Sympathetic Nervous System Là Gì

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StatPearls . Treasure Island (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 lớn regulate many functions và parts of the toàn thân. Colloquially, the SNS governs the "fight or flight" response while the PNS controls the "rest and digest" response. The main overall kết thúc effect of the SNS is khổng lồ prepare the body toàn thân for physical activity, a whole-toàn thân reaction affecting many organ systems throughout the toàn thân lớn redirect oxygen-rich blood to lớn areas of the toàn thân needed during intense physical demvà.<1>


Structure and Function

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

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

The effects in which SNS acts in direct contrast lớn 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 lớn mydriasis, allowing more light khổng lồ 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, & rate of conduction, allowing for increased cardiac output lớn supply the body toàn thân with oxygenated blood.
In the lungs, bronchodilation (beta-2) & decreased pulmonary secretions (alpha-1, beta-2) occur to allow more airflow through the lungs.
In the stomach and intestines, decreased motility (alpha-1, beta-2) và sphincter contraction (alpha-1), as well as contraction of the gallbladder (beta-2), occur lớn slow down digestion to divert energy to lớn other parts of the body toàn thân.
The exocrine và endocrine pancreas (alpha-1, alpha-2) decreases both enzyme và insulin secretion.
In the urinary bladder, there is relaxation of the detrusor muscle và 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 & water secretion. 

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


There is svào constriction through the alpha-1 receptor in arterioles of the skin, abdominal viscera, and kidney, & 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 khổng lồ allow for glucose 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 increase sweating & erection of hair to lớn help cool down the body.

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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 size the dorsal neural folds as the folds themselves khung the neural tube.<5>


Physioxúc 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 and become less sensitive; there is a compensatory increase in cardiovascular SNS activity and a reduction in PNS activity. However, both sympathetic & parasympathetic nervous activity khổng lồ the iris decreases with aging, which is consistent with the general decline of peripheral somatic nerve function.<6> Retìm kiếm 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, amuốn 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 and adnexa at varying levels, most commonly of the neck, resulting in increased parasympathetic input đầu vào. It presents with the classic triad of ipsilateral ptosis, pupillary miosis, and facial anhidrosis. It can be a complication of nechồng surgeries that damage the sympathetic input.<9> There are even reports after minimally invasive thyroidectomy.<10> For more information on Horner syndrome, please refer lớn our accompanying article.<11>

Hyperhidrosis, otherwise known as excessive sweating, is a comtháng indication for minimally invasive thoracic sympathectomy. Hyperhidrosis is excessive sầu sweating beyond the organism’s physiological need to sweat khổng lồ have sầu a temperature within an adequate range. Removing the sympathetic input lớn the part of the body toàn thân affected by hyperhidrosis is an acceptable & well-tolerated treatment.<12> Thorascoposic sympathectomy can also be useful to 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 và pathological processes, pheochromocytoma, erections và priapism, diabetic neuropathy, và 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, and 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 và result in devastating consequences to lớn the penis. This condition can result from spinal cord or caudomain authority equimãng cầu injury as the sympathetic đầu vào is damaged, và the parasympathetic tone dominates.<15> Nevertheless, SNS also contributes to lớn 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 and 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 và causes dysfunction of many systems, including the heart, the Gastroenterol tract, the & genitourinary system, & sexuality. As it is well established that hyperglycemia is the primary driver of this diabetic complication, the clinician must establish early và sustained intensive sầu glycemic control to lớn prsự kiện or delay the onmix và slow the progression of autonomic dysfunction. However, this strategy seems khổng lồ be more effective in type 1 versus type 2 diabetic patients.<18> 

Lastly, orthostatic hypotension is a common problem caused by the failure of noradrenergic neurotransmission. It is defined as a drop of systolic blood pressure by at least đôi mươi 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, và autonomic neuropathies that damage the SNS.<20>