SA NODE:

1. Specialized cells high in the right atrium next to the superior vena cava
2. Pacemaker of the heart with an inherent rate of 60 – 100 beats per minute
3. Richly innervated by parasymapthetic (vagus nerve) and sympathetic system

BACHMANN”S BUNDLE:

1. Intra-atrial tract connecting right and left atrium
2. Conducts impulse from the right to the left atrium

AV JUNCTION:

1. Situated at the anterosuperior portion of the interatrial septum near the tricuspid annulus,  the AV Junction is comprised of the compact AV node and the fast and slow AV nodal pathways on the  atrial side along with the Bundle of HIS on the ventricular side.
2. Inherent rate is 40 -60
3. Innervation by sympathetic as well as parasympathetic nervous system.
4. Slows conduction to allow atrial contents to empty into ventricles before ventricular contraction.

BUNDLE OF HIS AND BUNDLE BRANCHES:

1. Two main branches of the Bundle of HIS – Right and Left Bundle Branches
2. Right Bundle Branch is long and slender.
3. Left Bundle Branch is broad and fans out with 3 fascicles generally.  a. posterior   b.  anterior superior  c.  intraventricular septum
4. Conduction velocity through the HIS-Purkinje system is 3-4 times faster   than through the myocardium
5. HIS and Bundle Branches are supplied by the Left Anterior Descending  and the Posterior Descending coronary arteries.

ACCESSORY  ATRIOVENTRICULAR  CONNECTIONS:

Rapidly conducting accessory AV pathways exist on either right or left side

1.  Usually capable of antegrade and retrograde conduction – about 75% of patients exhibit pre-escitation (manifest bypass tract) on 12 lead ECG in sinus rhythm while 25% conduct retrograde only (concealed bypass tract) and < 2-4% conduct antegrade only.
2. Clinical syndrome: Wolff-Parkinson-White (WPW)  

Slowly conducting accessory AV pathways exist only on the right side

1. Antegrade only conduction via Mahaim fibers – pre-excitation evident on 12 lead only during PSVT – Mahaim PSVT
2. Retrograde only conduction – pre-excitation is NEVER evident on 12 lead ECG is sinus rhythm or in PSVT – Permanent form of Junctional Reciprocating Tachycardia (PJRT)

To see animation of the conduction system of the heart  Click Here .  

There are also several quizes on the cardiac cycle.  Click Here and look at the menu on the left for the quizes.

Labeling exercise of the internal structures of the heart Click Here