NEUROTRANSMITTER 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 & parts of the toàn thân. Colloquially, the SNS governs the "fight or flight" response while the PNS controls the "rest & digest" response. The main overall over effect of the SNS is lớn prepare the body for physical activity, a whole-body reaction affecting many organ systems throughout the body to redirect oxygen-rich blood khổng lồ areas of the body toàn thân needed during intense physical dem&.<1>


Structure & Function

The sympathetic nervous system is composed of many pathways that persize 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 khổng lồ L2) with the cell bodies distributed in four regions of the gray matter in the spinal cord bilaterally & symmetrically.<1><2> As opposed khổng lồ the parasympathetic nervous system, the first-order neurons of the SNS are short before synapsing on postsynaptic neurons found within sympathetic ganglia. Similar to the PNS, the neurotransmitter used at this junction is acetylcholine. This acetylcholine activates nicotinic receptors. These postganglionic neurons then travel to their effector sites và release the neurotransmitters epinephrine or norepinephrine, except for sympathetic innervation of sweat glands & the arrectores pili muscles, the small muscles attached lớn hair follicles, which use acetylcholine as their postganglionic neurotransmitter.<3> These neurotransmitters act on adrenergic receptors. Ahy vọng the adrenergic receptors are alpha-1 (coupled khổng lồ a Gq và works through the IP3/Ca2+ pathway), alpha-2 (coupled to lớn Gi and works through decreasing the cAMP pathway), & beta-1 và beta-2 (coupled to lớn Gs và works through increasing the cAMPhường 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 và involve sầu many organ systems and 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 to lớn 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 lớn supply the body with oxygenated blood.
In the lungs, bronchodilation (beta-2) và decreased pulmonary secretions (alpha-1, beta-2) occur lớn allow more airflow through the lungs.
In the stomach và intestines, decreased motility (alpha-1, beta-2) and sphincter contraction (alpha-1), as well as contraction of the gallbladder (beta-2), occur khổng lồ slow down digestion lớn divert energy khổng lồ other parts of the toàn thân.
The exocrine and endocrine pancreas (alpha-1, alpha-2) decreases both enzyme và insulin secretion.
In the urinary bladder, there is relaxation of the detrusor muscle and contraction of the urethral sphincter (beta-22) to lớn help stop urine output during sympathetic activation.
The kidney (beta-1) increases renin secretion to lớn increase intravascular volume.
The salivary glands (alpha-1, beta-2) work through small volume potassium and water secretion. 

Actions of SNS which do 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, and kidney, and weak constriction through the alpha-1 và beta-2 receptors in the skeletal muscle.
In the liver, increased glycogenolysis & gluconeogenesis (alpha-1, beta-2) occur lớn allow for glucose to lớn be available for energy throughout the body toàn thân.
In the spleen, there is a contraction (alpha-1).
Sweat glands and arrector pili muscles (muscarinic) work to lớn increase sweating and erection of hair to 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 and non-neural ectoderm. They form the dorsal neural folds as the folds themselves size the neural tube.<5>


Physioxúc tích và ngắn gọn 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 & a reduction in PNS activity. However, both sympathetic và parasympathetic nervous activity lớn the iris decreases with aging, which is consistent with the general decline of peripheral somatic nerve sầu 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, ahy vọng other disease processes, in both age-related hypertension and 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 input.<9> There are even reports after minimally invasive sầu thyroidectomy.<10> For more information on Horner syndrome, please refer to our accompanying article.<11>

Hyperhidrosis, otherwise known as excessive sầu sweating, is a comtháng indication for minimally invasive thoracic sympathectomy. Hyperhidrosis is excessive sweating beyond the organism’s physiological need khổng lồ sweat to lớn have a temperature within an adequate range. Removing the sympathetic đầu vào to the part of the body affected by hyperhidrosis is an acceptable and well-tolerated treatment.<12> Thorascoposic sympathectomy can also be useful to treat severe Raynaud syndrome, defined as episodic vascular spasms and 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, 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 sản phẩm of parasympathetic activity. At resting state, the SNS predominates, và the penis remains flaccid. However, if the sympathetic fibers to the penis are damaged or compromised, a sustained erection of over 4 hours, called priapism, can occur & result in devastating consequences khổng lồ the penis. This condition can result from spinal cord or caudomain authority equimãng cầu injury as the sympathetic input is damaged, và 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 comtháng causes of sympathetic nerve sầu neuropathy. This sympathetic denervation can lead khổng lồ 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 and causes dysfunction of many systems, including the heart, the Gastroenterol tract, the & genitourinary system, and sexuality. As it is well established that hyperglycemia is the primary driver of this diabetic complication, the clinician must establish early and sustained intensive sầu glycemic control khổng lồ prevent or delay the onmix & 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.<19> It is caused by a wide variety of disease processes, including but not limited to lớn pure autonomic failure, multiple system atrophy, và autonomic neuropathies that damage the SNS.<20>

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