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The autonomic nervous system is divided into two parts: 1) the parasympathetic, or craniosacral, division and 2) the sympathetic, or thoracolumbar, division.

The parasympathetic division is associated with the cholinergic system, while the sympathetic division is associated with the adrenergic system.

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In the sympathetic division, four neurotransmitters are released:
1) acetylcholine (ACH), which is released at the ganglia,
2) norepinephrine (NE), which is released at the adrenal medulla and neurons,
3) epinephrine (EPI), which is produced by the adrenal medulla, and
4) dopamine.

In the parasympathetic nervous system, the neurotransmitter acetylcholine (ACH) is released at both the ganglia and the postganglionic nerve endings.

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Adrenergic (sympathetic) receptors:

alpha 1:  found on glands, smooth muscles, eyes, most arteries and veins

alpha 2: found on the central (brain) and peripheral nervous systems

beta 1: found on the heart

beta 2: found on skeletal muscle arteries, coronary arteries, smooth muscle, uterus, and lungs

Cholinergic (parasympathetic) receptors:

muscarinic: located on cardiac, smooth muscle (visceral organs), and glands

nicotinic 1 (also known as nicotinic-nerve, Nn): located at ganglia of parasympathetic nerves and sympathetic nerves

nicotinic 2 (also known as nicotinic-muscle, Nm): located at the neuromuscular junction of skeletal muscle

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Adrenergic (sympathetic) receptor effects when stimulated:

alpha 1:  smooth muscle contraction, vasoconstriction of arteries and veins, pupillary dilation from contraction of iris muscle

alpha 2: decreases release of norepinephrine from adrenergic nerve ending, relaxes contraction of smooth muscle, vasodilation of arteries and veins

beta 1: end organ is the heart; increase in heart rate, force of contraction and conduction

beta 2 (by EPI only): bronchiolar smooth muscle undergoes bronchodilator, relaxation occurs at the uterus, and vasodilation occurs at the coronary and skeletal muscle blood vessels

Cholinergic (parasympathetic) receptor effects when stimulated:

muscarinic: decreases heart rate, bronchoconstriction, increase in digestion, GI secretions of the digestive system, constricts pupils

nicotinic 1 (also known as nicotinic-nerve, Nn): stimulates sympathetic and parasympathetic postganglionic neurons

nicotinic 2 (also known as nicotinic-muscle, Nm): skeletal muscle contracts

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Adrenergic Drug Classes (Sympathetic Division)

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Adrenergic Drug Classes (Sympathetic Division)

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Sympathomimetics: mimics the sympathetic system

alpha-1 adrenergic agonists: stimulate the alpha-1 andenergic receptors, causing:

contraction of smooth muscle and blood vessels, increasing blood pressure, vasoconstriction of nasal passage

ex: phenylephrine (Neo-Synephrine®)

adrenergic agonists: stimulates all alpha and beta receptors, causing:

increase in blood pressure by vasopressor-vasoconstriction, stimulation of heart, bronchodilation

ex: epinephrine (Adrenalin®)

beta-1 adrenergic agonists: stimulate different receptors depending on dosage--

low dose: stimulates dopaminergic receptors, causing vasodilation & increased renal blood flow

moderate dose: stimulates beta-1 receptors, increasing force of contraction and cardiac output

high dose:  stimulates alpha-1 receptors, causing vasoconstriction & increase in blood pressure

drug example: dobutamine

beta-2 adrenergic agonists: stimulate beta-2 receptors

causes relaxation of the smooth muscle of uterus and lungs (bronchodilation)

drug examples:

albuterol (Proventil®, Ventolin®)

terbutaline (Brethine®)

formoterol (Foradil®)

Sympatholytics: drugs that work against the sympathetic system

alpha-1 adrenergic blocking drugs bind to alpha-1 receptors and blocks them, causing

relaxation of smooth muscle, vasodilation of blood vessels (decreases blood pressure), and relaxes smooth muscle of uterus and increases urine flow

drug examples

prazosin (Minipress®)

terazosin (Hytrin®)

doxazosin (Cardura®)

selective beta-1 blockers:  binds to beta-1 receptors and blocks them

decreases force of contraction, cardiac output (lower blood pressure), heart rate, and conduction

drug examples

atenolol (Tenormin®)

metoprolol (Lopressor®)

non-selective beta blockers: blocks beta-1 and beta-2 receptors

decreases force of contraction, heart rate, conduction, and cardiac output, thus lowering blood pressure (may also cause bronchoconstriction)

drug examples:

nadolol (Corgard®)

propranolol (Inderol®)

timolol (Blocadren®)

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Parasympathomimetics: drugs that mimic the parasympathetic system

direct-acting cholinergenic agonists: stimulates muscarinic receptors; mimics ACH but effect depends by drug

acetylcholine: constricts pupil

pilocarpine (Pilocar®): constricts pupils, decreases fluids in eye and intraocular pressure

bethanechol (Urecholine®): stimulates muscarinic receptors of bladder & GI, preventing constipation and urinary retention

indirect-acting cholinergic agonists: inhibits acetylcholinerase, the enzyme that breaks down ACH in synapses; increases ACH

ex: neostigmine (Prostigmin®)

Parasympatholytics: drugs that work against the parasympathetic system

Anticholinergics: binds to a cholinergic receptor and blocks it (competitive antagonism:

examples

atropine: increases heart rate, decreases vagus nerve activity, slows digestion and urination

dicyclomine (Bentyl®): decreases GI secretions and motility

ipratropium bromide (Atrovent®): bronchodilation