Conduction system

The cardiac conduction system comprises:

* The sino-atrial node, located in the epicardial groove of the lateral part of the sulcus terminalis at the junction of the superior vena cava and the right atrium.

* The internodal tracts, sometimes described in three parts as the anterior (James), middle (Wenckebach) and posterior (Thorel) tracts.

Factors affecting the slope of phase 4 depolarisation o s Increased slope

S Arterial hypoxia r Hypercarbia

T« Acute hypokalaemia m Hyperthermia

Catecholamines Sympathomimetic drugs

Reduced slope Vagal stimulation Acute hyperkalaemia Hypothermia Positive airway pressure

Classification of cardiovascular calcium channels

L- type calcium channels T-type calcium channels Tetrodotoxin-sensitive calcium channels Sarcoplasmic calcium release channels Ryanodine receptor

Inositol 1,4,5-triphosphate (IP3) receptors

* The atrio-ventricular node, which is located in the base of the atrial septum at the apex of a triangle formed by the tendon of Todaro, the tricuspid valve annulus and the coronary sinus.

* The bundle of His, which penetrates the central fibrous body beneath the noncoronary cusp of the aortic valve.

* The bundle branches.

* The Purkinje fibres.

The vagus nerve provides beat-to-beat control of the sino-atrial node. This is mediated by M2 type muscarinic cholinergic receptors coupled to G-protein-activated inward rectifying potassium channels. Respiratory sinus arrhythmia is completely abolished by muscarinic receptor blockade with atropine.

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