Cardiology I – A Review of Cardiovascular Anatomy and Physiology
by Cwanza A. Pinckney, M.D.
Sign Up
Table of Contents
- Course Home
- Objectives
- Matching Exercise
- Introduction
- Anatomy: The Heart
- Anatomy: The Vessels
- Physiology: Introduction
- Physiology: Review of Circulation
- Physiology: The Cardiac Cycle
- Physiology: Innervation of the Heart
- Cardiac Electrophysiology: Introduction
- Cardiac Electrophysiology: Electrolytes
- Cardiac Electrophysiology: Cardiac Automaticity
- Cardiac Electrophysiology: The Cardiac Conduction System
- Cardiac Electrophysiology: Myocardial Contraction
- Sample Cases
- Case 1: Cardiac Tamponade
- Figure 1: Assessment
- Figure 2: Management
- Case 2: Cocaine Overdose
- Figure 3: Assessment
- Figure 4: Management
- Case 3: Hemorrhagic Shock
- Figure 5: Assessment
- Figure 6: Management
- Conclusion
- References
- Bibliography
Cardiac Electrophysiology: Cardiac Automaticity
Introduction:
Automaticity of the heart is produced by spontaneous and repetitive depolarization of certain cardiac cells, known as pacemaker cells. The depolarization of these cells leads to the generation of action potentials, which are electrical signals that are propagated throughout the cardiac muscle tissue and result in contractions. Understanding depolarization is essential to comprehending cardiac electrophysiology as a whole.
Depolarization:
-Introduction:
The process of depolarization occurs at the cellular level and involves changes in the electrical environment of a cell. This electrical environment is determined by the difference in the concentrations of charged particles between the inside and outside of the cell.
-Resting Membrane Potential:
At rest, the inside of a myocardial cell has a charge that is more negative than its surroundings. Therefore, the cell is said to have a negative resting membrane potential. This potential is normally around -90 millivolts, meaning the charge inside the cell is 90 millivolts lower than the surrounding environment.
-Anions and Cations: The resting membrane potential is the result of a difference in the concentration of cations (which are particles with a positive charge) and anions (which are particles with a negative charge) between the inside of the cell and the outside of the cell. The two most abundant charged particles within a cell are proteins (which have a negative charge) and the positively charged Potassium ion (K+). The most abundant charged particles located outside of a cell are the positively charged Sodium ion (Na+), and the positively charged Calcium ion (Ca+).
-Creation of the Negative Resting Membrane Potential:
The unbalanced distribution of particles on either side of the cell membrane is maintained by passive diffusion (across a selectively permeable membrane), as well as by active diffusion (which involves specialized membrane pumps). Passive diffusion as you may remember is a force that tends to move particles from areas of higher concentration to areas of lower concentrations so as to achieve equilibrium. Selective permeability means that a membrane will allow certain particles to easily pass, while it may restrict passage of other particles. Active diffusion establishes concentration gradients through the use of energy-requiring membrane-bound pumps.
-Passive Diffusion: Due to the high concentration of Potassium ions inside a cardiac cell, a concentration gradient exists that leads to the passive diffusion of Potassium ions out of the cell. This concentration gradient, however, is balanced by the negative electrical gradient (created by the intracellular protein anions) that tends to pull the positively charged Potassium ions back into the cell. At a certain electrical charge, these two forces tend to balance out. This is known as the Potassium resting membrane potential.
Similarly, Sodium ions are present in large concentrations outside the cell and will tend to diffuse into the cell. However, at rest, the cell membrane is relatively impermeable to Sodium and, therefore, Sodium ions remain in high concentrations outside the myocardial cell. Consequently, a Sodium resting membrane potential is created.
-Active Diffusion: Membrane potentials are not purely determined by passive concentration gradients and electrical forces; they are also determined by processes involving the active diffusion of ions. One such process involves the Sodium/Potassium pump, which is, perhaps, the most physiologically important pump in our body. It exports three Sodium ions out of the cell in exchange for pumping two Potassium ions into the cell. This results in maintaining Potassium at high concentrations within a cell, as well as maintaining Sodium at high concentrations outside the cell. In addition, due to the three positive charges being pumped out of a cell for every two positive charges pumped in, there is a net efflux of positive charge from the cell.
Since a cell membrane is relatively permeable to both the Sodium and Potassium ions (whether by active or passive diffusion), a resting membrane potential will exist that falls between the individual Sodium and Potassium resting membrane potentials.
-Depolarization and the Generation of an Action Potential:
As mentioned, at a steady state, the inside of a cardiac cell has a negative charge, while the outside of the cell has a positive charge. This difference in charge normally measures around -90 millivolts in a myocardial cell. If a myocardial cell remains at this negative value, then it will not contract. However, if something causes the inside of the cell to become more positive, a myocardial contraction may be produced.
As mentioned in the introductory paragraph, certain myocardial cells are capable of spontaneous and repetitive depolarization. These cells, known as pacemaker cells, have unstable resting potentials, which are caused by a slow influx of positive Sodium and Calcium ions while at rest. As the cell slowly depolarizes, it reaches a point at which a rapid alteration in the permeability of the membrane will occur. Once this happens, positive charges (from Sodium and Calcium) flood the cells and produce an action potential. This action potential then leads to contraction of cardiac muscle fibers.
After an action potential is generated, the cell quickly returns to its resting membrane potential. Positive ions then begin to leak into the pacemaker cell again. This will lead to another action potential and, therefore, the repetitive firing cycle continues.