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