Understanding the cellular action potential can help EMS providers determine why patients present with certain clinical findings
As cellular action potentials are the foundation for normal physiologic function, it warrants an abbreviated revisit before moving on to how magnesium and chloride influence this process. [Watch a video describing how magnesium and chloride influence cellular action potentials below.]
Inside the schoolhouse is where the normal physiologic function is initiated.
Sodium is the optimistic extracellular cation (positively charged ion) that loves the outdoors and sunshine. Sodium is the party guy that facilitates action wherever it resides. Potassium is the pessimistic intracellular cation (positively charged ion) that loves the control and predictability of being inside the schoolhouse. Calcium is the awkward freshman that tags along with sodium for the purpose of opening doors for sodium. The resting potential is when available sodium and calcium are outside the school and available potassium is inside the school.
Understanding cellular action potentials help EMS providers understand what is causing their patients to present with specific clinical findings.
When resting potential has been attained, the school bus driver with a bad attitude and poor work ethic comes in the form of an electrical impulse and tells sodium to move into the schoolhouse. If calcium holds the door, sodium will readily move into the schoolhouse, causing depolarization. Depolarization is the activation of a physiologic function. Once sodium moves into the schoolhouse, the bus driver tells potassium to move out, causing repolarization. Repolarization is the deactivation or cessation of depolarization. During these two processes, calcium gets stuck at the door because potassium moves out immediately after sodium has moved in. After potassium has moved out of the schoolhouse, calcium moves in to be with sodium. At this point, the school bus driver has performed his two jobs and leaves the schoolyard.
After the school bus driver leaves, sodium hates being inside the school and potassium hates being outside. With their mutual perspectives, sodium moves back outside, potassium moves back inside and calcium follows sodium wherever it goes. This is referred to as the return to resting potential. When the cell regains the state of resting potential, the school bus driver is instructed to come back to the schoolhouse and reinitiate the process of depolarization and repolarization.
[Read more: The first article in the “Let’s get cellular,” series addresses normal cellular action potentials in-depth and provides the foundational concept for this series on cellular physiology]
Magnesium is another cation (positively charged ion). Think of magnesium as a big bully. Magnesium particularly enjoys bullying calcium because it is young and vulnerable. When magnesium comes to school, it grabs onto calcium and prevents it from opening the door for sodium. As a reminder, sodium is a door snob and may or may not go through a door if it isn’t held open. This bullying results in less sodium moving into the schoolhouse because calcium isn’t holding the door.
Since sodium is the facilitator of action, it needs to move inside the schoolhouse to initiate a physiologic function. The influence magnesium has on cellular action potentials will decrease the physiologic effect of cells that are being affected. This is because calcium will be held by magnesium and can’t open the door for sodium to readily move into the schoolhouse.
Chloride is an anion (negatively charged ion). Think of chloride as a Debbie Downer who approaches any situation with a negative attitude. Wherever Debbie Downer resides, she will negatively overwhelm the positivity associated with any cation. Because Debbie Downer is so negative, school administrators don’t want her to interact with students inside the schoolhouse. Because of her negativity, the doors get locked to prevent her from entering the schoolhouse. Even though the doors are locked, Debbie Downer gets inside the schoolhouse because she finds a special entryway that school administrators cannot lock. Once inside the schoolhouse, her negativity prevents the school from initiating its associated physiologic function.
Moving chloride inside the schoolhouse will offset the positivity of cations that are residing within the school. By decreasing the positivity inside the schoolhouse, the cell will not achieve its cellular action potential. This is another mechanism the cell can utilize to decrease physiologic function that is independent from the influence of sodium.
CELLULAR ACTION POTENTIALS’ RELEVANCE TO THE EMS PROVIDER
Understanding cellular action potentials help EMS providers understand what is causing their patients to present with specific clinical findings. Magnesium decreases the influx of sodium and thus diminishes physiologic function. Magnesium is one mechanism that decreases the influx of sodium by inhibiting calcium’s ability to hold the door open for sodium. Chloride, an anion, accomplishes a similar end through a totally different process. By offsetting the positivity of cations, cells are prevented from achieving adequate polarity to initiate a cellular action potential.
This article has introduced the influence magnesium and chloride may have on patients who present with decreased physiologic processes, and what occurs when a patient is prescribed or administered the medication magnesium sulfate. The decreased influence of calcium minimizes depolarization associated with the decreased influx of sodium. You have also been introduced to the mechanism of benzodiazepine medications, such as Versed, Valium and Ativan. A benzodiazepine increases the influx of chloride and thus decreases physiologic function by offsetting the positivity of cations.
There are multiple ways cells can increase or decrease their ability to control physiologic functions. Physiologic function revolves around the initiation and spread of depolarization by bringing sodium into the cell membrane. Magnesium decreases cellular action potentials by preventing calcium from holding doors open for sodium. The anion of chloride decreases cellular action potentials by offsetting the positivity of cations through its negativity.
1. Adams, M. (2017) Pharmacology for Nurses A Pathophysiologic Approach. New York, NY: Pearson.
2. Jahanen-Dechent, W., Ketteler, M. (2012) Magnesium basics. Clinical Kidney Journal, 5(Suppl_1), 3-13.
3. Jentsch, T., Stein, V., Weinreich, F., Zdebik, A. (2002) Molecular Structure and Physiological Function of Chloride Channels. Physiologic Reviews, 82(2), 503-568.
4. Sheppard, D., Welsh, M. (1999) Structure and Function of the CFTR Chloride Channel. Physiologic Reviews, 79(1), S23-S45.
5. Volp, S. (2013) Magnesium in Disease Prevention and Overall Health. Advances in Nutrition, 4(3), 378S-383S.
6. Wecker, L. (2010) Brody’s Human Pharmacology Molecular to Clinical. Philadelphia, PA: Mosby Elesvier.