Before reading this section, make sure you check out Understanding the Action Potential. It covers the basics of this next section and it is suggested that you read it before proceeding on with the cardiac action potential.
The heart muscle cells share many things in common with nerve cells. Namely, they can generate action potentials. Each region of the heart has cells with slightly different action potentials. The graph below shows a simplified shape of an action potential generated by the ventricular cardiomyocyte (muscle cell from the ventricle), the most common cell of the heart.
From the picture
you should be able to tell that the action potential of the heart muscle cell
lasts a great deal longer than that of the nerve cell that we learned about
earlier. In order to pinpoint
the voltage changes present, we must separate the various parts of the cardiac
action potential into phases. Phase 0 is the immediate depolarization
that sends the voltage past the zero millivolt level, making it positive. This
is due to the sudden increase in membrane permeability to sodium ions and decrease
in potassium permeability (check out Understanding
the Action Potential for a more in depth description). Once the high
sodium permeability decreases, slight repolarization
occurs. This moment when the voltage declines makes up Phase 1. The membrane
potential then reaches a steady point at around zero millivolts. This is called
the plateau of the action potential, and it makes up the gist of Phase 2
as well. There is a reason for this moment of steadiness in the voltage. The
inward flow of calcium ions is equal to that of the outward flow of potassium
ions. So, why doesn’t the voltage just remain at zero? Well, because of the
falling membrane potential, the calcium permeability declines while the potassium
permeability increases. This initiates repolarization once again, and it makes
up Phase 3. The voltage decreases to its original value where it will
remain steady until the next action potential is generated (Phase 4.). In
muscle cells the generation of action potential is associated with contraction.
Learn more about it in the next section.