Adrenergic System: Definition, Stimulation & Effects

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

The adrenergic system involves a lot of nerves, hormones, neurotransmitters, tissues, and organs. Learn about the important fundamentals behind this system and what it does.

The Adrenergic System

The part of your body that uses epinephrine and norepinephrine as biochemical messengers is sometimes referred to as the adrenergic system. By the way, epinephrine is also called adrenaline and norepinephrine is also called noradrenaline, just in case you see those terms elsewhere. We'll be sticking to (nor)epinephrine as our terms of choice in this lesson for consistency's sake.

The adrenergic system involves many different components and this lesson will go over its basic parts as well as its important effects.

Neurotransmitters & Receptors

There are several key things to understand about the adrenergic system. The first is that the nerve fibers that release norepinephrine as their main neurotransmitter are called adrenergic neurons. The receptors stimulated by either norepinephrine or epinephrine are called adrenergic receptors or adrenoceptors. Drug that activate the adrenergic receptors are called sympathomimetics. On the other hand, drugs that block the activation of these same receptors are known as sympatholytics.

This vial contains epinephrine. Epinephrine is both a naturally occurring biochemical in the body and it is synthetically produced for medical purposes. Norepinephrine is also used medically but far less so than epinephrine.

Primarily, adrenergic neurons (nerve cells) release norepinephrine as their neurotransmitter. Adrenergic neurons can be found in the central nervous system (the brain and spinal cord), as well as the sympathetic nervous system. The receptors these adrenergic neurons stimulate are called alpha 1, alpha 2, beta 1, beta 2, and beta 3. Some of these can be further subdivided into more receptor subtypes, but this is beyond this lesson's scope.

The Effects Of The Adrenergic System

The alpha adrenoceptors are more likely to be stimulated by epinephrine than norepinephrine. Alpha 1 receptors are mainly located on the organs and tissues stimulated by these two biochemicals, while alpha 2 receptors are mainly located on adrenergic nerve endings. Why? Well, when norepinephrine is released by the adrenergic nerve's ending, it turns around, so to speak, and sticks to the alpha 2 receptors located there. This causes the inhibition of any further release of norepinephrine as too much of it can be dangerous.

Think of these alpha 2 receptors as kill switches found on some electronics. If those sensors sense the electronics are outputting too much heat and are getting overheated, the sensors signal the electronics to automatically power down to avoid crashing. This helps preserve the electronics for later use, much like the alpha 2 receptor feedback inhibition system preserves our body from over-stimulation by the norepinephrine.

While the beta adrenoceptors, in general, are more sensitive to epinephrine than they are norepinephrine, the beta 1 receptors are approximately equally sensitive to either norepinephrine or epinephrine.

So what does the adrenergic system do then? Its effects are wide ranging and depend on the organ or tissue stimulated, the predominant biochemical a receptor is sensitive to, as well as the more common receptor type(s) found on that organ or tissue.

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