The sympathetic is the antagonist of the parasympathetic and is - like this - a part of the vegetative (also: autonomous) nervous system.
The autonomic nervous system is important for the control of our organs and glands, it is therefore called autonomous, because we can not control it arbitrarily, it runs "on the side", without us being constantly aware (think, for example, only of breathing, digesting and Sweat)
In order to define the sympathetic with its tasks in a nutshell, one could say that it triggers all that constitutes an escape response (then, hundreds of years ago because of the tiger in the thicket, today it is perhaps rather stress or panic instead of "flight" because of an imminent examination or similar). By increasing the activity of the sympathetic, our bodily functions change as follows:
Now it has become clear WHAT the sympathetic triggers, but HOW he does it and WHERE in the body he is at all, remains to be clarified.
The sympathicus should not be thought of as a single "point" in the body. Rather, it is distributed over a fairly large part of the body. He has a place where his origin lies (that is, the cells that are a sort of command center) and a kind of rail system (the fibers that emanate from the cells and make what the command center commands "cell"). is forwarded to the recipient). The recipients of the commands are the organs on which the sympathicus acts (heart, lungs, gastrointestinal tract, vessels, eye, glands, skin).
The sympathetic is a thoracolumbar system, which means that its origins lie in the thoracic region (thorax (Latin) = thorax) and in the lumbar region ( lumbus (Latin) = loin). And in the side horn of the spinal cord. The cells of origin there are nerve cells (neurons), they send their information-forwarding nerve cell processes (axons) via intermediate stations to the organs that are to be controlled.
The intermediate stations are called ganglia ( ganglion (latin) = knot). Here are multipolar nerve cells. Multipolar means that they contain an information-relaying extension, the axon, and more than 2 information-receiving extensions, the dendrites.
There are two types of ganglia in the sympathetic system:
paravertebral ganglia (para = next to, ie ganglia next to the spine), which is referred to in German as the limiting strand (ganglia)
Prevertebral ganglia (pre = before, ie ganglia, which lie in front of the spine)
At these ganglion nerve cells, the information is switched from one cell to the next and then forwarded in its axon towards the organ. Switching the information that passes on a nerve cell always happens only in one of the two types of ganglia mentioned above, not both.
The order of the information line is thus:
Source cell in the spinal cord (1) - Multipolar nerve cell in a ganglion (2) organ
But what is the information? After all, the cell can not talk, but has to make it clear with electrical stimuli or a substance what it "wants". This substance is the so-called neurotransmitter.
Neurotransmitters are chemical messengers that - as the name implies - can relay information to different locations, so they are a kind of "messenger". A distinction is made between exciting (excitatory) and inhibitory (inhibiting) neurotransmitters.
The neurotransmitters serve the chemical information forwarding, while the electrical potentials that pass through the cell and its extensions (axons and dendrites), the electrical information forwarding serve. The chemical information forwarding is always important when the information is to get from one cell to another, because between cells is always a - albeit relatively small - gap, the information can not just skip.
When the electrical lead arrives at the "end" of the cell, its axon end, it causes a type of neurotransmitter to be released from the axon end. The axon end from which it is released is called presynapse ( pre = before, ie the synapse before the synaptic cleft). The neurotransmitter is distributed in the so-called synaptic gap, which lies between cell 1 (information line) and cell 2 (information reception), between which should be switched. After its release, the neurotransmitter "wanders" (diffuses) through the synaptic cleft to the extension of the second cell, the postsynapse ( post = post, ie the synapse after the synaptic cleft). It contains receptors designed specifically for this neurotransmitter. That's how he can bind to it. Its binding now generates an electrical potential at the second cell.
When switching the information from one cell to the next, the order of the information types is as follows:
electrically to the axon end of the first cell - chemically in the synaptic cleft - electrically from the binding of the neurotransmitter to the second cell
Cell 2 can now react in two ways by binding the neurotransmitter: Either it is energized and generates a so-called action potential or it is inhibited and the likelihood that it generates an action potential and thus excite other cells decreases. Which of the two pathways a cell takes depends on the type of neurotransmitter and the type of receptor.
Now one can specify what happens at the different "switch points" of the sympathetic: the first cell (cell of origin) in the spinal cord is excited by higher centers (eg the hypothalamus and the brainstem). The arousal continues through its entire axon to the first switch point (this is already in the ganglion). There, the neurotransmitter acetylcholine is released from the presynapse as a result of the propagated arousal. Acetylcholine diffuses through the synaptic cleft to the synapse of the second cell (postsynapse), where it binds to an appropriate receptor. This binding excites the cell (because acetylcholine is one of the excitatory neurotransmitters). Just as in the first cell, this excitement is transmitted back to the cell and its processes, and that to the receiver: the organ. There, as a result of the excitement, another neurotransmitter - this time it is norepinephrine - is released from the synapse of cell 2. This neurotransmitter then acts directly on the organ.
The sympathicus works with two different neurotransmitters:
The 1st (cell of origin - cell 2) is always acetylcholine
The second (cell 2 - organ) is always norepinephrine
The effect of the sympathetic is already indicated above and should be summed up again here in tabular form:
dilation of the pupil
Faster beating (increased frequency and increased contraction force)
Extension of the airways
Skin (includes sweat glands)
Increased sweat secretion; Putting up the hairs; Narrowing of the blood vessels (cold hands in case of excitement)
Decreased digestive activity
Blood vessels (except those of the skin and gastrointestinal tract)
Enlargement, so that more blood can flow per time
The sympathetic increases the heart rate, so the pulse increases. In addition, he has other effects on the heart, all of which enhance the performance of the heart throughout. Thus, the properties of the heart muscle cells are changed, which is why they can contract more, which allows the blood to be pumped with more force. The electrical properties of the nerve cells that lead to the muscle cells are also affected.
As a result, less irritation is already sufficient to trigger a full contraction of the heart muscle cells and also the transmission of excitation along the nerve cells is accelerated. However, for a muscle cell to be fully operational, it must completely relax between each contraction for a few milliseconds. The time to complete relaxation, also known as refractory period, is shortened by the sympathetic. In summary, the sympathetic nervous system has a stimulating effect, ie positive for heart rate ( chronotropy ), cardiac power ( inotropy ), excitation transmission ( dromotropy ), stimulation threshold ( bathmotropy ) and relaxation ( lusitropia ).
By increasing these functions, the heart can pump more and faster blood, which supplies the body with oxygen. The sympathetic nervous system ensures that the increased need for the brain and muscles is always covered.
The sympathetic nerve also plays a decisive role in the pupil. When it gets dark, the sympathetic nerve fibers that attract the eye are stimulated. This excites a muscle, which surrounds the pupil like a ring, dilator pupillae gennant. It contracts and widens the pupil in this way. The further the pupil, the more light can come into the eye and the better we can see in already low-light conditions.
But also on the lens in the eye of the sympathetic has influence. Here it is interesting to know a little the anatomy of the eye. The lens is suspended by fibers. These fibers are in turn attached to a muscle, the ciliaris muscle . It is aroused by the parasympathetic nervous system, the antagonist of the sympathetic nervous system. This completes the lens and we can see good nearby objects. The sympathetic, on the other hand, relaxes the muscle, causing the lens to flatten and allowing us to see better in the distance.
In order to explain the function of the sympathetic nervous system on the kidney, one must first of all go into the function of the kidneys. Among other things, these are responsible for maintaining the water and salt balance in the body. The water balance has a direct influence on the blood pressure, which we would be in the function of the sympathetic. As already mentioned above, the blood pressure is increased by the sympathetic. On the one hand the sympathetic nerve is directly constricting on the vessels, on the other hand it stimulates certain cells of the kidneys.
These cells produce the hormone renin . Renin is the first step in a long chain of events that culminates in the synthesis of the hormone angiotensin . If one translates the term angiotensin from the Greek, it means something like "vessel narrowing". In fact, it is the most effective substance the body can produce itself to constrict vessels. The tighter a vessel, the higher the pressure that must be built to allow blood to flow through it. This means that the effect of the sympathetic on the kidney is an increase in blood pressure. In the short term, this is a very meaningful mechanism. Unfortunately, we are now too much under too much stress these days, which is why this acute state of blood pressure elevation is turning into a long-term one. This results in chronic hypertension, which then often has to be treated with medication.
The sympathetic nerve is part of the autonomic nervous system, ie the nervous system, which works independently of the brain. It represents the activating part. This means that it reacts in situations that can potentially be dangerous and sets all bodily functions to a possible fight. Nowadays, people seldom get into life-threatening situations. Nevertheless, the sympathetic is used, and always when we are stressed.
The sympathetic nervous system is responsible for causing the heart to beat faster and increase blood pressure, ensuring increased blood supply. Our airways dilate so we get more oxygen. The vessels that supply the intestine with blood are narrowed to make the blood available to other organs, such as the brain, because digestion plays a minor role in stressful situations anyway. To see better, the pupils are wide. In addition, there is an increase in sweat production and energy reserves, such as fat deposits, degraded, so that energy-supplying substances such as fats and carbohydrates can be used in the muscle.
Hyperfunction of the sympathetic nervous system can be the cause and symptom of various diseases. For example, in the case of Raynaud's disease hyperfunction is the cause, in the case of pheochromocytoma the symptom. However, the effects on the body are the same in both situations, of course always in the context of deviations that can occur within a disease. In some cases, the blood pressure rises to such an extent that vessels close completely and the affected areas are slowly undersupplied. It can lead to massive sweats, restlessness, insomnia, severe headache and digestive difficulties. Depending on the condition, other specific symptoms may occur. All this explains why the correct diagnosis of some diseases can therefore be very difficult.
In contrast to the activating function of the sympathetic is the parasympathetic, which is responsible for the regeneration and digestion. After escaping the stressful situation, our body relaxes and begins to replenish energy reserves by stimulating digestion. For this purpose, the vessels to the intestine far and leave again more than just the minimum amount of blood through, which is needed to maintain the intestine. The vessels that lead from the intestine into the body are also extended, so that all absorbed nutrients can be processed and stored directly. The heartbeat slows down, the blood pressure drops and the diameter of the airways is reduced. The sympathetic and parasympathetic nervous system can only be active in parallel at the same time. Which of the two is mainly needed depends on our environment and personal feelings.
Further information can be found here: Parasympathicus