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!

Tags: cardiac physiology definitions, Cardiac Properties

This entry was posted on Sunday, October 11th, 2009 at 20:08 and is filed under Cardiac A & P, Cardiac Properties. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.