Lecture 17: Cardiovascular System
Electrical Activity and EKG
The Cardiac Action Potential Has a Prolonged Refractory Period
•Heart action potential has a prolonged spike (depolarized)
• Membrane is refractory for a long time
• This prevents summation and gives the heart time to fill
All Parts of the Heart Beat Spontaneously
• Heart muscle does not require stimulation by a nerve
• Nerves usually inhibit the heart beat; cutting the nerves -> heart speeds up
• Beat originates as a depolarization in the heart muscle cell itself (self stimulation)
• All parts of the heart can beat spontaneously
• Advantage: if one part of the heart is damaged another part can still produce a
beat
•Risk: beats originating outside of the pacemaker can produce life-threatening
arrhythmias
Normally the Heart Beat Originates in the SA Node
• The heart beat originates from the part of the heart with the fastest beat
• Normally this is the SA (sinoatrial) node of the right atrium
• The SA node is called the pacemaker
• Ectopic beats are those originating outside of the normal pacemaker
The Atria Are Electrically Insulated From the Ventricles
• The upper part of the heart (the 2 atria) is insulated from the lower part
• Electrical excitation can pass from the atria to the ventricles only at the AV node
The Heart Has Special Electrical Conducting Tissue
• Electrical excitation is passed through special conducting tissue from the AV node
to the ventricles
• Route: bundle of His -> bundle branches -> Purkinje fibers
Excitation is Delayed in the AV Node
• The excitation starts in the SA node and spreads across the atria
• When the excitation reaches the AV node there is a delay of about 0.1 seconds
before it passes into the bundle of His
• The delay allows the atria to contract before the ventricles are stimulated
•This results in better filling of the ventricles
• From the AV node impulses enter the Bundle of His and then travel along the
right and left bundle branches in the septum between the right and left ventricles
