Sympathetic nervous system là gì

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StatPearls . Treasure Islvà (FL): StatPearls Publishing; 2021 Jan-.


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Introduction

The sympathetic nervous system (SNS) is one of the two divisions of the autonomic nervous system (ANS), along with the parasympathetic nervous system (PNS), These systems primarily work unconsciously in opposite ways to lớn regulate many functions and parts of the toàn thân. Colloquially, the SNS governs the "fight or flight" response while the PNS controls the "rest và digest" response. The main overall end effect of the SNS is to prepare the body toàn thân for physical activity, a whole-body reaction affecting many organ systems throughout the body toàn thân khổng lồ redirect oxygen-rich blood to lớn areas of the body needed during intense physical demvà.<1>


Structure và Function

The sympathetic nervous system is composed of many pathways that perkhung a variety of functions on various organ systems. The preganglionic neurons of the SNS arise from the thoracic và lumbar regions of the spinal cord (T1 to lớn L2) with the cell bodies distributed in four regions of the gray matter in the spinal cord bilaterally and symmetrically.<1><2> As opposed to lớn the parasympathetic nervous system, the first-order neurons of the SNS are short before synapsing on postsynaptic neurons found within sympathetic ganglia. Similar khổng lồ the PNS, the neurotransmitter used at this junction is acetylcholine. This acetylcholine activates nicotinic receptors. These postganglionic neurons then travel to lớn their effector sites and release the neurotransmitters epinephrine or norepinephrine, except for sympathetic innervation of sweat glands & the arrectores pili muscles, the small muscles attached khổng lồ hair follicles, which use acetylcholine as their postganglionic neurotransmitter.<3> These neurotransmitters act on adrenergic receptors. Among mỏi the adrenergic receptors are alpha-1 (coupled khổng lồ a Gq & works through the IP3/Ca2+ pathway), alpha-2 (coupled lớn Gi and works through decreasing the cAMP. pathway), & beta-1 và beta-2 (coupled to lớn Gs & works through increasing the cAMPhường pathway).<4> Whether beta-1 và beta-2 are excitatory or inhibitory depends on the tissue on which it is located. These receptors are located on various parts of the body & regulate the actions of the SNS.

The functions of the sympathetic nervous system are expansive sầu & involve sầu many organ systems và various types of adrenergic receptors.

The effects in which SNS acts in direct contrast to the PNS function include the following:


In the eye, sympathetic activation causes the radial muscle of the iris (alpha-1) khổng lồ contract, which leads to mydriasis, allowing more light to lớn enter. Furthermore, the ciliary muscle (beta-2) relaxes, allowing for far vision to lớn improve sầu.
In the heart (beta-1, beta-2), sympathetic activation causes an increased heart rate, the force of contraction, and rate of conduction, allowing for increased cardiac output khổng lồ supply the body with oxygenated blood.
In the lungs, bronchodilation (beta-2) and decreased pulmonary secretions (alpha-1, beta-2) occur to allow more airflow through the lungs.
In the stomach và intestines, decreased motility (alpha-1, beta-2) & sphincter contraction (alpha-1), as well as contraction of the gallbladder (beta-2), occur lớn slow down digestion lớn divert energy to lớn other parts of the toàn thân.
The exocrine và endocrine pancreas (alpha-1, alpha-2) decreases both enzyme and insulin secretion.
In the urinary bladder, there is relaxation of the detrusor muscle and contraction of the urethral sphincter (beta-22) khổng lồ help stop urine output during sympathetic activation.
The kidney (beta-1) increases renin secretion khổng lồ increase intravascular volume.
The salivary glands (alpha-1, beta-2) work through small volume potassium và water secretion. 

Actions of SNS which vì chưng not oppose those of the PNS include the following:


There is strong constriction through the alpha-1 receptor in arterioles of the skin, abdominal viscera, và kidney, và weak constriction through the alpha-1 & beta-2 receptors in the skeletal muscle.
In the liver, increased glycogenolysis and gluconeogenesis (alpha-1, beta-2) occur to lớn allow for glucose to lớn be available for energy throughout the body.
In the spleen, there is a contraction (alpha-1).
Sweat glands và arrector pili muscles (muscarinic) work to lớn increase sweating and erection of hair lớn help cool down the body toàn thân.

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Embryology

Neurons of the peripheral autonomic nervous system, which includes both the sympathetic nervous system and parasympathetic nervous system, arise from neural crest cells that originate from between the neural và non-neural ectoderm. They khung the dorsal neural folds as the folds themselves form the neural tube.<5>


Physiongắn gọn xúc tích Variants

Aging has various effects on the sympathetic nervous system. Retìm kiếm has demonstrated that with increased age that baroreceptors of the heart decrease & become less sensitive; there is a compensatory increase in cardiovascular SNS activity và a reduction in PNS activity. However, both sympathetic & parasympathetic nervous activity to the iris decreases with aging, which is consistent with the general decline of peripheral somatic nerve function.<6> Research has also shown that baseline levels of noradrenaline levels increase with age resulting in an elevated basal SNS activation, while the reactivity becomes reduced with aging.<7> This increase in activation plays a role, aước ao other disease processes, in both age-related hypertension & heart failure.<8>


Surgical Considerations

Horner syndrome is a complication born from interruption of the sympathetic innervation khổng lồ the eye & adnexa at varying levels, most commonly of the nechồng, resulting in increased parasympathetic input đầu vào. It presents with the classic triad of ipsilateral ptosis, pupillary miosis, và facial anhidrosis. It can be a complication of neông chồng surgeries that damage the sympathetic đầu vào.<9> There are even reports after minimally invasive sầu thyroidectomy.<10> For more information on Horner syndrome, please refer to lớn our accompanying article.<11>

Hyperhidrosis, otherwise known as excessive sầu sweating, is a comtháng indication for minimally invasive sầu thoracic sympathectomy. Hyperhidrosis is excessive sầu sweating beyond the organism’s physiological need to sweat to lớn have sầu a temperature within an adequate range. Removing the sympathetic input đầu vào khổng lồ the part of the body toàn thân affected by hyperhidrosis is an acceptable và well-tolerated treatment.<12> Thorascoposic sympathectomy can also be useful lớn treat severe Raynaud syndrome, defined as episodic vascular spasms & digital ischemia secondary khổng lồ cold or emotional stimuli.<13>


Clinical Significance

The clinical significance of the sympathetic nervous system is vast as it affects many organ systems. Of the many physiological & pathological processes, pheochromocytoma, erections & priapism, diabetic neuropathy, and orthostatic hypotension are described below.

Pheochromocytomas are tumors that arise from chromaffin cells present in the adrenal medulla or paraganglion cells that secrete excess amounts of catecholamines (norepinephrine, epinephrine). Because of this catecholamine release, the symptoms are largely that of sympathetic activation, such as hypertension, tachycardia/palpitations, hyperglycemia, & diaphoresis.<14>

Erections are a hàng hóa of parasympathetic activity. At resting state, the SNS predominates, & the penis remains flaccid. However, if the sympathetic fibers khổng lồ the penis are damaged or compromised, a sustained erection of over 4 hours, called priapism, can occur and result in devastating consequences lớn the penis. This condition can result from spinal cord or caudomain authority equina injury as the sympathetic input đầu vào is damaged, and the parasympathetic tone dominates.<15> Nevertheless, SNS also contributes khổng lồ the normal sexual function of a man. Sympathetic stimulation of the male genitals causes sperm emission, which is sensed by the hypogastric nerve sầu.<16>

Diabetic autonomic neuropathy is one of the most common causes of sympathetic nerve sầu neuropathy. This sympathetic denervation can lead lớn impaired myocardial coronary blood flow và reduced myocardial contractility.<17> Diabetic neuropathy plays a crucial role in morbidity and mortality in patients with both type 1 và type 2 diabetes mellitus & causes dysfunction of many systems, including the heart, the Gastroenterol tract, the & genitourinary system, và sexuality. As it is well established that hyperglycemia is the primary driver of this diabetic complication, the clinician must establish early and sustained intensive glycemic control khổng lồ prevent or delay the onmix and slow the progression of autonomic dysfunction. However, this strategy seems to be more effective sầu in type 1 versus type 2 diabetic patients.<18> 

Lastly, orthostatic hypotension is a comtháng problem caused by the failure of noradrenergic neurotransmission. It is defined as a drop of systolic blood pressure by at least 20 mmHg or diastolic by 10 mmHg.

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<19> It is caused by a wide variety of disease processes, including but not limited khổng lồ pure autonomic failure, multiple system atrophy, & autonomic neuropathies that damage the SNS.<20>


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