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The sympathetic(adrenergic) nervous system is one division of the ANS. It is also known as the Thoracolumbar system. The thoracolumbar system is known for "Fight, Fright, Flight" because it is what regulates our organs and glands when we are in a situation filled with stress whether it be physical or mental causing our "fight or flight" response.

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There are four important neurotransmitters in the sympathetic system which are Acetylcholine(ACH), Norepinephrine(NE), Epinephrine(EPI), & Dopamine(DA).
Acetylcholine is a cholinergic neurotransmitter that is released at the ganglia from presynaptic nerve endings. It is present at the ganglia in both the sympathetic and parasympathetic systems.
Norepinephrine is an adrenergic neurotransmitter that is released at the postganglionic nerve endings, also some is released by the adrenal medulla & neurons.
Epinephrine is an adrenergic neurotransmitter that is released by the adrenal medulla.
Dopamine is a neurotransmitter that is important to both the sympathetic and central nervous system.

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Receptors of the sympathetic nervous system

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Receptors of the sympathetic nervous system

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The receptors of the sympathetic system are broken into two categories.

Ganglionic receptors:

present on ganglia

nicotinic receptors

activated by acetylcholine

Target receptors:

present on the target organs

activated by norepinephrine or epinephrine

There are four types of target receptors

Alpha-1 receptors:

stimulatory receptors

present on blood vessels, eyes, urinary bladder, kidney, most arteries and veins

activated by either postganglionic neurons or norepinephrine and epinephrine

when activated causes contraction of smooth muscles, which then leads to vasoconstriction, can also cause dilation of pupils

Alpha-2 receptors:

inhibitory receptors

present on nerve terminals

stimulated by norepinephrine and epinephrine

negative feedback system

when activated they decrease contraction of smooth muscles which then leads to vasodilation and decreases/regulates the release of norepinephrine

Beta-1 receptors:

stimulatory receptors

stimulated by norepinephrine and epinephrine, stronger effect from epinephrine

present in the heart

when activated they cause increase in heart rate(+chronotropic), the force of contractions (+inotropic), increase in conduction(+dromotropic)

Beta-2 receptors:

present in smooth muscle, lungs, uterus, skeletal muscle arteries, coronary arteries

stimulated by epinephrine

when activated causes smooth muscle relaxation, bronchodilation, uterine relaxation, relaxation of coronary arteries and skeletal muscles blood vessels-vasodilation

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Alpha-1 adrenergic agonists:

vasoconstriction of nasal passages which promotes decongestion

alpha-1 receptor is stimulated

- drugs:

1. ephedrine

2. phenylephrine (Neo-Synephrine®)

Adrenergic agonists:

stimulates all alpha and beta receptors

causes vasoconstriction and increases blood pressure due to stimulating alpha-1 receptor

stimulates beta-1 and causes cardiac stimulation

stimulates beta-2 causing bronchodilation

-drugs:

1. epinephrine(Adrenalin®)

Alpha-1 adrenergic blocking drugs:

binds to alpha-1 receptors and blocks

works against sympathetic system

causes relaxation of smooth muscles

causes vasodilation of blood vessels which then decreases blood pressure

increases urine flow due to the relaxation of the smooth muscle of the ureters

-drugs:

1. prazosin (Minipress®)

2. terazosin (Hytrin®)

3. doxazosin (Cardura®)

Beta-1 adrenergic agonists:

stimulates beta-1 receptors

low doses stimulate d-1 receptors in kidneys which then causes vasodilation and increased renal blood flow

moderate doses stimulate beta-1 receptors in the heart which then increases the force of contraction and the cardiac output

high doses stimulate alpha-1 receptors which then causes vasoconstriction

-drugs:

1. dobutamine(Dobutamine®)

Beta-2 adrenergic agonists:

stimulates beta-2 receptors

when stimulated it causes relaxation of the smooth muscles of the lungs and the uterus

relaxation of the smooth muscle of the lungs causes bronchodilation

-drugs:

1. albuterol (Proventil®, Ventolin®)

2. terbutaline (Brethine®)

3. formoterol (Foradil®)

4. salmeterol (Serevent®)

Beta adrenergic blocking drugs

Selective beta-1 blockers:

blocks only the beta-1 receptors

antagonizes norephinephrine and epinephrine at the beta-1 receptors

decreases force of contraction (-inotropic)

decreases cardiac output which also decreases blood pressure

decreases heart rate (-chronotropic)

decreases conduction (-dromotropic)

-drugs:

1. atenolol (Tenormin®)

2. metoprolol (Lopressor®)

Non-selective beta blockers:

blocks beta-1 and beta-2 receptors

decreases force of contraction (-inotropic)

decreases cardiac output which also decreases blood pressure

decreases heart rate (-chronotropic)

decreases conduction (-dromotropic)

can cause bronchoconstriction

-drugs:

1. nadolol (Corgard®)

2. propanolol (Inderal®)

3. timolol (Blocadren®)

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When the sympathetic system is stimulated

adrenal medulla releases epinephrine

vasoconstriction of the arteries occurs (excluding coronary arteries and arteries to skeletal muscle)

increase in heart rate, AV conduction

increases contractility

on the respiratory passages, bronchodilation occurs to allow more oxygen to get in

norepinephrine is released from postganglionic neurons

decreased GI motility, secretions

decrease in urination

dilation of the pupil (mydriasis)

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The parasympathetic(cholinergic) nervous system is another division of the ANS. It is also known as the craniosacral system. It is linked with "rest and digest". This system maintains conserving functions when the body is at rest. It only stimulates one specific body part/system whereas the sympathetic system has a full body response. The chief neurotransmitter of the parasympathetic system is acetylcholine. Acetylcholine is found at the ganglia and the postganglionic nerve endings. The postganglionic neurons are what release the acetylcholine.

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There are three main types of cholinergic receptors.
Muscarinic:

present on cardiac, smooth muscle, and glands

stimulated by acetylcholine

when activated these receptors decrease heart rate, cause bronchoconstriction, increase digestion, increase urination, increase GI secretion, or cause myosis

Nicotinic 1 (Nn):

present at the ganglia of parasympathetic nerves and sympathetic nerves

stimulated by acetylcholine

Nicotinic 2 (Nm):

present at the neuromuscular junction of skeletal muscle

causes muscular contraction

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Cholinergics/Parasympathomimetics

Direct acting cholinergic agonists:

directly stimulates or binds to muscarinic receptors

mimics acetylcholine

-drugs:

1. acetylcholine

2. pilocarpine(Pilocar®)

3. bethanechol(Urecholine®)

Indirect –acting cholinergic agonists:

inhibits the enzyme acetylcholinesterase which then destroys ACH in the synapse and causes an increase in ACH

-drugs:

1. neostigmine (Prostigmin®)

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“Beta1 Receptor.” Pathwaymedicine.org, pathwaymedicine.org/beta1-receptor.
“Muscarinic Receptor.” Pathwaymedicine.org, pathwaymedicine.org/Muscarinic-Receptor.
“Sympathetic Nervous System | Function, Definition & Symptoms.” The Human Memory, 11 Oct. 2019, human-memory.net/sympathetic-nervous-system/.
Dalessio, Alice. “Effects of Sympathetic and Parasympathetic Stimulation on Specific Organs - Medical Physiology.” Euroform Healthcare, 28 Apr. 2021, www.euroformhealthcare.biz/medical-physiology/effects-of-sympathetic-and-parasympathetic-stimulation-on-specific-organs.html.