Physiology of Excitatory and Conductive system of the heart including action potential in cardiac pacemakers and autonomic Control of Cardiac rhythm

1. Excitatory & Conductive
System of the Heart
Prof. Dr. Rashid Mahmood

2. Objectives
• At the end of the lecture student should be able to
• Describe the structure and functions of the conduction system of the heart
• List the sequence of events that occur in rhythmic excitation of the heart
• Explain the effects of sympathetic and parasympathetic stimulation on heart

4. Functions of Excitatory & Conductive System
of the Heart
1. Generation of rhythmical electrical impulses
2. Conduction of these impulses through heart
3. Atria should contract ahead of ventricular contraction
(about 1/6th sec)
4. All portions of ventricles should contract simultaneously
5. Heart should keep on contracting without extrinsic
stimulation

5. Specialized Excitatory & Conductive System of the Heart (in sequence)
1. SA Node
2. Internodal pathways & interatrial Pathways
3. Transitional fibers
4. AV Node
5. Bundle of His
i. Penetrating portion of AV Bundle
ii. Distal portion of AV Bundle
I. Right Bundle Branch
II. Left Bundle Branch
a. Left Anterior Fascicle
b. Left Posterior Fascicle
6. Purkinje Fibers
7. Ventricular Muscle

6. 1. S-A Node (sinoatrial node; sinus node)
SVC
Location: Near the junction of SVC &
superior posterolateral wall of the Rt.
Atrium
Size: 15mm long, 5mm wide & 1 mm thick
Diameter of the fibers = 2-7 um
Structure: Mainly comprised of P-Cells
(Stellate cells) & few
myofilaments.
P =Premature, Pale looking , Pace
maker cells
Action Potential:
RMP = -55 to -60 mv
Threshold level = -40 mv

7. SELF-Excitability of S-A Node
Due to presence of Ca++Na+ (T)
Channels or Na+ leak Channels
showing Diastolic
Depolarization
(“Funny Channels” leaky to
both Na+ and K+) (Funny
Current)
Rate of Generation of A.P =
60-80 /min
HCN: hyperpolarization-activated cyclic nucleotide-gated channels

8. Mechanism of Sinus Nodal Rhythmicity
1. Natural leakiness to
Ca++ (T-Type) & Na+
(Slow)
[“Funny channels, leaky
to Na+ & K+]
2. Voltage-gated fast Ca++
(L-Type) (& Na+)
Channels
3. Closing of fast Ca++ &
Opening of K+ Channels
4. K+ Channels remain
open (Hyperpolarization)
5. K+ Channels close (&
Natural leakiness

9. 2. Internodal pathways & interatrial Pathways
(Specialiazed Conduction fibers with fast speed)
Anterior (Bachman)
Middle (Wenchebach)
Posterior (Thorel)

10. 3. A-V Node • Location: Near the lower part of
the inter-atrial Septum Posteriorly in
the Rt. Atrium.
• Three Functional Regions:
• A-V Junction,
• Nodal Region,
• Nodal-His Region

11. 3. A-V nodal Delay
• A-V nodal Delay = 0.09 Sec
• Mechanism of Delay:
• Fiber Diameter is small & less
number of gap Junction in the
fibers -> Slow Conduction –
Decremental Conduction
• Advantage of A-V nodal
delay?
• Inherent Rate of Generation of
A.P = 40-60/min

12. 4. Bundle of His (AV Bundle)
AV Node
Penetrating portion of AV Bundle
Distal portion of AV Bundle
Right Bundle Branch Left Bundle Branch
Left Anterior Fascicle Left Posterior Fascicle
Purkinje Fibers

13. Time taken for the
impulse to travel through
different parts of
conduction system
• Atria to AV Node: 0.03 second
• AV Nodal Delay: 0.09 second
• AV Node to Ventricular Muscles:
0.04 second (main delay in
penetrating AV Bundle)
• Total time taken from SA Node
to Ventricular Muscles:
• 0.03+ 0.09+ 0.04= 0.16 second

14. Purkinje Fibers
• Largest fibers in the
Heart arising from the
Bundle Branches
9-18 um in Diameter
• High No of Gap Junction
( More Permeability)
• Function – Rapid
conduction of impulse
Throughout the Heart
• Discharge rate: 15-40/ min

15. Spread of impulse from Purkinje fibers into
Ventricular muscle
Interventricular Septum
Apex of the heart
Base of the heart

16. Control of Excitation and Conduction in the Heart
• The Sinus Node as the Pacemaker of the Heart
• Reason:
• Discharge rate of the sinus node is faster than the natural self-
excitatory discharge rate of either the A-V node or the Purkinje fibers

17. Abnormal Pacemakers—“Ectopic” Pacemaker
•some other part of the heart develops a rhythmical discharge
rate
abnormal sequence of contraction of the different parts of the
heart
weakness of heart pumping.
• Causes:
1. Development of excessive excitability and becoming the
pacemaker
2. Blockage of transmission of the cardiac impulse from the sinus
node to the other parts of the heart
Atria continue to beat at the normal rate of rhythm of the sinus node

18. Stokes-Adams syndrome
• After sudden A-V bundle block, the Purkinje system does not begin
to emit its intrinsic rhythmical impulses until 5 to 20 seconds
• person faints after the first 4 to 5 seconds because of lack of blood
flow to the brain

19. Autonomic Control of conducted system of heart
• Parasympathetic nerves (Vagi) mainly supply to SA and AV nodes,
and to a lesser to the muscles of the atria, and very little directly to
the ventricular muscle
• Sympathetic nerves supply to all parts of the Heart, especially to
ventricular muscle

20. Effects of Parasympathetic Stimulation
1. ↓ Rate of rhythm of SA
Node→
↓ Heart Rate
2. ↓ Excitability of AV
Junctional fibers→
↓ Conduction Velocity
3. ↓ Force of contraction
( By increasing membrane
permeability to K+ ions)

21. Ventricular Escape
• Strong parasympathetic
stimulation may completely
block transmission of
Cardiac impulse through AV
Node
• Ventricles may stop beating
for 5-20 seconds
• Purkinje fibers develop a
rhythm of its own @ 15-40
beats per minute
Mechanism of Vagal
Effects
• ↑ Permeability to K+ ions →
Hyperpolarization

22. Effects of Sympathetic Stimulation
•.
1.↑ Rate of rhythm of SA Node →
↑ Heart Rate
(Chronotropic Effect)
2. ↑ Excitability of AV Junctional
fibers (Bathmotropic Effect)
3.↑ Conduction Velocity
(Dromotropic Effect)
4.↑ .Force of Contraction
(Inotropic effect)

23. Mechanism of
Sympathetic
Effects
• Neurotransmitters:
Norepinephrine
• Receptors:
• Beta-1 adrenergic
receptors
• ↑ permeability to
sodium and
calcium ions

24. Summary slides

26. Specialized Excitatory & Conductive System of the
Heart (with their velocities)
1. SA Node (0.05 m/s)
2. Internodal pathways & interatrial Pathways (1 m/s)
3. Transitional fibers
4. AV Node (0.01-0.1 m/s; average: 0.05 m/s)
5. Bundle of His (1 m/s)
i. Penetrating portion of AV Bundle
ii. Distal portion of AV Bundle
I. Right Bundle Branch
II. Left Bundle Branch
a. Left Anterior Fascicle
b. Left Posterior Fascicle
6. Purkinje Fibers (1.5 – 4.0 m/s; mainly 4 m/s)
7. Ventricular Muscle (0.3-1 m/s)

30. Take home points
• Contractions in heart are controlled via well-regulated electrical signals
that originates in pacemaker cells in the sinoatrial (SA) node and are
passed via internodal and atrial pathways to atrioventrical (AV) node,
the bundle of His, the Purkinje system, and to all parts of the ventricle.
• Compared to physical myocytes, pacemaker cells have a slightly
different sequence of events. After repolarization to the resting
potential, there is a slow depolarization that occurs due to a channel
that can pass both Na+ and K+. As this “funny” current continues to
depolarize the cell, Ca++ channels are activated to rapidly depolarize
cell. The hyperpolarization phase is dominated by K+ current
• Sympathetic stimulation increases heart rate, force of contraction,
excitability and velocity of conduction while Parasympathetic
stimulation has opposite effects.

31. Learning resources
•Guyton and Hall (Text book of physiology), 13th edition
•Ganong (Text book of physiology), 24th Edition
© Prof. Dr. Rashid
Mahmood
31

32. Assessment Questions/ Activity
1. Draw the diagram of excitatory and conductive system of
heart
2. What are the effects of sympathetic and parasympathetic
stimulation on heart?
3. List the functions of Excitatory & Conductive System of the
Heart
© Prof. Dr. Rashid Mahmood 32