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Although cardiac muscle has a slow spontaneous rate of depolarization and contraction,
all normal excitation originates from the pacemaker in the sinoatrial node (SAN).
The SAN is located in the antero-medial wall of the right atrium. A wave of depolarisation
and consequent contraction rapidly spreads across both atria. It is unable to
cross the annulus fibrosus which electrically insulates the atria from the ventricles.
Continuation of wave of depolarisation occurs via the atrio- ventricular node
(AVN) which lies within the interatrial septum. Electrical activity in the AVN
is carried very rapidly through the annulus fibrosis and into the interventricular
septum via the bundle of His. The bundle of His soon divides into the right bundle
branch and the left main bundle branch.
- the right main branch supplies
the Purkinje network of the right ventricle.
- the left main branch splits
again after coursing only 1–2 cm
- this results in a left anterior fascicle,
which runs toward the anterior-superior papillary muscle, and a left posterior
fascicle, which courses toward the posterior- inferior papillary muscle
pathology, from whatever cause, affecting these bundle branches or fascicles may
manifest themselves on the surface ECG as bundle branch blocks (right or left),
or fascicular blocks (left anterior or left posterior)
- if there are multiple
sites of conduction delay result in multifascicular block (e.g., bifascicular
or trifascicular block)
The conducting system is a prime
example of structure being adapted to function so that:
- there is a
delay between atrial and ventricular contraction to permit efficient ventricular
- the papillary muscles are increasingly taut prior to ventricular
contraction to reinforce the AV valves
- the ventricular contraction wave
spreads rapidly from the apex to the base of the ventricles
Last reviewed 01/2018