The sympathetic nervous system (SNS) is a subdivision of the autonomic nervous system, which in turn belongs to the peripheral nervous system. In general, this system is responsible for activating the body's response to danger, such as running, hiding or fighting, the well-known fight or flight response. Rather, the parasympathetic nervous system controls behaviors such as reproduction or feeding..
The sympathetic nervous system works through interconnected neurons. These neurons are considered part of the peripheral nervous system, although there are also others that belong to the central nervous system.
Presynaptic or preganglionic sympathetic neurons found in the spinal cord communicate with postsynaptic or postganglionic sympathetic neurons found in the periphery. They do this through the so-called sympathetic ganglia, within which chemical synapses occur between both types of neurons..
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At synapses within sympathetic ganglia, preganglionic neurons release acetylcholine, a neurotransmitter that activates receptors on postganglionic neurons..
Once activated, postganglionic neurons release norepinephrine, and if they are activated for a longer time they release adrenaline. These neurotransmitters bind to receptors found in peripheral tissues and this causes the effects of the fight or flight response, the effects of which we will see in the next section..
The body of preganglionic neurons are found in the central nervous system, specifically in the spinal cord, which means that the function of the sympathetic nervous system is directly regulated by the central nervous system..
These neurons form efferent motor nerve fibers, which implies that they carry information from the central nervous system to the smooth muscles of the visceral organs:
The axons of several preganglionic neurons are known as preganglionic fibers and these project from the spinal cord (where the cell body is) to the sympathetic ganglia, which form the ganglionic sympathetic chain..
The ganglia represent the intermediate points of communication between the sympathetic central and autonomic systems..
Since preganglionic fibers travel small distances to reach the sympathetic chain, these are said to be very short nerve fibers.
When the preganglionic fibers reach the sympathetic ganglia, they communicate with the cell bodies of the second neurons: the postganglionic neurons, whose axons form the postganglionic fibers.
Such communication occurs through chemical synapses, since preganglionic fibers release chemical messengers..
The main chemical messenger released by these fibers is acetylcholine, which is specifically recognized and binds to receptors on the surface of the cell body of postganglionic fibers. Preganglionic fibers are said to be cholinergic because they secrete acetylcholine.
Finally, postganglionic neurons release norepinephrine or adrenaline, neurransmitters that reach their receptors in the visceral organs of our body, activating the effects that encompass the fight or flight response..
The functioning of all our organs is delicately controlled by the balance between the two divisions of the autonomic nervous system, that is, the sympathetic and parasympathetic nervous systems..
On some occasions these systems inhibit some functions and on others they activate them, which is why they are said to be antagonistic.
The sympathetic nervous system innervates the visceral organs. It is responsible for regulating the tone of blood vessels, heart rate, digestive tract functions (inhibition), pupil dilation, urination control (urine), etc..
One of the most prominent functions of the sympathetic nervous system is its participation in preparing our body for states of danger, emergency or stress, known as the "fight or flight" response..
The sympathetic division is responsible for the rapid involuntary responses we have when faced with something that scares us or that we know can be dangerous..
The "fight or flight" response is achieved by the sympathetic nervous system by stimulating the production of chemical messengers (neurotransmitters) such as norepinephrine and adrenaline. These neurotransmitters elicit various responses:
The sympathetic nervous system activates the fight or flight response when there is danger. All voluntary muscle movements are allowed, but functions that are not essential for survival are inhibited.
For example, if you are faced with a tiger, your body prompts you to run or hide, which would be the flight response. In another case, for example if you meet a small dog, you could fight and not hide.
It is interesting that these fight or flight responses are also activated when we observe possible dangers on television or in the cinema, not only when we are present before them..
The sympathetic nervous system acts through its postganglionic nerve fibers on most of the body thanks to the secretion of norepinephrine. This neurotransmitter has many physiological functions.
At the cardiovascular level, norepinephrine causes the constriction of blood vessels, which increases pressure and heart rate, that is, the acceleration of the contractions of the heart muscle.
There are preganglionic nerve fibers (derived from the central nervous system) that do not interact with other nerve fibers at the ganglionic level, but instead directly innervate the adrenal medulla, which is the central part of the adrenal glands.
The adrenal glands are endocrine glands that when stimulated by the sympathetic nervous system through these nerve fibers are capable of releasing the hormone adrenaline..
This hormone is the one that participates in the establishment of alertness, regulating visceral functions.
Its functions include the stimulation of the increase in blood sugar (release of energy in the form of glucose), the dilation of the pupil, the increase of irrigation to the muscular tissues, including the heart, etc..
The activity of the sympathetic nervous system on the male reproductive system causes vasoconstriction and loss of erection, so that during sexual arousal this system is inhibited.
In turn, this system has important implications during the ejaculation process (expulsion of seminal fluid), actively participating in it..
The sympathetic nerves have their origin in the vertebral column, beginning in the first thoracic segment of the spinal cord (T1) and extending to the second or third lumbar segment (L2) of the same.
These nerves are parallel to the spinal cord and are located on both sides of the spinal column. Their cell bodies reside in the central nervous system, but their axons extend into ganglia outside this system, where they come into contact with postganglionic cell bodies..
The neurons belonging to the postganglionic nerves of the sympathetic nervous system then extend until they reach their target organs, which are on which they exercise their functions..
However, the path of nerve fibers is not that simple, and there are some exceptions to the rule.
The preganglionic fibers of the sympathetic nervous system are actually nerve fibers belonging to the central nervous system that are dedicated to the functions of the autonomic nervous system (sympathetic division).
The route of the fibers destined to visceral control occurs by the interaction between pairs of neurons, one preganglionic and the other postganglionic.
This is the normal path that each postsynaptic or postganglionic fiber makes to go towards its target organs. There are then two lines that communicate with the spinal nerve:
There are some neurons that escape from "normality" in the routes of the nerves belonging to the sympathetic system. Why? Well, because these do not communicate with a neuronal pair, but they innervate a body structure directly.
What this means is that some nerve fibers derived from the spinal nerve, we could say that analogous to those of the white communicating branch of which we speak, do not go towards the ganglia, but rather extend directly towards their target tissues: the adrenal glands, where they exercise immediate functions.
We say that this is a “monosynaptic pathway” because there is only one synapse event: that of the axon terminal of neurons derived from the spinal cord with specific cells of the adrenal glands..
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