Sympathetic nervous system
Sympathetic nervous system | |
---|---|
Details | |
Identifiers | |
Latin | pars sympathica divisionis autonomici systematis nervosi |
Acronym(s) | SNS |
MeSH | D013564 |
TA98 | A14.3.01.001 |
TA2 | 6601 |
FMA | 9906 |
Anatomical terminology |
The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system.[1][2] The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.[3]
The autonomic nervous system functions to regulate the body's unconscious actions. The sympathetic nervous system's primary process is to stimulate the body's
The SNS has a major role in various physiological processes such as blood glucose levels, body temperature, cardiac output, and immune system function. The formation of sympathetic neurons being observed at embryonic stage of life and its development during aging shows its significance in health. While its dysfunction has shown to be linked to various health disorders.[5]
Structure
There are two kinds of
At the synapses within the ganglia, preganglionic neurons release
- Postganglionic neurons of muscarinic receptors, except for areas of thick skin, the palms and the plantar surfaces of the feet, where norepinephrine is released and acts on adrenergic receptors. This leads to the activation of sudomotor function which is assessed by electrochemical skin conductance.
- epinephrine. The synthesis and release of epinephrine as opposed to norepinephrine is another distinguishing feature of chromaffin cells compared to postganglionic sympathetic neurons.[8]
- Postganglionic sympathetic nerves terminating in the dopamine D1 receptors of blood vessels to control how much blood the kidney filters. Dopamine is the immediate metabolic precursor to norepinephrine, but is nonetheless a distinct signaling molecule.[9]
Organization
Sympathetic nerves arise from near the middle of the
To reach target organs and glands, the axons must travel long distances in the body, and, to accomplish this, many axons relay their message to a second cell through
Presynaptic nerves' axons terminate in either the
The postsynaptic cell then goes on to innervate the targeted end effector (i.e. gland, smooth muscle, etc.). Because paravertebral and prevertebral ganglia are close to the spinal cord, presynaptic neurons are much shorter than their postsynaptic counterparts, which must extend throughout the body to reach their destinations.
A notable exception to the routes mentioned above is the sympathetic innervation of the suprarenal (adrenal) medulla. In this case, presynaptic neurons pass through paravertebral ganglia, on through prevertebral ganglia and then synapse directly with suprarenal tissue. This tissue consists of cells that have pseudo-neuron like qualities in that when activated by the presynaptic neuron, they will release their neurotransmitter (epinephrine) directly into the bloodstream.
In the sympathetic nervous system and other components of the peripheral nervous system, these synapses are made at sites called ganglia. The cell that sends its fiber is called a preganglionic cell, while the cell whose fiber leaves the ganglion is called a
The ganglia include not just the sympathetic trunks but also the
Organ | Nerves[11] | Spinal column origin[11]
|
---|---|---|
stomach | T5, T6, T7, T8, T9, sometimes T10 | |
duodenum | T5, T6, T7, T8, T9, sometimes T10 | |
jejunum and ileum | T5, T6, T7, T8, T9 | |
spleen | T6, T7, T8 | |
gallbladder and liver |
|
T6, T7, T8, T9 |
colon
|
|
|
pancreatic head
|
|
T8, T9 |
appendix |
|
T10 |
bladder |
|
S2-S4 |
kidneys and ureters |
|
T11, T12 |
Information transmission
Function
Organ | Effect |
---|---|
Eye | Dilates pupil |
Heart | Increases rate and force of contraction |
Lungs | Dilates bronchioles via circulating adrenaline[13]
|
Blood vessels | Dilate in skeletal muscle[14] |
Digestive system | Constricts in gastrointestinal organs |
Sweat glands | Activates sudomotor function and sweat secretion |
Digestive tract | Inhibits peristalsis |
Kidney | Increases renin secretion |
Penis | Inhibits tumescence |
Ductus deferens | Promotes emission prior to ejaculation |
The sympathetic nervous system is responsible for up- and down-regulating many homeostatic mechanisms in living organisms. Fibers from the SYNS innervate tissues in almost every organ system, providing at least some regulation of functions as diverse as
The sympathetic nervous system is responsible for priming the body for action, particularly in situations threatening survival.[16] One example of this priming is in the moments before waking, in which sympathetic outflow spontaneously increases in preparation for action.
Sympathetic nervous system stimulation causes vasoconstriction of most blood vessels, including many of those in the skin, the digestive tract, and the kidneys. This occurs as a result of activation of alpha-1 adrenergic receptors by norepinephrine released by post-ganglionic sympathetic neurons. These receptors exist throughout the vasculature of the body but are inhibited and counterbalanced by beta-2 adrenergic receptors (stimulated by epinephrine release from the adrenal glands) in the skeletal muscles, the heart, the lungs, and the brain during a sympathoadrenal response. The net effect of this is a shunting of blood away from the organs not necessary to the immediate survival of the organism and an increase in blood flow to those organs involved in intense physical activity.
Sensation
The afferent fibers of the
General visceral afferent sensations are mostly unconscious visceral motor reflex sensations from hollow organs and glands that are transmitted to the
Relationship with the parasympathetic nervous system
Together with the other component of the autonomic nervous system, the parasympathetic nervous system, the sympathetic nervous system aids in the control of most of the body's internal organs. Reaction to stress—as in the flight-or-fight response—is thought to be elicited by the sympathetic nervous system and to counteract the parasympathetic system, which works to promote maintenance of the body at rest. The comprehensive functions of both the parasympathetic and sympathetic nervous systems are not so straightforward, but this is a useful rule of thumb.[4][18]
Disorders
The dysfunction of the sympathetic nervous system is linked to many health disorders, such as
The sympathetic stimulation of metabolic tissues is required for the maintenance of metabolic regulation and feedback loops. The dysregulation of this system leads to an increased risk of neuropathy within metabolic tissues and therefore can worsen or precipitate
The sympathetic nervous system holds a major role in long-term regulation of hypertension, whereby the central nervous system stimulates sympathetic nerve activity in specific target organs or tissues via neurohumoral signals. In terms of hypertension, the overactivation of the sympathetic system results in vasoconstriction and increased heart rate resulting in increased blood pressure. In turn, increasing the potential of the development of cardiovascular disease.[21]
In heart failure, the sympathetic nervous system increases its activity, leading to increased force of muscular contractions that in turn increases the stroke volume, as well as peripheral vasoconstriction to maintain blood pressure. However, these effects accelerate disease progression, eventually increasing mortality in heart failure.[22]
Sympathicotonia is a stimulated
Heightened sympathetic nervous system activity is also linked to various mental health disorders such as, anxiety disorders and post-traumatic stress disorder (PTSD). It is suggested that the overactivation of the SNS results in the increased severity of PTSD symptoms. In accordance with disorders like hypertension and cardiovascular disease mentioned above, PTSD is also linked with the increased risk of developing mentioned diseases, further correlating the link between these disorders and the SNS.[25]
The sympathetic nervous system is sensitive to stress, studies suggest that the chronic dysfunction of the sympathetic system results in migraines, due to the vascular changes associated with tension headaches. Individuals with migraine attacks are exhibited to have symptoms that are associated with sympathetic dysfunction, which include reduced levels of plasma norepinephrine levels, sensitivity of the peripheral adrenergic receptors.[26]
Insomnia is a sleeping disorder, that makes falling or staying asleep difficult, this disruption in sleep results in sleep deprivation and various symptoms, with the severity depending on whether the insomnia is acute or chronic. The most favoured hypothesis for the cause of insomnia is the hyperarousal hypothesis, which is known as a collective over-activation of various systems in the body, this over-activation includes the hyperactivity of the SNS. Whereby during sleep cycle disruption sympathetic baroreflex function and neural cardiovascular responses become impaired. [27] [28]
However more research is still required, as methods used in measuring SNS biological measures are not so reliable due to the sensitivity of the SNS, many factors easily effect its activity, like stress, environment, timing of day, and disease. These factors can impact results significantly and for more accurate results extremely invasive methods are required, such as microneurography. The difficultly of measuring the SNS activity does not only apply to insomnia, but also with various disorders previously discussed. However, overtime with advancements in technology and techniques in research studies the disruption of the SNS and its impact on the human body will be explored further. [29] [30]
History and etymology
The name of this system can be traced to the concept of sympathy, in the sense of "connection between parts", first used medically by Galen.[31] In the 18th century, Jacob B. Winslow applied the term specifically to nerves.[32]
The concept that an independent part of the nervous system coordinates body functions had its origin in the works of Galen (129–199), who proposed that nerves distributed spirits throughout the body. From animal dissections he concluded that there were extensive interconnections from the spinal cord to the viscera and from one organ to another. He proposed that this system fostered a concerted action or 'sympathy' of the organs. Little changed until the Renaissance when Bartolomeo Eustacheo (1545) depicted the sympathetic nerves, the vagus and adrenal glands in anatomical drawings. Jacobus Winslow (1669–1760), a Danish-born professor working in Paris, popularised the term 'sympathetic nervous system' in 1732 to describe the chain of ganglia and nerves which were connected to the thoracic and lumbar spinal cord.[33]
See also
References
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