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Cardiac Anatomy - Cellular Properties

Cardiac Cell Properties

          Often times when we are studying how the heart works, we get caught up in very specific details. We focus on the exact meaning of a change in deflection, or the moment when a wave front passes by a recording electrode. We often forget to focus on how these little details work together to generate a normally functioning rhythm that keeps us alive and well.

          This section is going to take a look at some details as well as tie them into how they affect the big picture. The details we are going to focus on are properties of cardiac cells, specifically automaticity, excitability, and refractoriness. The big view will allow us to look at how these three principals combine to create one normal cardiac cycle.

• Automaticity

          Automaticity is one of the things that makes cardiac cells unique from most other cells in the body. This is the ability for a cell to spontaneously generate a depolarization. The onset of every electrical event starts with a single cellular depolarization triggered by automaticity. All sinus beats begin this way as do all focal ectopics.

          Cardiac depolarization begins with small current leaks across the cell membrane during phase 4, the resting phase, of a cell. As ions slowly transfer across the cell membrane, the difference in voltage between the inside and outside of the cell, know as the transmembrane potential, becomes greater. When the transmembrane potential of the initiating cell reaches the critical threshold of -90mv, spontaneous depolarization occurs. This triggers chemical gates in the cell wall to open allowing Na+ ions to flood the interior of the cell. This onset of depolarization is represented by phase 0 of the action potential. Note that this is also the point where the cardiac cell physically begins to contract. As these contractions combine throughout the cardiac structure, the various chambers contract forcing blood forward. This is the systolic phase of the cardiac cycle.

          Usually the transmembrane potential reaches this critical threshold faster in sinus cells than it does elsewhere in the heart. For this reason, the sinus node is usually in control of how fast the heart rate occurs. The sinus node receives input from the brain by way of the vagus nerve that helps regulate how often these depolarizations occur.

         If cells elsewhere in the heart are damaged in some way, it is possible that the current leakage across the cell membrane of these cells may allow these cells to reach the critical transmembrane potential faster than the sinus node. When this happens, an ectopic event occurs. An ectopic beat is any beat that originates outside of the sinus node. When two ectopic beats occur together, it is often referred to as a couplet. Three or more that occur together without becoming continuous are referred to as a salvo. When a continuous group of ectopics are strung together, it is referred to as an arrhythmia.

          It is important to note that not all cells depolarize at the same rate. There are differences in how rapidly cells throughout the heart undergo the depolarization process. Generally, the fastest depolarization occurs in the Purkinje fibers while the slowest depolarizations occur in the sinus and AV nodes.(1) This difference in the rate of depolarizations is part of what determines how fast the wave front of electrical activity proceeds through the heart. (Discussed below in Conduction Velocity)

• Excitability

          Once a cell depolarizes, the second property of cardiac cells comes into play. This is the ability of one cell that is depolarizing to initiate depolarization in surrounding cells and is referred to as excitability. Excitability occurs when neighboring cells respond to the opening of Na+ channels in the cell wall of the cell that initially depolarized. The neighboring cells respond by opening the Na+ channels in their own walls triggering further depolarizations. This process spreads from one cell to another in a continuous event that will eventually cause all the cells in the heart to depolarize unless block by some abnormality.

• Refractoriness

          Once a cell has depolarized, there is a period of time in which it will not respond to an outside stimulus. This is the onset of what is known as the refractory period. The refractory period correlates with phases 1-3 of the action potential and is controlled by the flow of CA+ ions across the cell membrane. Phase 1 and 2 of the action potential represent the absolute refractory period where the cell will not respond to any stimulus. In phase 3, a stronger stimulus may trigger an early depolarization. Note that stimulation of ventricular cells during phase three may lead to what is referred to as R on T phenomena. If an R wave (depolarization) occurs on the T wave (repolarization) in phase 3, it may trigger the onset of ventricular tachycardia.

          The refractory period of cardiac cells is an important control that helps direct where an electrical wave front will travel. Depolarization will move to cells that are ready to depolarize. The cells that are refractory will not respond to the electrical activity which prevents the signal from returning in the direction it originally came from. This process causes the depolarization to spread outward from the source until all the cells in the heart have depolarized. Without a refractory period, the cells of the heart would repeatedly depolarize any time a nearby cell depolarized. This would keep the cells fluctuating between a contracted or systolic state, and the diastolic or non-contracted phase so rapidly that there would be no effective blood flow. It is the refractory period that prevents this from happening.

          Consider a perfectly round chamber. At the top is the origin of electrical activity. At the onset, there is a single small spot that becomes active. The depolarization at this location spreads out like a ring moving away from the onset. The cells below the ring are in the resting phase and are ready to depolarize. The cells above the ring have already depolarized and are now refractory. This keeps the ring moving from the top of the chamber towards the bottom. Once the electrical wave front reaches the bottom of the chamber, it will arrive at the final cells that can be depolarized. Everything behind it is refractory.

 
Depolarization Sequence - A Look at Refractory Periods

          Consider a cardiac chamber that is shaped in an oval.  For the sake of this example, we will assume that all the tissue in this chamber conducts at an equal rate and from top to bottom.  A stimulus is introduced at the top of the chamber.

Chamber Stim

          The cells immediately adjacent to this stimulus are in the resting phase and are ready to depolarize. A ring of depolarization move outward from the point of stimulation heading in all directions at equal velocity.

          Note that if some regions had properties that caused slower conduction, the ring would extend less rapidly in those areas. Because our model has equal conduction velocities all around the chamber so the ring of depolarization moves evenly throughout the chamber.

Depolarization Starts

          At the leading edge of the wave front, the electrical wave front continues because the cells just ahead of it are in the resting, or quiescent phase and are capable of receiving a signal.

          Behind the front, the cells that have completed depolarization are now in the process of repolarizing. These cells can not accept a signal, thus preventing the wavefront from returning back in the direction it came from.

Wave front expands

          As the wave front approaches the midpoint of the chamber, the cells back at the location of the original point of stimulation have finished repolarizing and are ready to depolarize again.

          However, there is a large band of refractory cells between the cells that are depolarizing and the cells that have recovered. This prevents the newly recovered cells from depolarizing again.

          In front of the ring of depolarization, quiescent cells are triggered to respond to the electrical activity allowing the wave front to continue.

Refractory Period follows Depolarization
          Over two thirds of the chamber has depolarized. About 30% of the chamber is refractory. Depolarization approaches far side of Chamber
          

          The depolarization wave front reaches the bottom of the chamber. There are no more cells that are ready to accept a signal and the electrical activation terminates.

          The refractory period of the depolarized cells helped to keep the wave front of depolarization moving in one direction, preventing it from doubling back on itself.

No cells are left in front of Depolarization
                                       

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