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Cardiac Conduction System


 





Properties of the cardiac muscle


AUTOMATICITY

CONDUCTIVITY

CONTRACTILITY

EXCITABILITY

REFRACTORY PERIODS


1.  AUTOMATICITY

is the unique ability of the cells in the SA node (pacemaker cells) to generate an action potential without being stimulated.

Due to reduce permeability to K but still allow passive transfer of Ca ions allowing a net change to build

Exemplified by the SA node


2.  CONDUCTIVITY

ability of the heart cells to transmit electrical current from cell to cell throughout the entire conductive system.



3.  CONTRACTILITY

ability of cardiac muscle fibers to shorten and contract in response to an electrical stimulus.


4.  EXCITABILITY

ability of a cell to reach its threshold potential and respond to a stimulus or irritation.

The lower the stimulus needed to activate a cell, the more excitable the cell; conversely, the greater the stimulus needed, the less excitable the cell.

The presence of ischemia and hypoxia cause the myocardial cell to become more excitable.



5.  REFRACTORY PERIODS


Period of rest

(1) the ionic composition of the cells during different phases of the action potential, and

(2) the ability of the cells to accept a stimulus.

Types:

Absolute refractory period

Relative  refractory period



 Absolute Refractory period


the time in which the cells cannot respond to a stimulus.

The ionic composition of the cells is not in place to receive a stimulus.

Phases 0, 1, 2, and about half of phase 3 represent the absolute refractory period





Relative Refractory period


The relative refractory period is the time in which repolarization is almost complete and a strong stimulus may cause depolarization of some of the cells.

Some cells may respond normally, some in an abnormal way, and some not at all.

The second half of phase 3 represents the relative refractory period of the action potential



Non Refractory period

occurs when all the cells are in their resting or polarized state.

The cells are ready to respond to a stimulus in a normal fashion.

Phase 4 represents the non refractory period

The duration of each refractory period may vary in response to use of medications or recreational drugs, or presence of disease, electrolyte imbalance, myocardial ischemia, or myocardial injury.

5 Phases of Action Potential

DEPOLARIZATION

Phase 0: Rapid depolarization

REPOLARIZATION

Phase 1: Initial repolarization

Phase 2: Plateau state.

Phase 3: Final rapid repolarization.

Phase 4: Resting or polarized state.

5 Phases of Action Potential

Phase 0: Rapid depolarization

ventricular muscle fibers are activated between 60 and 100 times/min by an electrical impulse initiated by the sinoatrial (SA) node.

changes the RMP and allows a rapid inward flow of Na into the cell through specific Na channels.

cell become positively charged.

voltage inside the cell at the end of depolarization is about 30 mV. This event produces a rapid up-stroke in the action potential

Phase 1: Initial repolarization

channels for K open and permit K to flow out of the cell, an action which produces an early, but incomplete repolarization

slowed by the Phase 2 influx of Ca ions

Phase 1 is illustrated as a short downward stroke in the action potential curve just before the plateau

Phase 2: Plateau state

During this period, there is slow inward flow of Ca which in turn significantly slows the outward flow of K.

The plateau phase prolongs the contraction of the myocardial cells.

Phase 3: Final rapid repolarization

During the inward flow of Ca stops, the outward flow of K is again accelerated, and the rate of repolarization accelerates

Phase 4: Resting or polarized state

During this period, the voltage-sensitive ion channels return to their pre-depolarization permeability.

The excess Na inside the cell (that occurred during depolarization) and the loss of K (that occurred during repolarization) are returned to normal by the Na and K ion pumps. An additional Na and Ca pump removes the excess of Ca from the cell

Conductive System of the Heart
 
 
 

Pacemaker potential and its ionic basis

THE CONDUCTIVE SYSTEM

the sinoatrial node (SA node),atrioventricular node (AV node), bundle of His, the right and left bundle, branches, and the Purkinje fibers

Control of excitation and conduction
 
The conducting system of the heart  initiates the normal cardiac cycle and coordinate the contractions of cardiac chambers. Both atria contract together, as do the ventricles, but atrial contraction occurs first.
The conducting system provides the h
eart its automatic rhythmic beat. For the heart to pump efficiently and the systemic and pulmonary circulations to operate in synchrony, the events in the cardiac cycle must be coordinated.