Adrenergic means "having to do with adrenaline." Adrenaline is now called epinephrine (E) but the old terminology is still in use. Noradrenaline or norepinephrine (NE) is very similar to epinephrine though less potent; it is the neurotransmitter secreted by most postganglionic neurons of the sympathetic division (gold neurons with dashed axons).
The target tissues that respond to these compounds have adrenergic receptors in their membranes. There are two categories of receptors, alpha and beta, each of which has two subtypes. Both the alpha 1 (a1) and beta 1 (B1) are stimulatory (i.e., color-coded green) and the alpha 2 (a2) and beta 2 (B2) are inhibitory (i.e., color-coded red).
The illustrations to the right imply that norepinephrine is releases from the termimals of postganglionic neurons (orange bars); it then binds with the alpha 1 receptors in the target organs. Norepinephrine and epinephrine can also reach receptors by diffusing (arrows) out of nearby capillaries. Binding of epinephrine (E) or norepinephrine (NE) to this type of receptor stimulates the target tissue; accordingly alpha 1 receptors are color-coded green.
Alpha 1 receptors are characteristic of vascular smooth muscle, although its density varies thoughout the body. For example, it is sparce on the coronary vessels -- therefore not shown in this location in the model -- but quite abundant on blood vessels of the oral & nasal mucosae. It is also characteristic of GI & urinary sphincters. Contraction of these smooth muscle fibers encircling these tubular structures narrows them thus restricting the movement of substances through them.
Alpha 1 receptors are also found on arrector pili muscles attached to hair follicles; their contraction accounts for "goose bumps." They are also found on apocrine sweat glands in the armpits, groin and palms causing 'nervous sweat' due solely to hormones in the blood. Contraction of the dilator muscle of the iris, causing the pupil to dilate, is due to alpha 1 binding .
They are abundant in the brain and are associated with the pain perception. In the periphery they are mainly located on vascular smooth muscle of veins more so than on arteries. Their effect is inhibitory.
Their most notable location is in the terminal membranes of adrenergic neurons (i.e., those that secrete norepinephrine) where they are described as "presynaptic". They are a component of a negative feedback mechanism in which the NE secreted by that terminal binds to its own alpha 2 receptors and, via a second messenger system, inhibits further secretion of NE by that neuron.
These receptors are characteristic of the heart; they are associated with the conducting system (e.g., pacemaker) and the ventricular musculature. They are also found in eccrine sweat & salivary glands. Binding of epinephrine or norepinephrine causes excitatory responses in these tissues; thus beta 1 is color-coded green.
Epinephrine binds to beta 2 receptors causing inhibitory responses by the target tissues; thus beta 2 is color-coded red. Norepinephrine does not bind to them.
Of special interest is their location on circular smooth muscle of air passages and some arterioles. These receptors are also located throughout the visceral smooth muscle of the GI tract & urinary bladder.
Last update: 10/12/2005