When something frightens and alarms us, our body reacts by causing different alterations in the organism. Our breathing and heart quicken, our mouths dry up, our muscles receive more blood flow, our pupils dilate and our sphincters contract.

These are actions that we do unconsciously , which prepare us for action if necessary. These responses are controlled by the autonomic nervous system, and within this by the so-called 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 , being the element that controls the visceral reactions and reflexes. This autonomous 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 nodes originating in the spinal cord, 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 ganglion , usually functioning from the neurotransmitter known as acetylcholine. In the case of postganglionic neurons, which connect the ganglion and the target organ, the sympathetic system acts by emitting noradrenaline.

Main functions of the sympathetic nervous system

While the parasympathetic system is responsible for carrying out processes that save the body energy and the enteric system focuses on the normal management of the digestive tract, the sympathetic system has the main function of preparing the body to respond quickly to external stimulation , causing processes that involve the consumption of a large amount of energy in order to ensure survival.

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

In summary, the main functions of the sympathetic system can be considered to be found in the acceleration of the functions of the organism and the preparation for action in the face of possible threats . It also contributes to the existence of a homeostasis by regulating and avoiding an 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 cause the activation of this system, reactions that will be seen in the next section.

When the sympathetic one is activated: reactions it provokes

The main function of the sympathetic system is to activate the organism to facilitate the reaction to the stimuli. To do this, it activates a series of physiological reactions that prepare us to respond. It should be noted that this activation of the sympathetic system facilitates the fight or flight from threatening events , but its activation does not only occur in this type of situation. This system acts regularly to maintain the body’s homeostasis and participates in multiple processes that require physiological activation. Let’s see below some of the reactions it provokes.

Eye reflex

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

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 targets the muscles, preparing them for action and causing resources to be devoted to keeping the motor aspects of the body going.

In addition, blood pressure is regulated and increased so that blood flows through the vascular system faster 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 respond at that pace. In this way a balance is maintained even though conditions have changed in order of the sympathetic nervous system.

Discharge of adrenaline, noradrenaline and glucose

The sympathetic system also causes the release of adrenaline and noradrenaline into the blood through the kidneys, in order to increase physical and psychological activation . It also increases the release of blood glucose from the liver

Lung dilation

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

Decrease in gastrointestinal system performance

The digestive process consumes a great deal of energy on its own. In order to conserve this energy, the parasympathetic system greatly reduces and slows down the activity of the digestive tract and the glands that secrete digestive enzymes. At a buccal level, it also stops the production of saliva, which is why it is common for our mouths to dry out in stressful situations.

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 is usually delayed in situations of stress or tension, although this is not entirely impossible. In this way, all mental activity is concentrated on the most immediate objectives, reducing the importance of those that can be postponed precisely because these needs can be satisfied later without paying a price.

Ejaculation and orgasm

As indicated above, the sympathetic system is not only activated in dangerous situations, but it also participates in multiple physiological processes. An example of this is its participation in sexual relations , provoking ejaculation in men and orgasm in both sexes. However, just before this, a state of constant effort and stress typical of the rest of the situations in which the sympathetic nervous system intervenes does not favour the appearance of this phenomenon, with which there is an apparent paradox.

Sympathetic nervous system pathway

The sympathetic system is configured by two chains of twenty-three ganglia that 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 is as follows.

Point of origin: Spinal cord

The sympathetic system, together with the set of networks of the autonomic nervous system begins in the spinal cord , the brain nucleus located in the brain stem that controls the set of unconscious vital functions and in which this system originates. This is a neurovegetative structure of great importance for life . It is from this that the sympathetic ganglion chains will be projected, inervating the rest of the organism.

Cervical region

The first large region where we can find the first ganglia is in the cervical area. In this cervical trunk we can find three nodes , upper, middle and lower cervical, which connect with regions such as the ocular muscles, the meninges, the pituitary gland, and the vagus, glossopharyngeal and hypoglossal nerves, with which it is 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 nodes also play an important role in controlling the heart, as well as the thyroid.

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 nodes that govern the heart start from the upper and lower cervical nodes (although the latter are located at the level of the ribs), originating some cardiac nerves.

Lumbar Region

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

Pelvic region

This is the largest part of the sympathetic system, which runs in the pelvis. The two chains of ganglia meet in this area in the coccygeal ganglion . In this area, the pelvic plexus, there are four nodes which innervate the rectum and bladder . From these, other secondary plexuses emerge, which control the gallbladder, 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 Interamerican. Madrid.
  • Guyton, A. C. & Hall, J. (2006). Treatise on Medical Physiology. Elsevier; 11th edition.