This document summarizes cardiac automaticity and the types of cardiac action potentials. It describes that the sinoatrial node contains pacemaker cells that spontaneously depolarize through sodium and calcium influx, generating a prepotential. This prepotential triggers the fast response action potential throughout the heart. The fast response action potential involves sodium influx in phase 0, followed by potassium efflux in phase 3, causing repolarization.

1. Cardiac Automaticity
Dr. Radwa Mehanna MD PhD
Professor of Medical Physiology
Faculty of Medicine, Alexandria University
Rhythmicity of the Heart

2. Students’ learning outcomes
At the end of this session, the student will be able to:
1. Characterize the types of cardiac action potentials.
2. Define the ionic basis of cardiac resting, pre and action potentials.
3. Describe the characteristics of the fast- and slow response action potentials.
4. Explain the effect of autonomic innervation on SA node

5. Properties of the Heart
1- Rhythmicity
2-Conductivity
3- Excitability
4- Contractility

6. Origin of cardiac stimulation
• The heart is a myogenic organ; it does not need nerve
stimulation to initiate its contraction.
• It contains specialized cells, which discharge spontaneously
(automaticity or rhythmicity)

7. Rhythmicity
It is the ability of
certain cardiac
muscle cells
to spontaneously
depolarize and
generate an action
potential “pacemakers”

8. Autorhythmic cells
Pace Maker

10. Autorhythmic cells
Pace Maker

12. Not a true RMP
It is a prepotential

13. No resting membrane potential
Phase 4
through IF

14. Reach -50 mV
Phase 4

15. Phase 0
-40/-45 mv
Threshold potential

16. Phase 3

17. K+ outflux continues
and causes
hyperpolarization to
pace maker cells that
activates IF Na+
channels and starts
phase 4 again

18. Slow action
Potential
If ; Na+ Funny channels
Ica (T); Transient Ca channels
Ica(L) ; Long acting Ca channels
Ik; K delayed rectifier channels

19. Autorythmic cells and Prepotential
• They are the most excitable among the myocardial cell
(have less negative potential than the rest).
• They are spontaneously depolarized during the diastole
(relaxation) causing what is called pre-potential or diastolic
depolarization.
• Occurs by the influx of Na through the funny channels then
the Ca++ through T Ca++ channels
• This forms Phase 4 named pre potential
• Thus no RMP but instead there is pre potential

20. Pace maker
• SAN has the greatest
rhythmicity and initiates
the beat of the whole heart
• SAN 100/min
• AVN 60/min
• AVB 45/min

22. Fast Action potential
RMP -90 mv

24. Phase 0

25. Phase 0

26. Phase 1
Phase 2
Initial repolarization
Efflux of K+ with ikto
Plateau
Opened K+ and Ca2+ channels
Importance of plateau
qIt prolongs ARP of
the cardiac muscle to
prevent tetanus of the
heart.
qCa2+ triggers Ca2+
release and thus
muscle contraction

27. Phase 3
Phase 4
Interval from
end of
repolarization
till the
beginning of
another AP =
RMP
Opening of slow
and fast K+
delayed ractifier
channels

28. Fast Action potential
INa; Na+ channels
Ica(L) ; Long acting Ca channels
iKto ; transient K channels
Ik; K delayed rectifiers channels

29. Restoration of Ionic concentration
Na+/Ca2+ exchange 3 Na+/1 Ca2+
Na+/K+ ATPase pump 3 Na+/2 K+
ATP Ca2+ pump

31. Fast response
action potential
Slow response
action potential
MP -90 stable -60 unstable
AP Fast Slow
Deplarizing ion Na+ through voltage gated fast
sodium channels
Ca++ through L- type calcium
channels
Phases 5 3, no phase 1, and phase 2
(plateau)
Duration Long short
Amplitude overshoting short

32. Autonomic Innervation
increases the K+ conductance
Decreases (cAMP) in the cells, and this slows the opening of Ca2+

33. 1. What is the main cause of repolarization of
ventricular myocytes during phase 3 of their action
potential?
a) influx of Ca++
b) influx of K+
c) efflux of K+
d) influx of Na+
e) efflux of Na+
C

34. 2. Opening of which of the following channels causes
the spike phase (Phase 0) of the action potential of
the sinoatrial node pacemaker cells?
a) Fast voltage-gated Na+ channels.
b) Voltage-gated L- type Ca++ channels.
c) Voltage-gated T- type Ca++ channels.
d) Voltage-gated K+ channels.
B

35. 3. Phase 4 (the prepotential) of the pacemaker
potential of the heart results from:
a) K+ influx and Ca2+ influx
b) Na+ influx and Ca2+ influx
c) Na+ influx and Cl− influx
d) Cl− influx and K+ influx
B

36. 4. The plateau phase of the cardiac action potential
is due to:
a) opening of voltage-gated L- type Ca++ channels
b) opening of fast voltage-gated Na+ channels
c) opening of voltage-gated K+ channels
d) all of the above A