Cardiac Electrical Activity (Part 2)

Question 1

Where is the only place the the atria aren’t completely isolated from the ventricles?

Answer 1

Atrioventricular node and bundle of His

Question 2

What is the sequence of activation in the heart?

Answer 2

SA node
Internodal Tracts
Atria
Atrioventricular Node
Bundle of His
Bundle branches
Purkinje Fibres
Ventricular myocardium

Question 3

Sinoatrial node

Answer 3

SITE OF ORIGIN of normal cardiac activation
-Small area of specialised tissue in RA near SVC inlet
~15mm long
Cells continuous with atrial myocardial cells
Spontaneous depolarization at ~70bpm at rest

Question 4

Internodal Tracts

Answer 4

Cells are connected and aligned so the rapid conduction does occur (faster along then across)

Question 5

Atria

Answer 5

Activation spreads via gap junctions in the muscle cells (functional syncytium)

Question 6

Atrioventricular Node

Answer 6

Conduction velocity is very slow (0.05m/s), this delays ventricular action so that atria can fully contract (“top-up” ventricles)

Question 7

Bundle of His (AV bundle)

Answer 7

From AV node to ventricles, only entry point as myocardium of chambers are insulated from each other by a fibrous skeleton (AV-valves attach to this)

Question 8

Bundle Branches

Answer 8

Bundle of his splits into L and R bundle branches

Question 9

Purkinje Fibres

Answer 9

Bundle branches split into purkinje fibres which ramifies over endocardial surfaces.

Rapid conduction velocity in BB and PF’s: 2-4m/s

Question 10

Ventricular Myocardium

Answer 10

Activation spreads through the mucle cell to cel via gap junctions (functional syncytium)
The endocardial site of the Purkinje network means that excitation travels from endo to epicadium

Question 11

Why is the SA node usually the pacemaker?

Answer 11

Because it has the highest intrinsic rate. If the SA node fails, next pacemaker down will take over (AV node)

-Overdrive Suppression of other pacemakers, keeps other cells from taking over. (probably due to hyperpolarization)

Question 12

What site is especially subject to a block?

Answer 12

AV node, causing a very long A-V delay

Question 13

Why is the purkinje fibres conduction so rapid?

Answer 13

to activate all of ventricular endocardium synchronously, so ventricles pump effectively.

Question 14

Velocity is proportional to square root of the radius of cells. THerefore?

Answer 14

Purkinje fibres have a very large radius, AV node cells a smaller radius.

Question 15

Wolff-Parkinson-White (WPW) syndrome is?

Answer 15

Condition in which there is an extra electrical pathway in the heart, a bundle of muscle (“bundle of Kent”) that acts as an electrical connection between atria and ventricles.
The condition can lead to periods of rapid heart rate (tachycardia) and re-entrant arrhythmias (where the active wavefront reenters and re-excites tissue that has repolarised)