**Gap junctions allow the entire myocardium of the atria or the ventricles to contract as a single, coordinated unit. **
Numerous and larger mitochondria
Same myosin and actin arrangement and the same bands, zones, and Z discs. Smaller SR and therefore a smaller reserve of Ca++.
Cardiac Conduction System
The hearts source of electrical activity is a network of specialized cardiac muscle fibres called Auto-rhythmic fibres and they are self-excitable . They continuously trigger action potentials that trigger heart contractions. They even continue to stimulate the heart after the heart is removed from the body.
These fibres have two important functions :
Pacemaker : setting the rhythm of electrical excitation that causes contraction of the heart.
Conduction System : a network of specialized cardiac muscle fibres that provide a path for each cycle of cardiac excitation to progress through the heart. This system ensures cardiac chambers become stimulated to contract in a coordinated manner, making the heart an effective pump.
Cardiac action potentials propagate through the conduction system as follows:
1. Excitation begins in the Sinoatrial (SA) Node in the right atrial wall. SA node cells repeatedly depolarize to threshold and do not have a stable resting potential. This spontaneous depolarization is a Pacemaker Potential and when it reaches threshold an action potential is triggered. Cardiac APs propagate from the SA node throughout both atria via gap junctions and following the action potential, the atria contract. 2. APs conduct along atrial muscle fibres reaching the Atrioventricular (AV) Node located in the interatrial septum. 3. From the AV node, the AP enters the Atrioventricular (AV) Bundle (AKA Bundle of His ) and is the sight where APs conduct from atria to the ventricles. These fibres extend from the AV node through the atrioventricular septum as well as the interventricular septum.
4. After propagating along the AV bundle, the AP enters the right and left bundle branches.
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