SYMPATHETIC NERVOUS SYSTEM: STRUCTURE, FUNCTIONS - NEUROPSYCHOLOGY - 2024

The sympathetic nervous system (SNS) is a part of the autonomic nervous system, and the complement of the parasympathetic nervous system. It is mainly responsible for activating a type of response known as "fight or flight", which appears when we are faced with a potentially dangerous or threatening stimulus.

Like the rest of the components of the human nervous system, the SNS works by means of a series of interconnected neurons. Most of those that form it are normally considered a part of the peripheral nervous system, although some can also be embedded within the central one.

In addition to these neurons, the SNS is also made up of several ganglia, which connects the part of the same present in the spinal cord with the more peripheral components. This connection occurs through certain chemical interactions known as synaptic.

In this article we will study both what are the main components of the central nervous system, as well as its most important functions. Likewise, we will also see what their differences are with the parasympathetic nervous system, the other part of the autonomic.

Structure

The sympathetic nervous system is usually divided into two areas: the presynaptic (or preganglionic) neurons, which are those found in the spinal cord, and the postsynaptic or postganglionic neurons. The latter are located in the extremities and in the periphery of the central nervous system.

The most important part of the SNS is the synapses through which its neurons join. In those that connect them with the sympathetic ganglia, a substance known as acetylcholine is released, a chemical messenger that activates nicotinic acetylcholine receptors in postganglionic neurons.

In response to this stimulus, postganglionic neurons mainly release norepinephrine, a substance that is responsible for activating the body and that can cause the generation of adrenaline in the adrenal medulla if it is kept in the body for a long time.

Preganglionic neurons are generated in the teracolumbar region of the spinal cord, especially between the T1 and T3 vertebrae. From there, they travel to the ganglions, usually to the paravertebral ganglia, where they synapse with a postganglionic neuron.

This second type of neuron is much longer, and travels from the ganglions to the rest of the body. It is essential that they reach all corners, since the SNS has a very important role in maintaining the body's homeostasis.

Organization of the SNS

The sympathetic nervous system extends from the thoracic to the lumbar vertebrae; and it has connections to the thoracic, abdominal, and pelvic plexuses. Its nerves arise from the middle of the spinal cord, in the intermediolateral nucleus of the lateral gray column.

Thus, it begins at the first thoracic vertebra of the spine, and is believed to extend to the second or third lumbar vertebra. Because its cells begin in the lumbar and thoracic regions of the spine, the SNS is said to have a thoracolumbar flow.

Axon path

The axons of the neurons that are part of the SNS leave the spinal cord through the ventral root. From there, they pass close to the sensory ganglion, where they become part of the anterior branch of the spinal nerves.

However, they are soon separated from them by the connectors of the white branches, which are named after the thick layers of myelin that cover each axon. From there, they connect with either the paravertebral ganglia or the prevertebral ganglia. They both extend to the sides of the spinal cord.

To reach its target glands and organs, axons have to travel long distances throughout the body. Many of the axons transmit their information through synapses to a second cell, connecting to the dendrites of that cell. These second cells then send the message to its final destination.

The axons of the presynaptic nerves terminate in either the paravertebral ganglia or the prevertebral ganglia. There are four different paths these axons can take before reaching their destination; but in all cases, they enter the paravertebral ganglion at the level of their spinal nerve of origin.

After this, they can either synapse in this ganglion, ascend to a superior ganglion, descend to a paravertebral ganglion that is situated in a lower position, or descend to a prevertebral ganglion and synapse there with a postsynaptic cell.

The postsynaptic cells, after receiving the information, activate the effectors with which they are connected; for example, a gland, a smooth muscle… Because the paravertebral and prevertebral ganglia are close to the medulla, the presynaptic neurons are much shorter than the postsynaptic ones.

Other routes

An exception to the neuronal pathways mentioned above is sympathetic activation of the adrenal medulla. In this case, the presynaptic neurons pass through the paravertebral ganglia; or through the prevertebral. From there, they connect directly to the adrenal tissues.

These tissues are made up of cells that have characteristics similar to neurons. When activated due to the action of the synapse, they will release their neurotransmitter, epinephrine, directly into the bloodstream.

In the SNS, as in other areas of the peripheral nervous system, these synapses are made in places known as ganglia. These also include the cervical ganglia, which send axons to the head and chest organs, and the celiac and mesenteric ganglia (which send them to the stomach and peripheral organs).

Information transmission

In the SNS, information is transmitted affecting different organs in a bidirectional way. Thus, efferent messages can cause changes in different parts of the body simultaneously; for example, by accelerating the heart rate, decreasing the mobility of the large intestine, or dilating the pupils.

On the other hand, the afferent pathway collects information from different parts of the body and transmits it to the SNS, where it will be used to modulate responses and the production of hormones such as norepinephrine.

Features

The sympathetic nervous system is responsible for regulating many of the homeostatic mechanisms in living organisms. The axons of the SNS activate tissues in almost all body systems, taking over functions as diverse as pupillary dilation or kidney function.

However, the SNS is best known for its response to stress, popularly known as the "fight or flight state." The technical name for this bodily activation situation is "organism's sympathetic-adrenal response."

At the neuronal level, during this response, the preganglionic sympathetic fibers ending in the adrenal medulla expel acetylcholine. Thus, a large secretion of adrenaline (also known as epinephrine) is activated, in addition to norepinephrine to a lesser extent.

This secretion acts mainly in the cardiovascular system, it is regulated directly by the impulses transmitted through the sympathetic nervous system, and indirectly by catecholamines that are released through the adrenal medulla.

Effects on the body

The sympathetic nervous system is in charge of activating the body to be ready for action, especially in situations that pose a perceived risk to well-being or survival. It is also responsible for helping us wake up, thus regulating part of the sleep-wake cycle.

These receptors are throughout the body, but are inhibited and regulated by beta-2 adrenergic receptors, which are stimulated by adrenaline. The latter are found in the muscles, heart, lungs, and brain.

The end effect of this entire process is the passage of blood from organs that are not necessary for immediate survival, to those that are involved in intense physical activity. Thus, the body prepares itself either to face danger or to escape from it.

Sensation

Most of the effects produced by the sympathetic nervous system occur at the unconscious level. Therefore, except in the most extreme cases, it is very difficult to realize that it is being activated. Among other things, intestinal functions are regulated, the heart rate is increased, and muscle tone is increased.

However, on some occasions there are perceptible effects at the level of consciousness due to the activity of the central nervous system. Thus, at times of risk you may notice a feeling of emptiness in the stomach, heat on the skin, dry mouth, or the idea that time passes more slowly.

All these sensations are but a side effect of the body's preparation to escape or fight a danger, which can be both real and imagined. If this bodily response lasts for a long time, problems such as chronic stress or anxiety can appear.

Still, the function of the SNS is essential for the proper functioning of the body and the survival of the human species. Therefore, it is one of the body systems whose effects are most powerful on the entire body.

Relationship with the parasympathetic nervous system

Sympathetic nervous system: dilation of the pupil, inhibits salivary production, dilation of skeletal muscles, stimulates salivary secretion, dilates the bronchi, accelerates the heart rate, stimulates the release of glucose, inhibits pancreatic function, inhibits intestinal motility, contracts the rectum, inhibits the adrenal gland, inhibits urinal bladder, promotes vaginal contraction, and promotes ejaculation.

The SNS is just one of the two components of the autonomic nervous system, and it could not perform its functions without the help of the parasympathetic. Both have practically opposite effects on the body. In this section we will see what are the main differences between them.

"Fight and flight" vs. "Rest and digestion"

We have already seen that the SNS is responsible for preparing the body for a situation in which it has to face any type of danger. The parasympathetic nervous system, on the other hand, is responsible for the activity of the body at times when everything is going well.

Thus, when there is no danger nearby, the body is dedicated to saving energy for when it is necessary to use it. In this way, it will take care of digesting food, using nutrients to rebuild the body, and simply rest and relax.

Neural pathways

One of the most important characteristics of the SNS is that its neurons travel a relatively short path. In this way, they are able to activate the effector organs very quickly, to be able to give an adequate response to an imminent danger.

In contrast, neurons in the parasympathetic nervous system travel a much longer path and much more slowly. This is because it is not necessary for the effector organs to respond so quickly, since when it is activated there is no threat in the environment.

Rest vs. Activation

The SNS is the main one in charge of activating the organism when a person has to perform almost any type of action. Thus, its hormonal secretions wake us up in the morning, cause sexual arousal, activate us when it comes to exercising…

The parasympathetic nervous system, on the other hand, has the responsibility to mediate when the body needs to be relaxed. For this reason, it is the main one in charge of regulating sleep cycles, digestion, rest and rest.

General body response

The summary of the activity of the sympathetic nervous system could be an increase in tension and activity in the body. Digestion and excretion stop, muscles tense, and attention increases sharply. All of this leads us to be ready for action.

On the contrary, when the parasympathetic nervous system is activated, the body enters a state of deep relaxation. We find it more difficult to concentrate, the priority of nutrient processing increases, our muscles relax, and we generally feel much calmer.

It is important to maintain a proper balance between these two systems for the body to function properly. However, due to problems such as chronic stress, lack of sleep or anxiety, more and more people suffer from excess activation of the SNS.

conclusion

The sympathetic nervous system is a complex network of neurons that runs through our entire body and performs a very important function within our body. It is one of the most basic body components of all that exist.

Without the sympathetic nervous system, human beings would not be able to react adequately to dangers and we would not be able to survive. Therefore, its study and care are of great importance.

References

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