Introduction to CV Pharmacology - Orlando Health

CV Pharmacology
Autonomic Cardiovascular Innervation
In order to have a better understanding of how cardiovascular drugs work, a brief review of how
the heart and circulation are regulated by the autonomic nervous system is needed. The
autonomic nervous system is composed of two branches; the sympathetic nervous system (SNS)
and the parasympathetic nervous system (PNS). The SNS and the PNS normally work together
to help maintain homeostasis in the body, including the cardiovascular system.
The fibers of the sympathetic nervous system affect the organs and tissues in which they are
located in by releasing norepinephrine (noradrenalin). These fibers are known as adrenergic
fibers in reference to their relationship to norepinephrine.
Some drugs are known as sympathomimetics because they mime or mimic the actions of the
sympathetic nervous system. These drugs may also be known as adrenergic agents because they
have the effects that stimulate the adrenergic fibers.
The sympathetic nervous system innervates all chambers of the heart. Sympathetic stimulation
(the “fight or flight” response) results in peripheral vasoconstriction, an increase in heart rate,
faster speed of electrical impulses conducted through the heart (conductivity), and an increase in
the strength with which the heart pumps blood (contractility).
While fibers of the parasympathetic nervous system do exist in both the atria and the ventricles,
they have a pronounced effect on the atria, while causing a minimal effect on the ventricles. The
fibers of the parasympathetic nervous system affect the organs and tissues that they innervate by
releasing acetylcholine. These fibers are known as cholinergic fibers in reference to their
relationship to acetylcholine. Parasympathetic stimulation results in a decrease in heart rate, a
slowing of electrical conduction through the AV node, and a mild decrease in the pumping action
of the ventricles.
Properties of Cardiac Cells
Automaticity is the ability of the cardiac pacemaker cells to spontaneously initiate an electrical
impulse without being stimulated from another source such as a nerve.
Excitability is the ability of the cardiac cells to respond to an external stimulus, such as from a
chemical, mechanical, or electrical source.
Conductivity is the ability of the cardiac cell to receive an electrical stimulus and conduct the
impulse to another cardiac cell. The conduction can be altered by factors such as sympathetic or
parasympathetic stimulation, cardiac muscle damage, electrolyte imbalances and medications.
Contractility is the ability of the cardiac cell to contract in response to an electrical stimulus.
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