6-8. ALPHA AND BETA RECEPTOR SITES
It has been found that different effector organs have either alpha or beta
predominant receptor sites.
a. Alpha Receptors. Alpha-receptors are associated mainly with increased
contractibility of vascular smooth muscle and intestinal relaxation. Alpha-receptors
have been classified into two types.
(1) Alpha1. Alpha1 receptors are located at the postsynaptic effector sites to
stimulate transmitter release in smooth muscle (that is, contracts smooth muscle of
peripheral blood vessels.
(2) Alpha2. Alpha2 receptors are located presynaptic on axon terminals to
inhibit release of transmitter (norepinephrine). These predominate in the intestinal tract
to cause relaxation.
b. Beta Receptors. Beta-receptors are associated with vasodilation and
relaxation of nonintestinal smooth muscle and cardiac stimulation. Beta-receptors are
divided into two types (example: bronchial dilation).
Beta1. Beta1 receptors cause cardiac stimulation and lipolysis.
(2) Beta2. Beta2 receptors cause bronchodilatation, relaxation of blood
vessels (usually skeletal muscles), and muscle glycogenolysis.
6-9. EFFECTS PRODUCED BY THE SYMPATHETIC NERVOUS SYSTEM
The sympathetic nervous system produces a variety of physiological effects upon
the body. Listed below are some of these effects/responses:
Eye (Pupil). Mydriasis (dilation) of the pupil is produced by alpha stimulation.
b. Heart. Both an increase in heart rate and an increase in the contraction
strength of the heart are produced by beta stimulation.
c. Bronchi. Relaxation of the bronchial muscle is produced by beta2
(1) Blood vessels in skeletal muscle. Constriction or dilation is produced--
over the usual concentration range of physiologically released and circulating
epinephrine, the beta-receptor response (vasodilation) predominates in blood vessels of
skeletal muscle and liver. The alpha-receptor response (vasoconstriction) is obtained in
blood vessels of other abdominal organs.