Henry Family Blog

Cardiac muscles are branched, intercalated discs with gap junctions.  They are involuntary, striated and have a single central nucleus per cell. 

The Conduction System of the Heart

Pacemaker cells at the sinoatrial node (SA) fire spontaneously and act as a pacemaker.  They form the conduction system for the heart.  The SA node sets the pace since it is the fastest.  In 50msec, excitation spreads through both the atria and down to the AV node.  A 100msec delay at the AV node is due to smaller diameter fibers.  This allows the atria to fully contract filling ventricles before ventricles contract.  In 50msec, excitation spreads through both ventricles simultaneously.

Action Potential in Heart Muscle Fibers

First, there is rapid depolarization due to the Na+ inflow when the voltage gated fast Na+ channels open.  Nest, it plateaus due to the Ca2+ inflow when voltage gated slow Ca2+ channels open and some K+ channels close.  Finally, repolarization due to K+ outflow when voltage-gated K+ channels open.

In the heart muscle, the refractory period is longer and lasts more than the contraction itself.  No summation of contraction can occur.  Alteration between relaxation and contraction aids in blood flow, filling and ejecting in the ventricles.

Pathway of an Action Potential in the Heart 

SA node cells–> atria muscle cells–> AV node–> Bundle of His–> right and left bundle–> purkinjie fibers–> ventricular myocites

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