The sympathetic nervous system refers to a part of the autonomic, involuntary nervous system. It influences and innervates a number of organ and body functions. In doing so, it produces ergotropic effects, meaning that it increases the body’s readiness to perform and act according to the primal pattern of “fight or flight.”
What is the sympathetic nervous system?
Schematic diagram of the human nervous system showing the sympathetic and parasympathetic nervous systems Click to enlarge. The autonomic nervous system, that is, the nervous system that cannot be influenced at will, consists of the sympathetic nervous system, the parasympathetic nervous system, and the intestinal nervous system (enteric nervous system). Vital functions such as respiration, metabolism and digestion, but also blood pressure and salivation, etc. are subject to the autonomic nervous system. It is under the central control of the brain and hormone system and not only ensures organ functions that are optimally adapted to living conditions, but also a functioning alternation of stress and rest tone. The sympathetic and parasympathetic nervous systems act on almost all organs as antagonists or opponents. This antagonistic action enables a wide variety of bodily functions that adapt automatically to changing demands and cannot and do not have to be influenced and controlled voluntarily. In this antagonistic interaction, the sympathetic nervous system behaves ergotropically, i.e. the impulses emanate from it that put the body into increased readiness to perform and also cause the depletion of energy reserves. Both the sympathetic and the parasympathetic nerve pathways lead from the brain and spinal cord, i.e. the central nervous system, to the individual organs. For example, they end in the muscle cells of the heart, the intestinal wall, the pupil muscles or sweat glands. The autonomic nervous system, especially the sympathetic nervous system, immediately ensures an increase in blood pressure, for example, when getting up in the morning, in order to prevent dizziness and to prepare the body for alertness and performance. In intense heat, for example, it ensures activation of the sweat glands. This means that the flow of information is also the other way around, with nerve impulses being transmitted from the organs (e.g. from the heart, intestines or bladder) to the brain.
Anatomy and structure
The sympathetic nervous system comprises a widely ramified, complex network of nerves that is centrally controlled by the hypothalamus, the brainstem, and the formatio reticularis, a network of neurons in the brain. These send impulses to the sympathetic root cells located in the spinal cord. There, the core areas of the peripheral sympathetic nervous system – the so-called first neurons or sympathetic root cells – are located in the area of the thoracic and lumbar spinal cord, i.e. in the thoraco-lumbar system. These root cells, located in the lateral horn of the spinal cord, form the so-called nucleus intermediolateralis and the nucleus intermediomedialis. From there, fiber systems pass into the paravertebral ganglia, the collections of nerve cells adjacent to the spinal cord. These interconnected nerve cords are called the sympathetic border cord, or truncus sympathicus. This also extends into the cervical spine and sacral region. The three cervical ganglia are found in the cervical region. The lowest ganglion may already be interconnected with the first thoracic or thoracic ganglion (called the stellate ganglion). In this area, there are twelve thoracic ganglia on both sides of the spine in the aforementioned border cord. In the lumbar region, four ganglia run and in the sacral medulla, after the union of the last fibers, there is still a single, “unpaired” ganglion (the so-called ganglion impar). The neurotransmitter (transmitter of the nerve impulse) is acetylcholine in the first step. After the first switch, the second, so-called postganglionic neuron then transmits the impulse to the respective target organ by means of noradrenaline. The sweat glands and the adrenal medulla are an exception here, to which the impulse transmission also takes place by acetylcholine. However, there are also axons (nerve nuclei) that leave the sympathetic border cord without a switch and lead directly to the target organ (the intramural ganglia). The three sympathetic nerve fibers exiting the boundary cord in the thoracic region also constitute a special feature. They pass through the diaphragm and then in turn form three nerve plexuses (nerve plexuses), which then travel to the plexuses of the internal organs.Similarly, the nerve fibers that tone the cerebral blood vessels, travel to the pineal gland, or innervate the eyes originate in the sympathetic border cord of the thoracic medulla.
Function and Tasks
Thus, the sympathetic nervous system – together with its counterpart, the parasympathetic nervous system – controls vital processes largely without conscious awareness or voluntary influence. The target tissues of the sympathetic nerve pathways are in particular the smooth muscles, e.g. of the blood vessels or bronchi, as well as the glands. While the parasympathetic nervous system ensures general regeneration, the body’s reserve build-up and regular bodily functions at rest, the task of the sympathetic nervous system is to prepare the organism for increased physical performance. Developmentally speaking, it makes the body ready to fight or flee. The sympathetic nervous system causes the heartbeat to increase in frequency and contraction and the bronchial tubes to dilate for increased lung function and thus better oxygen supply. Blood pressure increases, as does blood flow and muscle tone in the heart and skeletal muscles. Glycolysis, i.e. energy consumption or energy production in the body, also increases and ensures an increasing, i.e. performance-enhancing, supply of energy to the cells. This is also accompanied by a general increase in metabolism. In short, it puts the body into an increased readiness to perform, which also varies in intensity depending on the intensity of the stress reaction. In addition to the increased readiness to perform, also called ergotropia, the sympathetic nervous system conversely ensures a reduction in processes that are not absolutely necessary in fight and flight, i.e. in stress. These include intestinal activity (reduced peristalsis and glandular secretion), but also blood flow to the skin (consequences: cold skin and hands, etc.) and mucous membranes, the intestines and kidneys, and even the brain, where the sympathetic nervous system causes vasoconstriction. But it also affects bladder function (thus enabling continence), the sexual organs (for orgasm and ejaculation), and glandular secretion (increasing sweat gland secretion, adrenal gland adrenaline secretion, and decreasing salivary and pancreatic secretion), as well as the internal eye muscles (in the form of pupil dilation).
Diseases and ailments
A disturbance in this finely tuned interplay of sympathetic and parasympathetic nervous systems can also have correspondingly complex consequences due to its far-reaching influence. When the balance in the autonomic nervous system is generally out of whack, the diagnosis “vegetative dystonia” is often used as an umbrella term for a range of symptoms:
Dysfunction of the involuntary nervous system in general, and of the sympathetic nervous system in particular, can be expressed in symptoms such as sleep disturbances, severe weight loss, cramps, nervousness, cardiovascular problems, or circulatory problems. When the cervical sympathetic nervous system fails, we speak of the so-called Horner’s syndrome, which causes very specific symptoms: this failure of the sympathetic nervous system causes pupil constriction (so-called miosis due to the failure of the dilatator pupillae muscle), drooping of the eyelid (ptosis due to the disturbed tarsalis muscle), and a lowered eyeball (enophthalmos due to the failure of the orbitalis muscle). In addition to this clear symptomatology in Horner’s syndrome, disturbances of the sympathetic nervous system can also trigger a variety of vegetative disturbances elsewhere. From pathologically altered breathing (dyspnea or hyperventilation) to altered vascular regulation (the so-called Raynaud’s syndrome) to pathological thermoregulation of the body (e.g. excessive sweating or freezing), vegetative dysfunctions or disorders of the sympathetic nervous system can find their expression. Disturbed bladder function in the form of irritable bladder or pathologically altered gastrointestinal regulation can also be indications of a disorder of the sympathetic nervous system, along with many other metabolic or organ functions. Hyperhidrosis (excessive sweating) may also indicate a disorder of the sympathetic nervous system. If the suffering for the affected person is too great and other therapeutic measures are not effective, individual ganglia of the sympathetic nervous system are cut or blocked in a sympathectomy to correct the disorder. This endoscopic transthoracic sympathectomy is also used for certain circulatory disorders.In addition, there are also generally benign tumor diseases of the sympathetic nervous system, the so-called ganglioneuromas. In principle, these can develop wherever sympathetic nerve cells run (in the peripheral nervous system, i.e. not in the brain). They occur mainly in the adrenal medulla, in the sympathetic ganglia adjacent to the spinal column, but also in the head and neck region, and less frequently in the bladder or intestinal and abdominal walls. Diseases of the sympathetic nervous system can also lead to altered pain regulation, as well as increased susceptibility to infections and impaired immune defenses.
