Nervous System Anatomy and Physiology: Pharmacologic Reactions

This lecture is a continuation of our previous nervous system’s anatomy and physiology. In this piece, we will go through the involvement of sympathomimetic and parasympathomimetic drugs to the peripheral nervous system.

As a quick review, the nervous system is primarily composed of the following:

  • Central nervous system (CNS)
  • Peripheral nervous system (PNS)

The peripheral nervous system is broken down into two parts, mainly:

  • Sympathetic nervous system
  • Parasympathetic nervous system

Sympathetic Nervous System

The sympathetic nervous system, also known as the fight or flight response, is related mainly to stress. This means that when stimuli trigger the sympathetic nervous system, the following occurrences, which are also signs of stress, are bound to happen:

  1. Increased heart rate
  2. Increased respiratory rate

Parasympathetic Nervous System

The parasympathetic nervous system, also known as the rest and digest response, is the opposite of the sympathetic nervous system. This is basically what happens when the body is not going into overdrive or utilizing the sympathetic nervous system.

The parasympathetic nervous system’s functions are:

  • Slows down the heart rate
  • Stimulates gastrointestinal activities
  • Relaxes the muscles of the urinary and excretory system

The Teeter-Totter Effect

Remember that the sympathetic nervous system and the parasympathetic nervous system are opposites, acting like a teeter-totter wherein, if one is turned on, the other is turned off automatically.

Sympathomimetic versus Parasympathomimetic

The sympathomimetic and parasympathomimetic drugs are like nicknames for the sympathetic and parasympathetic nervous system drugs, emulating their functions and actions inside the body.

The Sympathomimetic Drugs

Sympathomimetic drugs are sympathetic nervous system stimulants, jumpstarting the heart and lungs to increase their rates; thereby, boosting oxygen supply to numerous parts of the body and inducing better breathing.

Epinephrine is a typical example of a sympathomimetic drug. Epinephrine is adrenaline utilized in hospital settings, especially in emergency rooms.

The Parasympathomimetic Drugs

When the body is given parasympathomimetic drugs, other organs will be prioritized, and the following will happen:

  • Increased digestion
  • Increased mucus production
  • Increased blood flow to different organs

This means that there will be a cholinergic effect that can lead to better perfusion to the mucous membranes, gastrointestinal tract, and kidneys.

SNS and Adrenaline

Loading up adrenaline to a client will shunt blood to the two vital organs of the body – the heart and lungs. Blood pressure will shoot up, squeezing blood from the extremities to make sure that distribution will be focused on the system that matters most during distressing situations.

However, there are side effects that are expected to happen, namely:

  • Dry mouth
  • Constipation
  • Urinary retention
  • Confusion
  • Drowsiness

Hospital Scenario

A client who is in cardiac arrest comes into the emergency room. The health team is pumping the heart to perfuse the body. Are you going to give a parasympathomimetic or sympathomimetic drug?

To sustain life, the heart and lungs need to have adequate perfusion. Therefore, epinephrine and dopamine have to be given to raise the blood pressure and pulse, and shunt the required blood to the heart and lungs.

In our next discussion, we’ll be talking about agonist, adrenergic agonist, and antagonists.

What You Need to Know about Sympathomimetic Drugs

Here, we’ll be going right into the anaphylactic drugs.

While eating at a restaurant, your client has suddenly developed an anaphylactic reaction after devouring a plateful of mixed seafood. After a couple of minutes, the person goes into a severe allergic reaction. The moment a person experiences an anaphylaxis due to something he or she ate, inhaled, or came in contact with, the result would be decreased oxygen perfusion to the body.

A Background on Anaphylaxis

It doesn’t matter if it’s a heart attack, a stroke, a deep vein thrombosis, or a pulmonary embolism; for as long as the pathways for blood-rich oxygen are blocked, the body becomes compromised.

In the case of anaphylaxis, the blood vessels become wider, causing low pressure which is equated to low perfusion and essentially, low oxygen supply. Therefore, the goal of anaphylactic drugs is to reverse this consequence.

Sympathomimetic Drugs

The first type of anaphylactic drug is the sympathomimetic drug. Sympathomimetic drugs are meant to mimic the role of the sympathetic nervous system, acting on the fight and flight response. Tapping the sympathetic nervous system will cause increased blood flow to the three main organs in the body – the lungs, the heart, and the brain. Thereby decreasing the rest and digest phase and getting the required amount of oxygen where the body needs it. This is how sympathomimetic drugs are created – to imitate the parasympathetic response system.

The sympathomimetic drugs that are used in hospitals are:

  • Epinephrine
  • Isoproterenol
  • Ephedrine

Administration of either one of these drugs would immediately cause vasoconstriction, increasing perfusion by shunting blood to the brain, lungs, and heart.


Among these three kinds, epinephrine is the most common medication used in any healthcare setting, especially with emergency cases. Epinephrine is what’s given first during a cardiac arrest to jumpstart the heart that stopped beating. The body naturally creates epinephrine or what is also known as adrenaline; therefore, epinephrine is just adrenaline in a bottle.

How to Remember Epinephrine

Epinephrine or sympathomimetic drugs have several effects on the body, to quickly recall what these drugs to do the body, remember the acronym, NASCAR.

  • N – nervousness
  • A – anxiety
  • S – sugar in the blood is increased
  • C – cardiac arrest
  • A – allergic reaction
  • R – respiratory bronchodilator

A Summary

Sympathomimetic drugs stimulate the body’s oxygen and blood perfusion by imitating the sympathetic nervous system’s effects. Introduction of epinephrine to the body can cause adverse reactions like nervousness, anxiety, and increased blood sugar. Sympathomimetic drugs also cause headaches, palpitations, tremors, and dizziness.

Aside from cardiac arrest, epinephrine is also used for the anaphylactic reaction that can be caused by several factors like food, medication, latex, and insect stings. A secondary effect of sympathomimetic drugs is respiratory bronchodilation or lung expansion that is due to the shunting of blood to the respiratory tract.

So that’s sympathomimetic drugs in a nutshell. For those who want to brush up on the topic of sympathetic versus parasympathetic nervous system, we have a lecture dedicated entirely on that matter. Or if you just want to check out our other nursing-related videos and NCLEX® reviews, head on to our Simple Nursing website and YouTube channel.