Quiz 6: Drugs Affecting the Sympathetic Nervous System
CNS constitutes 2 structures - brain and spinal cord. It sends and receives information through the structures called nerves. Peripheral nervous system (PNS) constituting cranial and spinal nerves are divided into somatic and visceral nerves. Visceral nervous system, which is also called autonomic nervous system (ANS) is again divided into sympathetic and parasympathetic nervous systems. Sympathetic nervous system works in opposite to the parasympathetic nervous system. Both the systems activate certain functions and inactivate or reduce the rate of activity of certain functions. The sympathetic nervous system is involved in the regulation of the activities of the internal organs and glands when large amount of energy is to be spent. The situations of physical exertion, stress, anxiety and illness activate the sympathetic nervous system. The peripheral sympathetic nerves, the so called adrenergic nerves, release neurotransmitter norepinephrine (NE) that is able to produce a sympathetic stimulation.
Epinephrine (EPI) or adrenaline is a hormone and neurotransmitter secreted from the adrenal medulla at the time of fight or flight reaction. It is one of the catecholamines, which is released into the blood and reaches the respective receptors of internal organs to produce a sympathetic effect. The action of EPI, NE (norepinephrine) and beta-adrenergic drugs on the beta-1 receptor is to greatly increase the heart rate and thus to increase the blood pressure. As the force of contraction and the atrioventricular conduction increase, their action results in hypertension. In the lungs, epinephrine binds with the beta-2 adrenergic receptors to produce bronchodilation. Epinephrine is used for treating acute allergic reactions and asthma. As it can stimulate both alpha- and beta-adrenergic receptors, immediate administration subcutaneously, is the preferred method of treatment. But considering the beta-1 receptor binding and increased cardiac stimulation, epinephrine is not recommended in the case of chronic asthma treatment. A drug which does not activate beta-1 adrenergic receptors, but bind with beta-2 receptors, can be used for chronic asthma treatment. Thus, the resulting effect of the treatment can be confined to bronchodilation. The alpha-adrenergic drug terbutaline (Brethine) can be administered parenterally or in the form of tablets. As it does not bind with beta-1 receptors, it produces bronchodilation without causing cardiac stimulation.
The two major groups of adrenergic receptors are identified as alpha and beta-adrenergic receptors due to the variation in the action delivered by both epinephrine and norepinephrine. Some of the epinephrine and norepinephrine therapeutic action upon these receptors as follows: Elevated heart rate is the result of norepinephrine on receptors like beta-1. Hence, option (a) is not correct. Bronchodilation is the result of epinephrine on receptors like beta-2. Hence, option (b) is not correct. Contraction of urinary bladder is not the main effect of norepinephrine on receptors like alpha-1. Hence, option (d) is not correct Elevated force of muscle contraction in heart is the result of action of norepinephrine and epinephrine on receptors like beta-1. Hence, option (e) is not correct. Due to the action of epinephrine or norepinephrine contraction in blood vessels are generated or we can say vasoconstriction is produced. As a result of this contraction during a fight or flight, blood will be diverted from parts or organs that are not part of the reaction. Thus option (c) is correct.