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Airway, breathing and circulation (the ABCs) have been stalwart concepts since the inception of EMS training. These concepts are obviously important, and it is difficult, if not impossible to understand how these systems truly function without addressing what is happening at the cellular level of physiology.
In certain aspects, possessing a foundational understanding of the cellular physiology behind the ABCs is more relevant than the ABCs themselves. Human physiology can be a difficult concept to grasp. Due to the complexity and academic nature of physiology, many EMS providers may believe it is easier to avoid or skim over this concept. This article will introduce readers to cellular action potentials in anything but an academic manner.
Cellular action potentials
You may be asking yourself, “why is it important for EMS providers to understand the concept of cellular action potentials?” One of the answers to this question is quite simple. If our bodies are unable to achieve a cellular action potential, we would cease to exist as a living human being. Every time our heart beats, we take a breath or have a conscious thought, it is the result of a cellular action potential. In a nutshell, any normal or abnormal physiologic action revolves around achieving or hindering a cellular action potential.
Depolarization and repolarization
Before getting into the practical aspects of physiology, we need to take a short academic side trip. Cations are positively charged ions, anions are negatively charged ions. Depolarization is moving the extracellular cation sodium into the intracellular space. Think of depolarization as the activation of any body function. When systems within our body achieve depolarization, our heart beats, we take a breath or have a conscious thought, and so on.
Cells within our bodies are unable to maintain constant and sustained levels of depolarization. This is because cells have limited amounts of stored energy. Constant depolarization will result in a depletion of stored energy, and the cell will eventually lack resources to function. This is where repolarization comes into to play.
Repolarization is moving the intracellular cation potassium out to the extracellular space. Think of repolarization as the deactivation of any bodily function. This allows the cell time to replenish energy stores so it can be depolarized again.
Schoolhouse physiology theory and resting potential
To simplify this concept, we will look at the schoolhouse theory. Think of cells as schoolhouses rather than biological structures. These schools control all the functions that normally occur within our bodies. As an example, there are schools that control heart function, breathing, cognition and so forth. The teachers and administrators of the school recognize the importance of classrooms inside the schoolhouse. This is because inside the school is where most of the action and learning typically occurs.
Within our community, we have three types of students who regularly attend the schools. First and foremost, is sodium. Think of sodium (cation = positively charged ion) as an optimist within the student population. The positivity associated with sodium has the potential to create action wherever it’s located. When sodium comes to school, it prefers to spend the entire day outside of the building.
There is also potassium. Think of potassium (cation = positively charged ion) as the pessimist within the student population. Even though potassium is a positively charged ion, it tends to see the world from a glass-half-empty perspective. When potassium comes to school, it prefers to spend the entire day inside of the building.
Lastly there is calcium. Think of calcium as incoming freshman. Calcium feels awkward but wants to fit into the student body; it sees hanging out with sodium as a means to readily fit in. As a side note, sodium hates to open doors for itself. Since this is the case, sodium reluctantly allows calcium to tag along so long as calcium facilitates opening any door for sodium.
Under this illustration, sodium starts the day outside the schoolhouse, potassium starts inside the schoolhouse, and calcium hangs out wherever sodium is located. There is no activation of body function because sodium is outside of the school rather than inside. This is referred to as the resting cellular potential. Cells within our bodies expend a large amount of energy to achieve this state of resting potential.
Schoolhouse physiology theory and the school bus driver
As sodium stays outside and potassium stays inside, their parents are concerned the students will become one dimensional. To avoid this tendency, they hire a school bus driver. This school bus driver comes in the form of an electrical impulse. His job starts when there is more sodium outside the school in comparison to potassium inside the school. This school bus driver has only two responsibilities:
- First, to tell sodium to move into the schoolhouse.
- After sodium moves in, to tell potassium to move out of the schoolhouse.
Think of the school bus driver as an employee who has a bad attitude with poor work ethics. He consistently approaches these two jobs with minimal enthusiasm and effort.
Schoolhouse physiology theory and depolarization
When there is more extracellular sodium in comparison to intracellular potassium, the school bus driver is instructed to come to the school. As a reminder, he arrives in the form of an electrical impulse. It is this electrical impulse that tells sodium – the facilitator of action – to move into the schoolhouse. As agreed upon, calcium rushes up front to open the door and sodium moves into the schoolhouse. When the extracellular sodium moves into the schoolhouse, depolarization occurs. Depolarization results in whatever physiologic process the school controls. The heart will beat, breathing occurs, there is conscious thought, etc.
Schoolhouse physiology theory and repolarization
When most of the sodium has moved into the cell, the school bus driver tells potassium to move out of the school. When the intracellular potassium moves out of the schoolhouse, repolarization occurs. Repolarization results in deactivating whatever physiologic process the school controls.
As a side note, calcium gets stuck at the door during this process. This is like opening the door for your party at a busy restaurant. After your party goes through the door, there are people on the inside wanting to come out. Calcium, being a conscientious door holder, will wait until the intracellular potassium comes out of the schoolhouse before joining sodium. After potassium is told to move out of the schoolhouse, the school bus driver has completed his two duties and leaves the school yard.
Schoolhouse physiology theory and return to resting potential
After the school bus driver leaves, intracellular sodium realizes it stinks to be inside the schoolhouse. On the other hand, extracellular potassium realizes it stinks to be outside the schoolhouse. 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. Once the cell attains resting potential, the school bus driver is instructed to come back and initiate the process of depolarization and repolarization, with the resulting return to resting potential. This process continues indefinitely until we die.
Cellular action potential and pharmacology for EMS
Why is the concept of cellular action potentials important to EMS providers? Understanding cellular action potentials will help EMS providers understand what is causing their patients to present with specific clinical findings. As an example, let’s look at what causes a patient to present with an increased heart rate. This might be caused by increased levels of sodium moving into their cardiac cells. A bradycardia might be the result of too little sodium moving into those same cardiac cells.
Cellular action potentials also apply to the administration of medications. Medications that increase sodium influx will typically increase associated physiology. Medications that inhibit sodium influx will typically decrease associated physiology. Some medications can inhibit calcium from opening doors for sodium. This typically results in decreased physiology, as less sodium moves into the cell because calcium isn’t there to hold the door open.
Cellular action potentials have a direct effect on normal patient physiology as well as patients suffering from injury or disease. Medications prescribed to patients or medications which are administered by EMS providers obviously influence cellular action potentials. Understanding cellular action potentials will help EMS providers relate to what is occurring within their patients.
For those interested in learning more about the concept of cellular action potentials, watch the video below. To test your knowledge, take the quiz: Quiz: Depolarization and polarization — cellular action potential.
References
- Carmeliet E, Vereecke J: Adrenaline and the plateau phase of the cardiac action potential. Fluegers Arch 313: 300-315, 1969
- Hodgkin, A. L., and A. F. Huxley. 1952. A quantitative description of membrane current and its application to conduction and excitation in nerve. J. Physiol. 10:500-544.
- Isenberg G: Is potassium conductance of cardiac Purkinje fibres controlled by Ca++? Nature 253: 273-274, 1975
- Lecar, H., and R. Nossal. 1971a. Theory of threshold fluctuations in nerves. Relationships between electrical noise and fluctuations in axon firing. Biophys. J. 11:1048-1067.
