Sympathetic nervous system: anatomy, functions and route

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

These are actions that we do unconsciously, Which prepare us to act when needed. These responses are controlled by the autonomic nervous system and within it by what is called 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 that 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 it is made up of a chain of ganglia originating in the medulla oblongata, connecting to the spinal cord and the organs to which they innervate. Thus, we usually find preganglionic and postganglionic neurons.

Preganglionic neurons are those that connect the spinal cord and the ganglionUsually, it works from the neurotransmitter known as acetylcholine. With regard to the postganglionic neurons, which connect the ganglion and the target organ, in the sympathetic system, the action occurs from the emission of norepinephrine.

Main functions of the sympathetic nervous system

While the parasympathetic system is responsible for carrying out energy-saving processes in the body, 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 quickly 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 vigorous physiological reactions that allow survival, Being to allow the most important fight-flight reaction of its functions. These reactions will later be fought by the parasympathetic system, have a homeostatic balance that keeps the body in an optimal state of functioning according to external stimulation.

In summary, the main functions of the sympathetic system can be considered as acceleration of body functions and preparation for action in the face of possible threats. It also contributes to the existence of regular homeostasis and prevents excessive action of the parasympathetic system (which could lead to a too slow heart rate, for example).

However, it may be interesting to see what types of reactions cause this system to activate, which reactions will be discussed in the next section.

When the sympathetic is activated: reactions it causes

The main function of the sympathetic system is to activate the body to facilitate the reaction to stimuli. It does this by activating a series of physiological reactions that prepare us to respond. It should be noted that this activation of the sympathetic system facilitates fight or flight from threatening eventsBut its activation is not limited to such situations.

This system works regularly to maintain body homeostasis and participates in multiple processes that require physiological activation. Let’s see some of the reactions it causes below.

1. Eye reflex

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

2. Performance of 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 to the blood. This increase is directed to the muscles, preparing them for action. and devoting resources to maintaining the motor aspects of the body.

In addition, it regulates and increases blood pressure, so that blood circulates faster through the vascular system and reaches various organs earlier. Of course, this helps that these can offer a quick response to the needs of the moment, which in turn causes other parts of the body to do the same to adapt to this pace. In this way, a balance is maintained even if conditions have changed in the order of the sympathetic nervous system.

3. Secretion of adrenaline, norepinephrine and glucose

The sympathetic system also causes epinephrine and norepinephrine to be released into the blood by the kidneys, in order to increase physical and psychological activation. It also increases the release of blood glucose from the liver

4. Lung dilation

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

5. Decreased performance of the gastrointestinal system

The digestive process consumes a lot of energy on its own. In order to be able to conserve this energy, the parasympathetic system drastically reduces and slows down the activity of the digestive tract and glands which secrete digestive enzymes. In the mouth, it also stops the production of saliva, which is why it is common for our mouth to dry out in stressful situations.

6. Stop shedding

Faced with a possible danger, excretion can lead to a situation of vulnerability incompatible with survival. The sympathetic nervous system causes the sphincters to contract, which makes it difficult. Urinating or defecating are usually delayed processes in situations of stress or tension, although this is not entirely impossible. In this way, all mental activity is focused on the more immediate goals, reducing their importance to those that are reportable precisely because those needs can be met later without paying a price.

7. Ejaculation and orgasm

As noted above, the sympathetic system is not only activated in dangerous situations, but participates in multiple physiological processes. An example of this is their participation in sex, Causing ejaculation in men and orgasm in both sexes. However, just before that, a state of constant exertion and stress typical of other situations in which the sympathetic nervous system intervenes does not favor the appearance of this phenomenon, thus giving an apparent paradox.

Sympathetic nervous system route

The sympathetic system is configured from two chains of twenty nodes which circulate along and to both sides of the spine, innervating various organs and systems in its course.

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

1. Point of origin: spinal bulb

The sympathetic system, as well as all the networks of the autonomic nervous system begins in the medulla oblongata, Cerebral nucleus located in the trunk of the brain which controls all the unconscious vital functions and in which this system originates. It is a neurovegetative structure of great importance for life. It will be from this one from which will be projected the chains of sympathetic ganglia, innervating the rest of the organism.

2. Cervical region

The first large region where the first lymph nodes can be found is located in the cervical region. In this cervical trunk we can find three lymph nodes, Upper, middle and lower cervical, connecting to regions such as eye muscles, meninges, the pituitary and vagus, glossopharyngeal and hypoglossal nerves, which is related to the ability to control the intensity of light picked up by the eyes, the release 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.

3. Thoracic region

In the chest the sympathetic system can be found a dozen ganglia, which innervate the organs located in the corresponding areas. The lungs, heart and digestive tract are the most important parts. However, part of the lymph nodes that govern the heart start from the upper and lower cervical lymph nodes (although the latter are located in the ribs), causing certain nerves in the heart.

4. Lumbar region

Of great importance is the part of the sympathetic nervous system that crosses the lumbar region, Due to the large number of organs it innervates. Under normal conditions, five ganglia can be found in this area, from which the nerve fibers originate that they reach the solar plexus then the aorto-abdominal plexus. These plexuses innervate most of the intra-abdominal organs, having connections to the spleen, liver, diaphragm, and stomach, among others.

5. Pelvic region

It is the most complete part of the sympathetic system, which crosses the pelvis. The two chains of ganglia they are united in this area in the coccygeal ganglion. In this area, the pelvic plexus, you can find four nodes that rectum and bladder innervate. From these come other secondary plexuses, which control the gallbladder, prostate, and penis / vagina and clitoris.

Bibliographical references:

  • Kandel, ER; Schwartz, JH and Jessell, TM (2001). Principles of neuroscience. Fourth edition. McGraw-Hill Inter-American. Madrid.
  • Guyton, AC and Hall, J. (2006). Treatise on medical physiology. Elsevier; 11th edition.

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