This is a free website for Nuclear Medicine Technologists and Students who wish to broaden their understanding of Nuclear Cardiology Practices and Principles.
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This is a free website for Nuclear Medicine Technologists and Students who wish to broaden their understanding of Nuclear Cardiology Practices and Principles. |
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LESSON 3cTHE ACTION POTENTIALA voltage difference, or electrical potential, exists across the membranes of almost all cells in
the body. Electrolytes (concentrations of ionic solutions) in the fluid inside and outside of the cell
membrane make the flow of electrical current possible. To help us understand how the cardiac tissue generates and transmits electrical impulses,
we must observe the cell membrane operations at the atomic level. An atom should be electrically neutral. Under certain conditions, especially when in solution, some atoms tend to lose or gain an electron or two. The elements sodium and potassium, especially vital for nerve function, tend to lose an electron, and with it, one negative charge. This leaves a balance of one extra positive charge, and these sodium and potassium atoms become positive ions, or "cations". Calcium atoms lose two electrons, becoming cations bearing two positive charges.
A resting cell stimulated by an impulse from the cardiac conduction system begins a sequence of rapid changes in membrane potential affecting the permeability of the cell membrane and the movement of ions (electrical charge) into and out of the cell. This sequence of changes is called the action potential. Characteristics of the cell membrane such as selectively allowing ions through the membrane under certain conditions, ionic imbalance within and outside the cell membrane, the presence of certain neurotransmitters, and a specialized conduction system create a system that moves current through tissues that result in the contraction and relaxation of muscle tissue, repeatedly, to keep your heart beating for a lifetime. The following conditions explain the phenomenon of myocardial contraction:
An excess of positive charges accumulate on the outside of the cell membrane. When the threshold is reached, ionic gates open in the membrane. Sodium ions, and the positive charges carried with them, gush back into the cell. This rapid shift in positive charges is called depolarization, and it creates an electrical current. Quickly the ions are pumped back out again, during the stage termed repolarization. Depolarization in the SA node is the event that sparks off a heartbeat. Waves of depolarization speed along the cell membranes and conduction pathways through the heart. When they reach the terminals of the Purkinje fibers they leap across to the cardiac muscle fibers and set in motion the molecular ratchets of actin and myosin, which slide past each other to shorten the fiber. All this happens in less than a second. In the following illustration, the upper curves represent the action
potential for the specialized cells of the cardiac conduction system
and for a ventricular muscle cell during one complete cycle. Note
how the pacemaker cells fire off sequentially from SA node to
Purkinje fibers. Note also
the difference between pacemaker and myocardial cells; the pacemaker
cells automatically depolarize during diastole while myocardial
muscle cells, which remain polarized during diastole, require an
outside stimulus to depolarize.
The lower curves demonstrate the corresponding normal ECG.
PUTTING IT TOGETHER- THE CELL MEMBRANE POTENTIAL
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