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Sympathetic nervous system: functions and travel

Sympathetic nervous system: functions and travel

September 18, 2022

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

It's about actions that we do unconsciously , that prepare us for action if necessary. These responses are controlled by the autonomic nervous system, and within it 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 , being this the element that controls the visceral reactions and reflexes. This autonomous system is made up of both the sympathetic system and two more 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 medulla oblongata, connecting to the spinal cord and the organs they innervate. Thus, we usually find preganglionic and postganglionic neurons.

Preganglionic neurons are those that connect spinal cord and ganglion , usually functioning from the neurotransmitter known as acetylcholine. In regard to postganglionic neurons, which connect ganglion and target organ, in the sympathetic system the action is produced from the emission of noradrenaline.

Main functions of the sympathetic nervous system

While the parasympathetic system is responsible for making processes that involve the body energy savings and the enteric focuses on the usual management of the digestive tract, the sympathetic system Its main function is to prepare the body to respond with speed to external stimulation , causing processes that involve the consumption of a large amount of energy in order to ensure survival.


So, the sympathetic system causes a series of energetic physiological reactions that allow survival , being to allow the fight-flight reaction the most important of its functions. These reactions will later be combated by the parasympathetic system, having a homeostatic balance that keeps the organism in an optimal state of functioning according to external stimulation.

In summary, it can be considered that the main functions of the sympathetic system are found in the acceleration of the agency's functions and preparation for action against possible threats . It also contributes to the existence of a homeostasis when regulating and avoiding an excessive performance of the parasympathetic system (which could cause a too slow heart rate, for example).

However, it can 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 activates: reactions that provoke

The main function of the sympathetic system is to activate the organism to facilitate the reaction to stimuli. For this, it activates a series of physiological reactions that prepare us to respond. It must be taken into account that this activation of the sympathetic system facilitates the fight or flight of threatening events , but its activation is not only given to this type of situations. This system acts in a regular way to maintain body homeostasis and participates in multiple processes that require physiological activation. Let's see below some of the reactions it causes.

Eye reflex

The sympathetic system produces at the ocular level mydriasis or pupillary dilatation , 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 used constantly regardless of the relevance of the objective.

Performance in 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 for action and making resources to keep the motor aspects of the organism in motion.

In addition, it regulates and increases blood pressure, so that blood flows more quickly through the vascular system and arrives before the different organs. Of course, this contributes to their being able to offer a quick response to the needs of the moment, which in turn causes other parts of the body to do so to adapt to that rhythm. In this way an equilibrium is maintained even though the conditions have changed by order of the sympathetic nervous system.

Secretion of adrenaline, noradrenaline and glucose

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

Pulmonary dilatation

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

Decrease in the gastrointestinal system

The digestive process consumes a large amount of energy on its own. In order to be able to conserve that energy, the parasympathetic system reduces and greatly slows the activity of the digestive tract and the glands that secrete digestive enzymes. At the buccal level it also stops the production of saliva, which is why it is common for us to dry out in situations of tension.

Stops excretion

In the face of a possible danger, excretion can mean a situation of vulnerability incompatible with survival. The sympathetic nervous system causes the sphincters to contract, making it difficult. Urinating or defecating are usually delayed processes in situations of stress or tension, although it is not something entirely impossible. In this way, all mental activity focuses on the most immediate objectives, downplaying those that are postponable precisely because those needs can be met later without paying a price.

Ejaculation and orgasm

As we have indicated previously, the sympathetic system is not activated only in situations of danger, but participates in multiple physiological processes. An example of this is their participation in sexual relations , causing ejaculation in man and orgasm in both sexes. However, just before this a state of constant stress and stress typical of the rest of situations in which the sympathetic nervous system intervenes does not favor the appearance of this phenomenon, which gives an apparent paradox.

Travel of the sympathetic nervous system

The sympathetic system is configured from two chains of twenty-three ganglia that circulate along and on both sides of the spine, innervating in its path different organs and systems . These chains send nerve endings to both organs and the vascular system. The route that follows would be the following.

Point of origin: Spinal bulb

The sympathetic system, together with the set of networks of the autonomic nervous system start in the medulla , brain nucleus located in the trunk of the brain 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 will be from this where the sympathetic ganglia chains will be projected, innervating the rest of the organism.

Cervical region

The first large region where we can find the first lymph nodes is located in the cervical area. In this cervical trunk we can find three ganglia , upper cervical, middle and lower, which connect with regions such as the eye muscles, meninges, pituitary, and vagus, glossopharyngeal and hypoglossal nerves, which 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 ganglia also have an important role in the control of 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 zones. Lungs, heart and digestive tract are the most important elements . However, part of the ganglia that govern the heart start from the superior and inferior cervical ganglia (although the latter is at the level of the ribs), causing some cardiac nerves.

Lumbar region

The part of the sympathetic nervous system that runs 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 arise. reach the solar plexus and its continuation, the aortic-abdominal plexus . These plexuses innervate the majority of intra-abdominal organs, having connection with the spleen, the liver, the diaphragm and the stomach, among others.

Pelvic region

It is the most caudal part of the sympathetic system, which runs in the pelvis. The two ganglia chains join in this area in the coccygeal ganglion . In this area, the pelvic plexus, you can find four ganglia that right innervation and bladder . From these come other secondary plexuses, 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 Interamericana. Madrid.
  • Guyton, A. C. & Hall, J. (2006). Treaty of Medical Physiology. Elsevier; 11th edition.

Sympathetic Nervous System: Crash Course A&P #14 (September 2022).


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