Structure

Layers

From inside outwards

• Endothelium
• Myocardium
• Mesothelial epicardium
• Pericardium

Packmaker nodes

• SA node (predominant)
• AV node
• Purkinje fibres  (but doesn't involve Ca2+ channels)

Sinoatrial (SA) node

• 8mm long 2mm thick
• Lies in the groove where SVC joins RA

Atrioventricular (AV) node

• 22mm long, 10mm wide, 3 mm thick
• Sits posteriorly on the right side of the interatrial septum, near the ostiu of the coronary sinus
• As rate of atrial depolarisation increases, conduction through AV is slowed
• Refractory period increases at very high heart rate
• Common site for re-entry
• Fibrous aterioventricular ring separates atria and ventricles, and only electrical conduction pathway is via AV node.

Heart block

• 1st degree
  --> Abnormal prolongation of AV conduction time
• 2nd degree
  --> Only fraction of impulse pass through AV node
• 3rd degree
  --> No impulse pass through AV node

Effects on ANS on AV node

• Acetylcholine 
  * Hyperpolarise and slow/block AV conduction
• Catecholame
  * Decrease AV conduction time

Bundle of His

Passes subendocardially down the right side of the interventricular septum for about 1cm
--> Then divides into left and right bundle branches.

Left bundle branch branches out almost perpendicularly
--> Travels through the interventricular septum
--> Divides into thin anterior division and thick posterier division on the subendocardial surface of the LV

Purkinje fibres

• Subendocardial
• Broadest cells in heart
• Phase 1 more prominent
• Phase 2 longer
• Long refractory period
  * Blocks some ectopics
  * Inversely related to heart rate

Other notes

• Pericardial sac normally contains 5-30mLs of clear fluids