The autonomic nervous system plays an important role in regulating bodily functions, including heart rate, blood pressure, bronchial muscle movements, saliva secretion etc. (Webster, 2001, pp. 15). The functioning of the system is based on a network of neurons that release chemicals to transmit messages to the respective tissues groups. The receptors receive these chemicals and convert into functional responses that are carried out by the tissues (Duvall and Kershner, 2006, pp. 17). Different Drugs have different Effects on Nervous System. The autonomic nervous system can be divided into two categories:
- Sympathetic Autonomics Nervous System
- Parasympathetic Autonomics Nervous System (Webster, 2001, pp. 15)
When drugs are administered into a body, they affect the organs or tissues that are supplied with neurotransmitters by the autonomic nervous system. The response of both sympathetic and parasympathetic divisions can be controlled based on the configuration and quantity of drug used. These drugs can be classified into four categories:
- P+ agents: Stimulation of the parasympathetic autonomic nervous system
- P- agents: Blocking of the parasympathetic autonomic nervous system
- S+ agents: Stimulation of the sympathetic autonomic nervous system
- S- agents: Blocking of the sympathetic autonomic nervous system
1. P+ DRUG AGENTS
P+ drugs act in two ways. They either act on receptors (direct action) and affect the bodily response, or act on neurotransmitters (indirect action) and affect the release of chemicals (Webster, 2001, pp. 17). Most of the effects of P+ drugs are pharmacological in nature. In cases when large doses of P+ drugs are administered, the patient may have difficulty in producing saliva and urinating. From this point, if the amount if further increased, it may affect the neuromuscular system causing paralysis.
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P+ drugs can affect the cardiovascular system, both directly and indirectly. When it acts in a direct manner, the cardiovascular output is decreased as a result of decrease in heart rate and myocardial contractility. On the other hand, indirect action of P+ drugs causes increase in the cardiovascular output as well as the heart rate. As the cardiovascular response of direct and indirect action of P+ drugs is negative, the overall effect depends on the quantity and concentration of drug administered (Olshansky et al, 2008). In general, P+ drugs cause an overall decrease in cardiac output and blood pressure.
P+ drugs cause an increase in the spontaneity and activity of the muscles of the gastrointestinal tracts. As the muscles are excited, the level of secretion is also increased.
Effects on the Eye
P+ drugs lead to constriction of the pupil (also known as myosis), and may even cause loss of visual accommodation due to paralysis of circumferential tissues of the eye (also known as cycloplegia). As the intraocular pressure around the ciliary muscles is decreased, P+ drugs are generally used to cure eye diseases such as glaucoma.
P- DRUG AGENTS
P- drug agents occupy the acetylcholine receptor sites, and block the acetylcholine receptors that form protein receptor complexes in the cell membranes of intestinal muscles, glandular tissues, and the circumferential muscles around the heart (Duvall and Kershner, 2006, pp. 17). Drugs made by these agents are referred as Psychoactive Drugs in Clinical Ideology.
In large doses, P- drug agents help preventing the blocking of cranial nerves, thus accelerating cardiac activity (Olshansky et al, 2008). This is particularly useful in avoiding cardiac arrest when a high dose of anesthesia is administered. On the other hand, in small doses, the effect is opposite, causing the heart rate and activity to decrease. The overall effect on the cardiovascular system is a function of both direct and indirect action of the P- drug agents.
Effects on Nervous System
Depending on the quantity and concentration of drug administered, P- drug agents can either stimulate or slow down the functioning of the central nervous system. There are two types of P- drug agents, tertiary and quaternary (Kluwer, 2009). While the tertiary agents are water soluble and can easily penetrate the central nervous system; the quaternary agents are no soluble in water, thereby having less chance of entering the brain and having negative effects on the central nervous system.
Effects on Exocrine Glands
P- drug agents inhibit the flow and quantity of secretions from the respiratory, intestinal, and urinary glands. This allows P- drug agents to be useful in certain dental procedures that involve control of salivation and drying of mouth.
Effects on Muscles
P- drug agents decrease the spontaneity and activity of involuntary muscles, primarily around respiratory and intestinal tracts. As such, these drugs can be used in the treatment of asthma and spasms. However, large dosage of P- drugs can cause constipation due to decrease in activity of esophagus and gastric glands.
Effects on the Eye
P+ drugs lead to dilation of the pupil (also known as mydriasis), and may even cause loss of visual accommodation due to paralysis of circumferential tissues of the eye (also known as cycloplegia). The mydriasis effects can be used for ophthalmological purposes, as the pupil dilation effect allows one to closely examine the retina. Similarly, cycloplegic effects can help in taking eye measurements.
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S+ DRUG AGENTS
There are different types of S+ drug receptors, the most important of which are: alpha and beta receptors (Webster, 2001, pp. 21). Effects of S+ drugs vary depending on the individual stimulation of the alpha and beta receptors respectively.
Effects on Nervous System
S+ drug agents, particularly amphetamines, activate the central nervous system to a great extent and increase body alertness. High doses of such drug agents can lead to higher levels of anxiety, restlessness, muscle twitching, etc.
The effect of S+ drugs on the cardiovascular system depends on the combination of direct and indirect actions. In general, S+ drug agents cause an overall increase in the contraction of cardiac muscles. When the blood pressure increases, the heart rate is decreased. This causes the alpha receptors to constrict and the beta receptors to dilate; causing expansion of the pulse rate (Greenstein and Greenstein, 2007).
S+ drug agents that affect the alpha receptors lead to constriction of skin vessels; whereas, S+ drug agents that affect the beta receptors lead to dilation of skeletal muscles. Therefore, it can be said that while alpha receptor drug agents increase the resistance to blood flow, the beta receptor drug agents cause an overall decrease.
S+ drug agents affecting beta receptors lead to a decrease in the activity of respiratory tracts. This makes them useful in the treatment of asthma and severe throat allergies.
S+ drug agents activating the beta receptors slow down the release of insulin and accelerate the breaking of glycogen in the liver (glycogenolysis) (Greenstein and Greenstein, 2007). As the lipids and fatty acids start breaking down rapidly, the overall metabolic rate is increased.
Effects on the Eye
S+ drug agents lead to pupil dilation (mydriasis) and decrease in the fluid pressure of the lens. This makes them useful in the treatment of ophthalmologic conditions, such as glaucoma.
S- DRUG AGENTS
S- drug agents have the ability to block all S- drug receptors, or can selectively inhibit the activity of alpha and beta receptors individually.
Alpha S- Agents
S+ drug agents that activate the alpha receptors, lead to narrowing of the blood vessels as their muscular walls are contracted (Kluwer, 2009). Administering alpha S- drug agents prevents this from happening and lead to a decrease in the resistance to blood flow. As the blood pressure is decreased, the cranial nerves are activated, thereby, leading to a decrease in heart rate. Alpha S- agents also block the pupil dilation effects of S+ drug agents. Common uses of alpha S- agents include treatment of vascular spasms, hypertension, hypertrophy etc.
Beta S- Agents
Beta S- agents inhibit the beta receptors in the sympathetic autonomic nervous system. While the S+ drug agents that activate beta receptors cause expansion of blood vessels, respiratory tracts, increase in heart rate, etc.; beta receptor blockers produce the opposite effect, thereby causing an increase in heart rate, constriction of respiratory tracts, etc (Kluwer, 2009). Beta S- drug agents are particularly useful in the treatment of hypertension, migraine, hyperthyroidism etc.
The autonomic nervous system acts as an involuntary control mechanism for many bodily functions. When drugs are administered into the body, they directly affect the receptors responsible for tissue and glandular response. The results are subjected to the Nursing Management of the lab. In this report, effects of different types of drug agents on different functional systems of the body were discussed. Mechanism of activity, effect on receptors, and treatment were also studied in brief. It was found that concentration of a particular drug agent can have varying effects on a single group of receptors. The response of the tissues and glands is a combination of drug agents acting on receptors in the autonomic nervous system, and the neurotransmitters releasing the chemicals.
- Duvall, B. and Kershner, R. M. (2006), Ophthalmic Medications and Pharmacology, 2nd ed., Thorofare, NJ: SLACK Incorporated.
- Greenstein, B. and Greenstein, A. (2007), Concise Clinical Pharmacology, London, UK: Pharmaceutical Press.
- Kluwer, W. (2009), Clinical Pharmacology Made Incredibly Easy, 3rd ed., Ambler, PA: Lippincott Williams & Wilkins.
- Olshansky, B. Sabbah, H. N. Hauptman, P. J. and Colucci, W. S. (2008), Contemporary Reviews in Cardiovascular Medicine, Circulation 118: 863 – 871.
- Webster, C. R. L. (2001), Clinical Pharmacology, Jackson, MA: Teton NewMedia