What Are Adrenergic Systems?

Fight-or-Flight Response

• The acute stress response, commonly known as the "fight-or-flight response," is mainly attributed to the action of the hormones in the adrenergic system. In the event of a perceived threat, the adrenergic system primes the body for either fighting or fleeing by releasing specific catecholamines. Adrenaline, a well-known hormone involved in the fight-or-flight response, causes an increase in heart rate and lung action, blood vessel dilation in muscles, bladder relaxation and pupil dilation.

Excitation and Relaxation

• Further study of the adrenergic system led to the discovery of alpha and beta receptors, types of adrenergic hormone receptors located all over the body. Both alpha and beta receptors are positioned right at the junction across which nerve impulses travel from a neuron (nerve cell) to another neuron, gland cell, muscle cell or cell from any other organ. Generally, an alpha receptor mediates the excitation of the cell receiving the nerve impulse, while a beta receptor mediates its relaxation.

Effects on Specific Organs

• In an attempt to further understand the body's responses to stimuli, researchers have studied more specific types of alpha and beta receptors. Alpha receptors divide into two groups: A1, a receptor found in the heart, liver and smooth muscle; and A2, found in the pancreas, nerve termini, vascular smooth muscle and platelets. Beta receptors, on the other hand, come in three types. The first, B1 receptors, are found in the heart; B2 in the lungs, liver, uterus, gastrointestinal tract, and skeletal and vascular smooth muscle; and B3 in fat cells.

Treatment of Various Conditions

• Knowing the action of the different types of alpha and beta receptors aids the development of drugs that target certain ailments. These drugs act either as agonists or antagonists. An agonist attaches itself to a receptor site and stimulates its response, while an antagonist reverses the effect of an agonist, blocking that response. Examples include B2 receptor agonists for asthma treatment and A1 receptor antagonists for blood pressure regulation.