Lecture about the electrical activity of the heart

1. Electrical Activity of the Heart
Dr.Tarig Eltoum Yagoub, MSc,MD,FCCP
Associate Professor & Consultants Pulmonologist

2. Electrical Potential
• Electrical difference across cell membrane
– Resting Membrane potential (RMB) in all tissue
– Action Potential (AP) in excitable tissues only
• What is excitable tissues?
– Are tissues that respond to stimulus by producing action potential (electrical
signal)
• 2 Excitable tissues in the body
– Nerves
– Muscles

3. Resting Membrane Potential
• A transmembrane electrical gradient (potential) is maintained, with
the interior of the cell negative with respect to outside the cell

4. Why Resting Membrane Potential?
• Due to unequal distribution of ions inside & outside cell:
• Na+
higher outside than inside cell
• K+
higher inside cell than outside
• Ca+
much higher outside than inside cell

5. What causes unequal distribution of ions?
• Maintained by ion selective channels, active pumps & exchangers

6. Action Potential (Signal)
• Action potential (AP) is the rapid reversal of membrane potential.
• AP can move along excitable tissues

7. Action Potential Animation

8. Types of Muscles
• Three muscle types in the body
– Smooth
– Skeletal
– Cardiac
• Cardiac muscle is called Myocardium
– Forms the chambers of the heart
– Part is specialized and forms the conduction system
• Cardiac Muscle has special prosperities
– Histological Prosperities
– Electrical Properties

9. Types of Muscles

10. Histological Properties of Cardiac Muscle Fibers
• Cardiac muscle has 3 important histological properties
1. Branching
2. Heart function as syncytium
3. Atrial syncytium and ventricular syncytium

11. Histological Properties of Cardiac Muscle Fibers
• Exhibit branching; Adjacent cardiac cells are joined end to end by
specialized structures known as intercalated discs

12. Histological Properties of Cardiac Muscle Fibers
• Heart function as syncytium ;When one cardiac cell undergoes an
action potential, the electrical impulse spreads to all other cells that
are joined by gap junctions so they become excited and contract as a
single functional unit (syncytium)

13. Histological Properties of Cardiac Muscle Fibers
• Atrial syncytium and ventricular syncytium

14. Electrical Properties of Cardiac Muscle Fibers
• The heart muscle has 4 important electrical properties
1. Auto-rhythmicity
2. Excitability
3. Conductivity
4. Contractility

15. Auto-rhythmicity
• Auto-rhythmicity / automaticity is the ability to initiate a heart beat
continuously & regularly without external stimulation
• But the strength and rate of contraction are modified by the
autonomic nervous system input

16. Excitability
• Excitability is the ability to respond to a stimulus of adequate
strength and duration (i.e. threshold or more) by generating a
propagated action potential

17. Conductivity
• Conductivity is the ability to conduct action potential through the
cardiac tissue

18. Contractility
• Contractility is the ability to contract in response to stimulation.

19. Cardiac Action Potential
• Cardiac AP has 5 phases (4,0,1,2,3)
–4 Resting Membrane Potential
–0 Depolarization
–1 Overshoot
–2 Plateau
–3 Repolarization
• Differs from APs in other excitable tissues because cardiac AP has a
plateau phase

20. Phases of Cardiac Action Potential

21. Phase 4:Resting Phase (RMP)
• Cardiac cells remain inactive until stimulated (RMP)
• Associated with diastole and stimulation causes the following phases.

22. Phase 0:Depolarization phase:
• Opening of fast Na channels and rapid depolarization
• Drives Na+
into cell changing membrane potential

23. Phase 1:Initial rapid repolarization
• Closure of the fast Na+
channels
• Phase 0 & 1 together

24. Phase 2:Plateau Phase
• Due to balance between inward movement of Ca+
& outward movement of K +
Corresponds to ST-Segment of the ECG.
• Normally blocks any premature stimulator signals

25. Phase 3:Repolarization
• K+
channels remain open.
• K +
channels finally close when membrane potential reaches certain level
• Corresponds to T-Wave on the ECG

26. Pacemaker Cell AP
• Differences between nonpacemaker & pacemaker cell AP
• Pacemaker Cells - Slow, continuous depolarization during rest
• Continuously moves potential towards threshold for a new action potential (called a phase 4
depolarization)

27. What is an ECG?
• The electrocardiogram (ECG) is a representation of the electrical
events of the heart during cardiac cycle.
• Each electrical event has a distinctive waveform.