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You are watching: The sympathetic nervous system is to ____ as the parasympathetic nervous system is to ____.

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



The forgiveness nervous mechanism (SNS) is one of the two divisions of the autonomic nervous system (ANS), in addition to the parasympathetic nervous system (PNS), these systems mostly work unconsciously in opposite methods to regulate plenty of functions and parts that the body. Colloquially, the SNS governs the "fight or flight" an answer while the PNS controls the "rest and also digest" response. The main all at once end result of the SNS is come prepare the body for physical activity, a whole-body reaction affecting countless organ solution throughout the body to redirect oxygen-rich blood to locations of the human body needed throughout intense physical demand.<1>

Structure and also Function

The sorry nervous device is written of plenty of pathways that perform a range of features on assorted organ systems. The preganglionic neurons of the SNS arise native the thoracic and also lumbar regions of the spinal cord (T1 to L2) v the cell bodies distributed in 4 regions the the gray matter in the spinal cord bilaterally and symmetrically.<1><2> As protest to the parasympathetic worried system, the first-order neurons the the SNS space short prior to synapsing on postsynaptic neurons found within sympathetic ganglia. Similar to the PNS, the neurotransmitter provided at this junction is acetylcholine. This acetylcholine activates nicotine castle receptors. These postganglionic neurons then travel to your effector sites and release the neurotransmitters epinephrine or norepinephrine, other than for forgiveness innervation of sweat glands and the arrectores chili muscles, the little muscles attached to hair follicles, which usage acetylcholine as their postganglionic neurotransmitter.<3> These neurotransmitters plot on adrenergic receptors. Among the adrenergic receptors are alpha-1 (coupled to a Gq and works with the IP3/Ca2+ pathway), alpha-2 (coupled to Gi and works through decreasing the cAMP pathway), and beta-1 and beta-2 (coupled come Gs and also works through increasing the cAMP pathway).<4> Whether beta-1 and also beta-2 space excitatory or inhibitory counts on the tissue on which that is located. This receptors are located on various parts the the body and also regulate the actions of the SNS.

The features of the sympathetic nervous mechanism are expansive and involve countless organ systems and various species of adrenergic receptors.

The effects in which SNS acts in direct comparison to the PNS duty include the following:

In the eye, sympathetic activation reasons the radial muscle that the iris (alpha-1) to contract, which leader to mydriasis, allowing an ext light come enter. Furthermore, the ciliary muscle (beta-2) relaxes, allowing for much vision come improve.
In the love (beta-1, beta-2), forgiveness activation causes an boosted heart rate, the pressure of contraction, and also rate the conduction, allowing for increased cardiac output to supply the body through oxygenated blood.
In the lungs, bronchodilation (beta-2) and decreased pulmonary secretions (alpha-1, beta-2) occur to allow an ext airflow with the lungs.
In the stomach and intestines, reduced motility (alpha-1, beta-2) and also sphincter convulsion (alpha-1), and contraction the the gallbladder (beta-2), happen to sluggish down cradle to divert energy to various other parts that the body.
The exocrine and also endocrine kidneys (alpha-1, alpha-2) decreases both enzyme and insulin secretion.
In the urinary bladder, over there is be safe of the detrusor muscle and also contraction the the urethral sphincter (beta-22) to assist stop to pee output during sympathetic activation.
The kidney (beta-1) increases renin cheap to boost intravascular volume.
The outstanding glands (alpha-1, beta-2) work through small volume potassium and water secretion. 

Actions of SNS which execute not protest those that the PNS include the following:

There is strong constriction v the alpha-1 receptor in arterioles that the skin, abdominal muscle viscera, and kidney, and weak constriction with the alpha-1 and also beta-2 receptors in the skeleton muscle.
In the liver, increased glycogenolysis and gluconeogenesis (alpha-1, beta-2) occur to permit for glucose to be obtainable for power throughout the body.
In the spleen, over there is a convulsion (alpha-1).
Sweat glands and also arrector chili muscles (muscarinic) job-related to boost sweating and erection the hair to help cool under the body.


Neurons of the peripheral autonomic worried system, which consists of both the sympathetic worried system and also parasympathetic worried system, arise indigenous neural crest cells the originate from in between the neural and also non-neural ectoderm. They kind the dorsal neural folds together the crease themselves form the neural tube.<5>

Physiologic Variants

Aging has actually various impacts on the sympathetic nervous system. Research has demonstrated that v increased period that baroreceptors that the love decrease and become less sensitive; over there is a compensatory boost in cardiovascular SNS activity and a palliation in PNS activity. However, both sympathetic and also parasympathetic nervous task to the iris decreases with aging, i beg your pardon is constant with the general decrease of peripheral somatic nerve function.<6> Research has likewise shown the baseline levels of noradrenaline levels boost with period resulting in an elevated basal SNS activation, when the reactivity becomes reduced with aging.<7> This boost in activation theatre a role, amongst other an illness processes, in both age-related hypertension and also heart failure.<8>

Surgical Considerations

Horner syndrome is a complication born from interruption of the sympathetic innervation come the eye and also adnexa at differing levels, most typically of the neck, result in increased parasympathetic input. It presents through the classic triad of ipsilateral ptosis, pupillary miosis, and facial anhidrosis. It have the right to be a complication that neck surgeries that damages the sorry input.<9> there are also reports after minimally invasive thyroidectomy.<10> For much more information on Horner syndrome, please describe our accompanying article.<11>

Hyperhidrosis, otherwise known as extreme sweating, is a typical indication because that minimally invasive thoracic sympathectomy. Hyperhidrosis is too much sweating beyond the organism’s physiological have to sweat to have actually a temperature within an enough range. Removing the sympathetic input come the part of the body affected by hyperhidrosis is an acceptable and also well-tolerated treatment.<12> Thorascoposic sympathectomy can likewise be advantageous to treat major Raynaud syndrome, characterized as episodic vascular spasms and digital ischemia an additional to cold or emotionally stimuli.<13>

Clinical Significance

The clinical meaning of the forgiveness nervous device is vast as the affects numerous organ systems. Of the countless physiological and pathological processes, pheochromocytoma, erections and also priapism, diabetic neuropathy, and orthostatic hypotension are defined below.

Pheochromocytomas space tumors the arise native chromaffin cells existing in the adrenal medulla or paraganglion cells that secrete excess quantities of catecholamines (norepinephrine, epinephrine). Because of this catecholamine release, the symptoms are greatly that of forgiveness activation, such together hypertension, tachycardia/palpitations, hyperglycemia, and also diaphoresis.<14>

Erections space a product that parasympathetic activity. At resting state, the SNS predominates, and also the prick remains flaccid. However, if the sympathetic yarn to the prick are damaged or compromised, a continual erection of over 4 hours, called priapism, can take place and an outcome in devastating aftermath to the penis. This problem can result from spinal cord or cauda equina injury as the sympathetic input is damaged, and also the parasympathetic tone dominates.<15> Nevertheless, SNS additionally contributes to the normal sexual duty of a man. Sorry stimulation of the masculine genitals reasons sperm emission, i beg your pardon is sensed by the hypogastric nerve.<16>

Diabetic autonomic neuropathy is one of the most common causes of sorry nerve neuropathy. This sympathetic denervation have the right to lead to impaired myocardial coronary blood flow and reduced myocardial contractility.<17> Diabetic neuropathy theatre a vital role in morbidity and mortality in patients through both type 1 and form 2 diabetes mellitus and also causes dysfunction of many systems, including the heart, the Gastroenterol tract, the and genitourinary system, and also sexuality. As it is well created that hyperglycemia is the main driver of this diabetic complication, the clinician must create early and also sustained intensive glycemic manage to protect against or hold-up the onset and slow the development of autonomic dysfunction. However, this strategy appears to be more effective in type 1 versus kind 2 diabetic patients.<18> 

Lastly, orthostatic hypotension is a typical problem caused by the fail of noradrenergic neurotransmission. That 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 selection of disease processes, including however not minimal to pure autonomic failure, multiple system atrophy, and autonomic neuropathies that damage the SNS.<20>