1. Lecture 2 Physiology of the heart
Objectives
– Describe each of the components of the cardiac conduction system
– Describe the sequence of excitation of the cardiac
conduction system
–– Explain how the cardiac muscle action potential is generated
– Identify the components of a normal EKG, and discuss the physiologic relevance of each component

2. Main Components of the Cardiac Muscle Conduction System
SA node, or pacemaker
AV node
AV bundle, or bundle of His
right and left bundle branches
conducting myofibers (Purkinge
fibers)

3. Conducting system in the heart

4. The Pacemaker
the pacemaker, or SA node, is a mass of cells in the right atrial wall
pacemaker cells spontaneously discharge action potentials at a rate of / minute or more
in the intact animal, autonomic nerves
modify the rate of discharge, so that the
resting “heart rate” is about 70 b/min

5. Sequence of Excitation of Heart Muscle
the atria and ventricles must contract in a coordinated fashion
the sequence of cardiac muscle excitation is such that the first event is
1- depolarization of the SA node
2- the impulses then travel down and across both atria, causing atrial muscle fiber contraction

6. Sequence of excitation of heart muscle, Con’t.
at the AV border there is band of non-conducting tissue
the impulse must pass through the AV node
and through this tissue mass in order to reach the ventricles
the presence of the non-conducting tissue slows the impulse by about 0.1 sec
this gives the atria time to fully empty before
the ventricles begin to contract.

7. Sequence of excitation of heart muscle, Con’t.
The right and left AV bundles are connected
by the short Bundle of His
This bundle transmits the action potential into
the muscle of both ventricles via purkinje fibers
the sequence of excitation is such that the
lower portions of the ventricles contract first,
pushing the blood upwards

8. Conduction pathway

9. Cardiac Muscle Action Potential
an action potential is the result of depolarization of a muscle or nerve cell
the action potential propagates along nerves and into the muscle fibers, causing muscle
contraction
the cardiac muscle action potential has three
distinct phases; rapid depolarization; the
plateau; and repolarization

10. Action potential

11. Electrical and Mechanical Correlations

12. The Refractory Period refractory means “unresponsive”, or stubborn
in physiology, it refers to the period of time when the muscle or nerve cell is unresponsive to stimulation
the absolute refractory period refers to the time when the cell will not respond regardless of the strength of stimulus
the relative refractory period refers to the time when the cell will respond only if the stimulus is “suprathreshold”
The absolute refractory period in skeletal and heart muscle lasts about as long as the action potential (~
250 ms in heart muscle)

13. Heart Physiology: Intrinsic Conduction System
Autorhythmic cells:
– Initiate action potentials
– Have unstable resting potentials called
pacemaker potentials
– Use calcium influx (rather than sodium)
for rising phase of the action potential

14. Pacemaker potential and action potential

15. Intrinsic conduction system, Action potential

16. Electrical activity of the heart

17. ECG ; Electrocardiograph

18. ECG ; Electrocardiograph “Con't”

20. Recording of ECG