. in stressful or tense situations, although this is not entirely impossible. In this way, all mental activity is concentrated on the most immediate objectives, downplaying the importance of those that can be postponed precisely because these needs can be satisfied later without paying a price.

7. Ejaculation and orgasm

As we have indicated above, the sympathetic system is not only activated in situations of danger, but also participates in multiple physiological processes. An example of this is its participation in sexual intercourseIt is involved in sexual intercourse, causing ejaculation in men and orgasm in both sexes. However, just before this, a state of constant effort and stress typical of the other situations in which the sympathetic nervous system intervenes does not favor the appearance of this phenomenon, thus giving rise to an apparent paradox.

Pathway of the sympathetic nervous system

The sympathetic system is made up of two chains of twenty-three ganglia that circulate along and on both sides of the spinal column, innervating different organs and systems. circulate along and on both sides of the spinal column, innervating different organs and systems along the way..

These chains send nerve endings to both organs and the vascular system. The route that follows would be as follows.

1. Point of origin: Medulla oblongata

The sympathetic system, together with the set of networks of the autonomic nervous system begins in the medulla oblongataThis is the cerebral nucleus located in the brainstem that controls the set of unconscious vital functions and in which this system originates. It is a neurovegetative structure of great importance for life.. It is from here that the sympathetic ganglion chains will project, innervating the rest of the organism.

2. Cervical region

The first large region where we can find the first ganglions is located in the cervical area. In this cervical trunk we can find three ganglions, upper, middle and lower cervical.They connect with regions such as the ocular muscles, the meninges, the hypophysis, and the vagus, glossopharyngeal and hypoglossal nerves, which are linked to the ability to control the intensity of light captured by the eyes, the emission of hormones and the ability to swallow. Some of these ganglia also play an important role in the control of the heart, as well as the thyroid.

3. Thoracic region

In the thorax the sympathetic system can be found a dozen ganglia, which innervate the organs located in the corresponding areas. Lungs, heart and digestive tract are the most important elements.. However, part of the ganglia that govern the heart originate from the superior and inferior cervical ganglia (although the latter is located at the level of the ribs), originating some cardiac nerves.

4. Lumbar region

The part of the sympathetic nervous system that passes through the lumbar region is of great importance due to the large number of organs in the lumbar region.due to the large number of organs it innervates. Under normal conditions, five ganglions can be found in this area, from which nerve fibers arise that reach the solar plexus and its and its continuation, the aortic-abdominal plexus, are found in this area.. These plexuses innervate most of the intra-abdominal organs, having connection with the spleen, liver, diaphragm and stomach, among others.

5. Pelvic region

This is the most caudal part of the sympathetic system, which runs through the pelvis. The two chains of ganglia unite in this area in the coccygeal ganglion.. In this area, the pelvic plexus, four ganglia can be found that innervate the rectum and bladder. innervate the rectum and bladder. Other secondary plexuses emerge from these, which control the gall bladder, prostate and penis/vagina and 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). Treatise on Medical Physiology. Elsevier; 11th edition.