The SA node acts as the pacemaker of the heart by spontaneously generating action potentials that spread through the atria. The atrioventricular node then slows conduction to allow time for blood to fill the ventricles before they contract. The excitation wave travels through specialized conduction pathways in the ventricles before activating the purkinje fibers and ventricular muscle. The autonomic nervous system controls heart rate and conduction through the atrioventricular node. An ECG records the electrical activity of the heart by detecting the P, QRS, and T waves which represent atrial depolarization, ventricular depolarization, and ventricular repolarization, respectively.

1. CARDIAC EXCITATION
By Tayyab khan
kmu

2. SA node is called the pacemakerof the heart
SA node is composed of a group of specialized cardiac muscle cells
Instead they are the cells that gained a property to generate
spontaneous action potentials.

4. Heart rate (HR) can be called as;
the rate at which action potentials are initiated in
SA node

5. HRHR is under control of autonomic nervous systemis under control of autonomic nervous system
Sympathetic nervous system Parasympathetic nervous system
HR HR

6. Once the action potential is initiated in SA
node, the depolarization wave spreads
outward in all directions and covers all the
atrial muscle cells.

7. • The spreading wave of depolarization reaches to
atrioventricular node (AV node) before it reaches to the
ventricles.
• AV node is composed of non-contractile cardiomyocytes
(a plate of cartilage and fibrous material) which are
specialized to conduct signals slowly.
• Because of its histological properties AV node also acts
as an electrical insulator.
• By retarding the electrical wave, it allows time for slower
moving mechanical events and for blood to move from
atria to ventricles.

9. • The excitation wave
travels in the
ventricles in an order
• The order is,
a) Septum, b) Apex,
c) free walls, d) base of
ventricles.

10. • This pathway to the ventricles begins with the common
bundle of His, a tract of specialized myocytes
• Then excitation is delivered downward into the
interventricular septum.
• Here it separates into left and right bundle branches
• High-speed Purkinje fibers carry the depolarization wave
to the contractile ventricular cardiac muscle cells.
• Similar to atrial muscle cells, depolarization wave is
conducted from cell to cell via gap junctions in ventricles.

13. • The term “chronotropy” is used to define the heart rate.
• Positive chronotropy is used for increased heart rate (positive
chronotropes increase heart rate)
• Negative chronotropy is used for reduction in heart rate
(negative chronotropes decrease the heart rate)

14. SYMPATHETIC NS
NOREPINEPHRINE
ββ – 1– 1 adrenergic receptors
cAMP
(+) chronotropy(+) chronotropy
Increased HRIncreased HR
PARASYMPATHETIC NS
ACETYLCHOLINE
M2M2 receptors
cAMP
(-) chronotropy(-) chronotropy
Decreased HRDecreased HR

15. • The term “dromotropy” is used to define the conduction speed
in AV node.
SYMPATHETIC NS
PARASYMPATHETIC NS
(+) dromotropy(+) dromotropy
Increased rate of conductionIncreased rate of conduction
through the AV nodethrough the AV node
(-) dromotropy(-) dromotropy
Decreased rate of conductionDecreased rate of conduction
through the AV nodethrough the AV node

17. • To activate the various regions in the heart fully, the waves of electrical activity travels
through the structures of the heart.
• ECG captures a series of snapshots of these electrical events to provide information
about their tim ing , dire ctio n and the m ass o f tissue s involved.
• Waves in the ECG recording are created by excitation and recovery of different
regions of the heart.

18. • Pwave: When the wave of depolarization spreads across the
atria, P wave is recorded in ECG.
• P wave refers the atrial excitation (atrial depolarization).
• When the wave of excitation moves slowly through the AV
node and crosses from atria to ventricles via the bundle of His,
a q uie t brie f pe rio d fo llo ws the P wave .
• QRS complex: Ventricular depolarization produces the QRS
complex
• QRS complex refers the ventricular excitation.
• It is composed of three components:
a) Qwave: Excitation of interventricular septum,
b) Rwave: the apex and the free walls
c) S wave: regions the base.
• T wave: Ventricular repolarization registers on the ECG
recording as the T wave.
• In other words the T wave is ventricular recovery.

22. HEIGHT amount of muscle involved
INTERVALS duration of electrical event