Introduction to Electrophysiology;

What are they doing in there?

For Self Test on the cardiac cycle Go Here 

Self Test Part 2 of the cardiac cycle Go Here

Simple Lessons In Cardiac Contracility, Cardiac Pressures,  The Cardiac Output And Starlings Curve Made Easy

ACTION POTENTIAL:

Phase o:  Cell is stimulated, cell menbrane is more permeable to sodium ions.

Phase 1:  Brief period of rapid repolarization:  sodium influx ends in fast channels: potassium flows out

Phase 2:   Plateau phase: influx of calcium and sodium through slow channels which sometimes occurs in Phase 0 and Phase 1:   Responsible for the refractory period during whic the depolarized stae is maintained.  Calcium’s role is related to the contractility of the heart.

Phase 3:  Negativity restored due to increased loss of intracellular potassium.

Phase 4:  Return to resting membrane potential (-90 mV).  Normal distribution of Na+ and K+ are restored.

Need a review of cardiac action potentials?  These instructional videos are fabulous!

Definitions of the Properties of Cardiac Function:

Automaticity:  the ability of certain cells in the heart to initiate electrical impulses spontaneously

Excitability:  the ability of the cardiac cells to respond to stimulation

Conductivity:  the ability to transmit an impulse through specialized conduction system and atrial / ventricular muscle

Refractoriness:  the inability to undergo repeat stimulation until after a certain period of time has elapsed

Contractility:  the ability of the fibers to shorten when stimulated, resulting in the contraction of muscle creating a pump action

Depolarization and Repolarization:

Polarized Cell:  The cell at rest.  The interior of the cell has a negative charge with respect to the outside.  Normal distribution of K+ and Na+ is maintained.

Depolarization:  The cardiac cell is stimulated when Na+ enters the cell and an inward electricla current is produced.

Repolarization:  The cell recovers as K+ leaves and outward electrical current is produced

Click Here  for animation of this process.

Refractory Periods:

Absolute Refractory Period (ARP):  a relatively long period following excitation during which cells cannot respond to another stimulus, regardless of its strength.  This period of time is roughly the duration of systole.

Relative Refractory Period (RRP):  a narrow window of time near the beginning of the ARP during which stimulus strength must be above normal and the response from which is less than normal.

Vulnerable Period:  not a refratory period but a window of time during the refractory phase that the heart is prone to develop fibrillation in response to a premature beat delivered at that time; generally corresponds to the top of the T wave.

Supernormal Period (SNP):  also not a refracctory period, but is an interval of time during which it may be possible for a premature stimulus to conduct with better than expected behavior.

A review of the action potential of the heart is appropriate here. You will love these instructional videos!