Receptor Agonists and Antagonists
A large variety of drugs and natural substances can selectively activate or block specific cholinergic or adrenergic receptors. Agonist : is a substance that binds to and activates a receptor, in the process mimicking the effect of a natural NT or hormone.
Example:
Antagonist : is a substance that binds to and blocks a receptor, thereby preventing a natural NT or hormone from exerting its effect.
Example:
Autonomic Tone
Most body organs receive innervation from both divisions of the ANS and typically work in opposition to one another. This balance is called autonomic tone and is regulated by the hypothalamus.
Sympathetic Responses
During physical and emotional stress, the sympathetic division dominates the parasympathetic. Increased sympathetic activities favors body functions that require vigorous physical activity and rapid production of ATP. Emotions such as fear, embarrassment, or rage stimulate the sympathetic division.
Think “E” situations: exercise, emergency, excitement, embarrassment.
Activation of the sympathetic division and release of hormones by the adrenal medullae set into motion a series of events called the Fight-or Flight Response . The effects of sympathetic activation are longer lasting and more widespread than the parasympathetic.
Parasympathetic Responses
This response enhances the Rest and Digest activities. Parasympathetic stimulation supports body functions that conserve and restore body energy during times of rest and recovery. Between periods of exercise, parasympathetic impulses to digestive glands and smooth muscle of the GI tract predominate and allow energy-supplying food to be digested and absorbed.
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