What does a moving car have to do with the autonomic nervous system and the heart?
I love learning (and teaching) with the use of metaphors.
They help me associate difficult subjects with something that I interact with on a more regular basis. For example, when I first learned about the autonomic nervous system and its effects on the heart, it was bulky and complex, making it difficult to understand.
Figuratively I needed a hammer – to break this convoluted mess into chunks – and then, with the help of a metaphor, put it back together in a way that made sense to me. The metaphor that did it was the concept of a moving car (the heart) and how it responds to the gas and break pedals (autonomic nervous system neurotransmitters).
Let me share it with you.
The moving car is like the beating heart – it’s already in motion and responding to the controls.
Let’s say we want to increase the car’s speed.
There are two options (given the car is an automatic):
- We can push on the gas pedal – and the car will respond by going faster. This is like the sympathetic nervous system. Using epinephrine, dopamine and norepinephrine, the heart responds by increasing its rate. Examples of drugs include Epinephrine, Norepinephrine and Dopamine (you could have guessed that was coming…).
- You could also try taking your foot off of the break pedal (or removing the break’s influence on the moving vehicle). This is like the parasympathetic nervous system. Removing the affect of acetylcholine, will reduce the amount of parasympathetic innervation to the heart – further allowing it to speed up. Can anyone say Atropine?!
To make things a bit more interesting, think about how much you can push the gas pedal versus how much you can remove your foot off the break – way more room to manipulate the gas pedal than the break. Perhaps that explains why there is a max dosage of Atropine, but not so for epinephrine. However, as is the case, as soon as we hit a speed limit that becomes difficult for the car to handle, we realize we should slow down, before we are ARRESTED (insert morbid joke here).
So how do we get this moving car to slow down?
As you might imagine, we will use the break pedal as well as the gas pedal, but this time in opposite manner. In order to slow the car down we want to push on the break pedal. This is like flooding the nervous system with acetylcholine which causes the heart rate to drop.
An example of a chemical that will do this is an organophosphate. A term you might be familiar with is vagal tone – acetylcholine blush is just that – too much of this is a bad thing. Stomping on the break pedal may sound like a semi-reasonable and effective way to slow the vehicle, but in essence it is much better for the car if you ease off on the gas pedal, versus jamming the break. The same holds true for the heart – easing off of the gas pedal would be like limiting the amount of sympathetic innervation to the heart. Drugs like beta blocking agents would be an example of this.
I could probably go on and on about this, but let’s wrap this up. As I mentioned at the start, this was a very helpful way for me to visually remember the major components of the autonomic nervous system – perhaps it will be the same for you!!
As always, stay safe and be well,
Bruce Hoffman is a critical care nurse, paramedic and current graduate student. He works as both a clinician and educator in Connecticut, Massachusetts and Maryland, with background in the division of critical care (ICU, ER, Cardiology, and Flight). He enjoys professional gigs in clinical and distance medical education, advocacy, leadership, consultation and blogging. He is a frequent and national lecturer for a host of Emergency Medical Services and Critical Care continuing education programs. He remains a member of his hometown ambulance service where he has served in a variety of administrative and operational roles. In his spare time, Bruce enjoys spending time with his wife Stephanie as well as traveling, hiking and biking.