The document discusses the generation and transmission of cardiac impulses. It describes the specialized conduction system of the heart including the sinoatrial node, atrioventricular node, bundle of His, and Purkinje fibers. It explains the differences between action potentials in pacemaker cells versus muscle cells. Key differences include the ion channels involved and slower rates of depolarization in pacemaker cells. The document also summarizes the transmission of impulses through the heart and reasons for delays at different parts of the conduction system.

1. CARDIAC IMPULSES
Genesis and transmission of
CARDIAC IMPULSES
Pondicherry university
Department of Biochemistry and Molecular Biology

2. Conduction System
• Auto-rhythmic Fibers : Specialized cardiac muscle
fibers called which has an inherent and rhythmical
electrical activity. Automaticity and rhythmicity.
• Sinus Node ( SA node) : Sinoatrial node, a small
flattened ellipsoid strip, 3mm wide -15mm long –
1mm thick -3 to 5µm diameter, Located in superior
posterolateral wall of R.atrium, near opening of
superior vena cava. Connected directly to atrial muscle
fibres. Capability of self-excitation.
• Internodal pathway: Connects between two nodes
• Auto-rhythmic Fibers : Specialized cardiac muscle
fibers called which has an inherent and rhythmical
electrical activity. Automaticity and rhythmicity.
• Sinus Node ( SA node) : Sinoatrial node, a small
flattened ellipsoid strip, 3mm wide -15mm long –
1mm thick -3 to 5µm diameter, Located in superior
posterolateral wall of R.atrium, near opening of
superior vena cava. Connected directly to atrial muscle
fibres. Capability of self-excitation.
• Internodal pathway: Connects between two nodes

3. • Atrioventricular Node (AV Node) : Less Gap
Junctions between nodal cells, more resistance.
Comparatively slow conduction.
• A-V Bundle / Bundle of His : Impulses travels only
in one direction.
• Purkinje Fibers: Very high permeability gap
junctions. velocity of 1.5 - 4.0 m/sec (6x that of
ventricular muscle & 150x in A-V nodal fibers).
Synchronize Right and Left ventricular contraction
Cont..Conduction System
• Atrioventricular Node (AV Node) : Less Gap
Junctions between nodal cells, more resistance.
Comparatively slow conduction.
• A-V Bundle / Bundle of His : Impulses travels only
in one direction.
• Purkinje Fibers: Very high permeability gap
junctions. velocity of 1.5 - 4.0 m/sec (6x that of
ventricular muscle & 150x in A-V nodal fibers).
Synchronize Right and Left ventricular contraction

4. cont.. Conduction System

5. Parts of Conducting System Velocity of impulse
Atrial Muscle 0.3 m/sec
Ventricular Muscle 0.3 – 0.5m/sec
AV Node 0.05 m/sec
Cont..Conduction System
AV Node 0.05 m/sec
Internodal fibers 1m/sec
Left & Right Bundle Branches 2 m/sec
Purkinje Fibers 1.5 - 4 m/sec

6. Are cardiac action potentials different?

7. SA NODE ACTION POTENTIAL (AP)
• Found in SA nodal cells. These cells has no true
resting potential. Their potential is called as
Pacemaker potential (-60mV)
• Depolarizing is primarily by slow Ca++ currents
instead of by fast Na+ currents.
• No fast Na+ channels and currents operating in
SA nodal cells. This results gradual
depolarization. Hence called as slow response
action potential
• Pacing rate is 100 times/minute
• Found in SA nodal cells. These cells has no true
resting potential. Their potential is called as
Pacemaker potential (-60mV)
• Depolarizing is primarily by slow Ca++ currents
instead of by fast Na+ currents.
• No fast Na+ channels and currents operating in
SA nodal cells. This results gradual
depolarization. Hence called as slow response
action potential
• Pacing rate is 100 times/minute

8. Cont…SA NODE AP
F-type Sodium channel
(Funny type)Voltage gated
Potassium channel
Types of Channels
SA Nodal Cell
T- type Calcium Channel
(Transient type)
L- type Calcium Channel
(Long lasting type)

9. Na+
Threshold
Na+
• F –type Sodium channel opens
causing inward movement of Na+.
• Depolarization in membrane potential
• Responsible for inherent leakiness of
Na+ in SA node. (Automaticity)
• These channels works at -60 mV
instead of -90 mV in normal Na+
channels. Hence called as funny
channels/ funny currents

10. Na+
Threshold
Na+
Ca2+ (T)
• Around -50 mV Voltage gated T- type
Ca2+ channel opens. (T-Transient)
• As the potential become positive their
activity decreases.
• Active for very short time.
•Na+ permeability decreases gradually
with increase in potential positively.

11. Na+
Threshold
Na+
Ca2+ (L)
• This gradual depolarization is known
as a Pacemaker potential .
•Around -40 mV Voltage gated L- type
Ca2+ channel opens. (L-Long lasting)
• Active for long time.
• Ca2+ movement through this channel
is not rapid, making slower rate of
depolarization.
Ca2+ (T)

12. Na+
Threshold
Na+
•Voltage gated T- type Ca2+ channel
closes.
• As the potential become positive and
their activity decreases.
• L- type Ca2+ channel primarily
responsible for depolarization
• Na+ permeability is considerably less
due to high positive potential.
Ca2+ (L)

13. Na+
K+
Threshold
Na+
K+
•Voltage gated L- type Ca2+ channel
closes.
• Voltage gated K+ channel opens.
• Out ward movement of K+ ion causing
hyperpolarization
• hyperpolarized state is necessary for
pacemaker channels to become
activated.

14. Na+
Threshold
Na+
• Voltage gated K+ channel become
inactive as the potential become
negative again.
• a slow decline in the outward
movement of K+
• Na+ F- type channel become prominent
and cycle repeats

15. SA NODE AP : Summary

16. AV Node AP
• Similar to AP of SA Node.
• determined primarily by changes in slow
inward Ca++ and K+ currents, and do not
involve fast Na+ currents.
• have intrinsic pacemaker activity produced by
the same ion currents as in SA nodal cells.
• Pacing rate is 40 – 60 times /minute
• Similar to AP of SA Node.
• determined primarily by changes in slow
inward Ca++ and K+ currents, and do not
involve fast Na+ currents.
• have intrinsic pacemaker activity produced by
the same ion currents as in SA nodal cells.
• Pacing rate is 40 – 60 times /minute

17. Myocyte AP
• Found in muscle cells heart
• Unlike nodal cells myocytes have a true resting
membrane potential (-90 mV)
• Threshold voltage of -70 mV cause rapid
depolarization.
• Types of channels
- Fast Na+ channel
- L- type Ca2+ channel
- Transient Outward K+ channel
- Voltage – gated K+ channel
- K+ leaky channel
• Found in muscle cells heart
• Unlike nodal cells myocytes have a true resting
membrane potential (-90 mV)
• Threshold voltage of -70 mV cause rapid
depolarization.
• Types of channels
- Fast Na+ channel
- L- type Ca2+ channel
- Transient Outward K+ channel
- Voltage – gated K+ channel
- K+ leaky channel

18. K+
•K+ leaky channel is open at resting membrane potential-90 mV

19. Na+ enters
Fast Na+ channel
Na+ enters
Fast Na+ channel
Na+
• fast Na+ channel opens
around -70 mV
• Rapid depolarization due to
fast Na+ channels

20. Na+ enters
Fast Na+ channel
Na+ enters
Fast Na+ channel
Na+
• Transient outward K+
channel become active
making a small repolarization
of +5 mV.
K+

21. Na+ enters
Fast Na+ channel
Ca2+ enters
L-type Ca2+ channel
Na+ enters
Fast Na+ channel
Ca2+
• Repolarization is delayed and
there is a plateau phase in the
action potential.
•Ca2+ inward = K+ outward
K+

22. Na+ enters
Fast Na+ channel
Ca2+ enters
L-type Ca2+ channel
Na+ enters
Fast Na+ channel
Ca2+
• Transient outward K+
channels closes
• Membrane potential
depolarizes to about -40 mV

23. Na+ enters
Fast Na+ channel
Ca2+ enters
L-type Ca2+ channel
K+ exits
Voltage gated K+
channel
Na+ enters
Fast Na+ channel
K+
K+
K+ exits
Voltage gated K+
channel
• Ca2+ channel got inactivated
• hyperpolarization (-55 mv) happens
due to voltage gated K+ channel.
• K+ leaky channel become activated

24. Na+ enters
Fast Na+ channel
K+ exits
Voltage gated K+
channel
Ca2+ enters
L-type Ca2+ channel
Na+ enters
Fast Na+ channel
K+ exits
Voltage gated K+
channel
K+
Outflow of K+ restores the negative resting membrane potential (-90 mV).

25. Myocyte AP: summary

26. • action potential in non-pacemaker cells is primarily
determined by relative changes in fast Na+, slow
Ca++ and K+ currents.
• Fast sodium channel opens around -70 mV, allows rapid
depolarization, rate of depolarization is higher than
nodal cells.
• After 1msec fast sodium channel closes. Ca2+ channels
opens around -5 mV and stays open for 0.25sec
• During plateau phase Ca2+ move from the interstitial
fluid into the cytosol. This inflow causes even more
Ca2+ to pour out of the sarcoplasmic reticulum and T-
tubules into the cytosol .This helps in muscle
contraction
• At the end of plateau of AP, Ca influx stops while efflux
into sarcoplasmic reticulum & T-tubules occur &
contraction ends
Myocyte AP and Contraction
• action potential in non-pacemaker cells is primarily
determined by relative changes in fast Na+, slow
Ca++ and K+ currents.
• Fast sodium channel opens around -70 mV, allows rapid
depolarization, rate of depolarization is higher than
nodal cells.
• After 1msec fast sodium channel closes. Ca2+ channels
opens around -5 mV and stays open for 0.25sec
• During plateau phase Ca2+ move from the interstitial
fluid into the cytosol. This inflow causes even more
Ca2+ to pour out of the sarcoplasmic reticulum and T-
tubules into the cytosol .This helps in muscle
contraction
• At the end of plateau of AP, Ca influx stops while efflux
into sarcoplasmic reticulum & T-tubules occur &
contraction ends

27. Transmission of cardiac impulses

28. Transmission of cardiac impulses

29. Transmission of cardiac impulses

30. Transmission of cardiac impulses

31. Transmission of cardiac impulses

32. Transmission of cardiac impulses

33. AV nodal Delay
Part of conducting
system
Reasons for delay Delay
(sec)
AV node less number of gap
junctions
0.09 sec
AV bundle Resistance in AV bundle 0.04 sec
Internodal pathways Transmission time 0.03 secInternodal pathways Transmission time 0.03 sec
• A total delay of 0.16 sec. This allows time for the atria to empty their
blood into the ventricles before ventricular contraction begins. This
increases the efficiency of the pumping action of the heart.
• It is primarily the AV node and it’s adjacent fibers that delay this
transmission into the ventricles

34. Parts of conducting system Pacing rate
SA Node 75 times/min
AV Node 60-50 times/min
Bundle of His & Purkinje fibers 20-30 times/min
Who is the real pacemaker?
Bundle of His & Purkinje fibers 20-30 times/min
• The discharge rate of the sinus node is faster than discharge rate of either the A-V
node or the Purkinje fibers.
• the sinus node discharges again before either the A-V node or the Purkinje fibers can
reach their own thresholds
• rate of rhythmical discharge in SA Node is faster than any other part

35. Reference
• Widmaeir. P.E, Raff.H, Strang .T.K- Vander’s Human Physiology
the mechanisms of body function. 11th edition. McGraw-Hill
Higher Education
• Gerard J. Tortora, Bryan Derrickson. Principles of Anatomy and
Physiology, 13th Edition. John Wiley & Sons, Inc.
• Kim E. Barrett, Scott Boitano, Heddwen L. Brooks, Susan M.
Barman, Ganong’s Review of Medical Physiology, 23rd edition.
McGrawHill Medical
• Arthur C. Guyton, John E. Hall. Text Bookof Medical Physiology
11th Edition. Elsevier Saunders, Elsevier Inc
• Widmaeir. P.E, Raff.H, Strang .T.K- Vander’s Human Physiology
the mechanisms of body function. 11th edition. McGraw-Hill
Higher Education
• Gerard J. Tortora, Bryan Derrickson. Principles of Anatomy and
Physiology, 13th Edition. John Wiley & Sons, Inc.
• Kim E. Barrett, Scott Boitano, Heddwen L. Brooks, Susan M.
Barman, Ganong’s Review of Medical Physiology, 23rd edition.
McGrawHill Medical
• Arthur C. Guyton, John E. Hall. Text Bookof Medical Physiology
11th Edition. Elsevier Saunders, Elsevier Inc