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Drugs Affecting Peripheral Nervous System #2. Fight or Flight versus the Parasympathetic Pig. Adrenergic Nerve Endings. The adrenergic nerve ending synthesizes and releases norepinephrine (NE)

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Drugs Affecting Peripheral Nervous System #2

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Drugs Affecting Peripheral Nervous System #2

Fight or Flight versus the Parasympathetic Pig

Adrenergic Nerve Endings

  • The adrenergic nerve ending synthesizes and releases norepinephrine (NE)

  • Released norepinephrine stimulates adrenergic receptors located on cardiac and smooth muscle membranes to initiate sympathetic responses

  • In emergency situations the adrenal medulla releases epinephrine (EPI) which also stimulates adrenergic receptors and sympathetic activity

  • NE and EPI are inactivated by neuronal reuptake back into the nerve ending for reuse or metabolism by monoamine oxidase (MAO)

    • MAOI??

  • How do the various tissues distinguish a response to norepinephrine?

  • Why does the Fight or Flight response excite some tissues and inhibit others?

Alpha-Adrenergic Receptors

  • Alpha-adrenergic receptors are located on smooth muscle membranes and cause smooth muscle contraction when stimulated by NE, EPI, or drugs

  • Alpha receptor stimulation causes vasoconstriction of most arteries and veins

Alpha-Adrenergic Receptors Cont’d

  • Alpha receptor stimulation of the radial muscle of the pupil causes pupillary dilation or mydriasis

  • Contraction of the smooth muscle sphincters in the urinary and intestinal tracts inhibit urination and intestinal motility

Beta-Adrenergic Receptors

  • Beta-1 receptors are located primarily in the heart and cause increased heart rate, force of contraction, and atrioventricular conduction

  • Beta-2 receptors are mostly located on smooth muscle and cause relaxation of smooth muscle, especially bronchiolar and uterine smooth muscle

  • Beta-3 receptor stimulation also causes vasodilation and increased blood flow to coronary and skeletal muscle blood vessels

Table 2.2


Norepinephrine and Epinephrine (Catecholamines)

  • Norepinephrine is released from adrenergic nerve endings and primarily stimulates alpha and beta-1 receptors, it is not effective at beta-2 receptors

  • Epinephrine is released from the adrenal medulla into the blood where it travels as a hormone to stimulate all alpha and beta receptors

Alpha-Adrenergic Drugs

  • Alpha-adrenergic drugs are used clinically to cause vasoconstriction of blood vessels

  • In hypotensive states, drugs such as norepinephrine and metaraminol, are given by IV injection to raise blood pressure

  • In allergies and colds, drugs such as phenylephrine (non-catecholamine) are administered as nasal sprays or drops, and oral tablets to decongest nasal and ocular tissues

Alpha Adrenergic Drugs and Indications

Beta-Adrenergic Drugs

  • Beta drugs are used clinically to stimulate the heart, bronchodilate respiratory passageways, and relax the uterus during preterm labor

  • Selective beta-2 drugs are usually administered by oral inhalation for the control of asthma

  • Epinephrine is injected subcutaneously in allergic and cardiac emergencies to stimulate the heart, promote bronchodilation, and maintain blood pressure

Beta Adrenergic Drugs and Indications

Indirect and Mixed Agents

  • Indirect Agents cause release of norepinephrine from nerve ending

    • Do not bind directly to adrenergic receptor

  • Mixed Agents cause release of norepinephrine from nerve ending and bind directly to adrenergic receptor

  • Table 2.1

Adverse Effects Caused by Adrenergic Drugs

  • The main adverse effect caused by alpha drugs is excessive vasoconstriction resulting in hypertension or excessive dryness of the eyes and nasal sinuses when used as decongestants

  • Overstimulation with beta drugs causes excessive cardiac stimulation and increased blood pressure

  • Adrenergic drugs can also cause CNS stimulation resulting in tremors, restlessness, and anxiety


Cholinergic Nerve Endings

  • Cholinergic nerve endings synthesize and release acetylcholine (ACH)

  • Released ACH stimulates cholinergic receptors located on smooth, cardiac, and skeletal muscle membranes to initiate a variety of effects

  • ACH is inactivated by the enzyme acetylcholinesterase

Classification of Cholinergic Receptors

  • There are three main types of cholinergic receptors

  • Cholinergic receptors, also referred to as muscarinic, are associated with the parasympathetic nervous system

  • Nicotinic-neural receptors are located on autonomic ganglia

  • Nicotinic-muscle receptors are located on skeletal muscle

Cholinergic Receptor Actions

  • Cholinergic receptors are located on smooth and cardiac muscle membranes in association with parasympathetic nerve endings

  • Parasympathetic nervous activity is associated with body functions during rest and restoration of energy such as eating and digestion

  • Parasympathetic activity also controls the elimination of waste products from the urinary and intestinal tracts

Direct Cholinergic Agonists

Direct-Acting Cholinergic Drugs

  • Direct-acting cholinergic drugs are similar to ACH and stimulate receptors like ACH

  • Direct-acting drugs have longer durations of action than ACH and are clinically useful

  • These drugs are used in opthalmology as miotics and in the treatment of glaucoma

  • Bethanechol stimulates urinary bladder contraction and is taken orally to treat nonobstructive urinary retention

Indirect-Acting Cholinergic Drugs

  • Indirect-acting drugs increase ACH levels at receptors by inhibiting the enzyme acetylcholinesterase

  • These drugs primarily increase ACH at cholinergic and nicotinic-muscle receptors

  • Drugs classified as reversible inhibitors of acetylcholinesterase are the most widely used

  • These drugs are also referred to as anticholinesterase drugs

Adrenergic Blockers

Cholinesterase Inhibitors

Cholinergic Antagonists

Ganglion Blockers

Autonomic Ganglia

  • The neural synapses between autonomic pre- and postganglionic nerve fibers are referred to as autonomic ganglia

  • ACH is the neurotransmitter released at all pre- ganglionic fibers and functions to transmit the nerve impulse to the postganglionic fibers

  • The receptor on the postganglionic nerve membrane at the ganglionic site is the nicotinic-neural (Nn) receptor

Drug Actions on Autonomic Ganglia

  • Drugs that bind to and stimulate Nn ganglionic receptors activate both sympathetic and parasympathetic autonomic nerves

  • Drugs that block the Nn receptors inhibit the activity of both autonomic divisions

  • With the exception of nicotine, there are few ganglionic drugs of clinical importance

Nicotine and Ganglionic Stimulation

  • Nicotine, the main active ingredient in tobacco products, is a mild ganglionic stimulant

  • Both sympathetic and parasympathetic ganglionic Nn receptors are stimulated by nicotine

  • The main effects of nicotine via smoking are an increase in cardiac and gastrointestinal activity

  • Nicotine also produces effects in the brain related to the pleasures of smoking and the development of drug dependency

Smoking Deterrents

  • Nicotine containing gum (Nicorette) and transdermal patches (Habitrol, Nicoderm, Nicotrol, ProStep) are used to aid in quitting the smoking habit

  • The gum or patches are used in decreasing amounts over an 8–12 week time period

  • Tobacco products should not be used while using these products

Ganglionic Blocking Drugs

  • Ganglionic blocking drugs bind to the Nn receptor and block the effects of ACH at the ganglia

  • The activity of both autonomic divisions is decreased

  • The main effects of blockade are decreased blood pressure, heart rate, gastrointestinal, and genitourinary activity

  • The only clinical use of ganglionic blockers is to lower blood pressure during surgery or in the treatment of severe hypertension

Adverse Effects

  • Ganglionic blockers reduce the activity of the entire autonomic nervous system to produce a wide variety of adverse autonomic effects

  • Adverse effects include hypotension, decreased cardiac output, constipation, urinary retention, blurred vision, and dry mouth

  • A significant number of patients cannot tolerate these drugs, which accounts for the limited clinical use

Neuromuscular Transmission and Blockade

Local Anesthetics

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