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CHAPTER I: CARDIAC EXCITATION
Asst. Prof. Dr. Emre HamurtekinEMU Faculty of Pharmacy
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SA node is called the pacemaker of 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.
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Heart rate (HR) can be called as;the rate at which action potentials are
initiated in SA node
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HR HR is under control of autonomic nervous is under control of autonomic nervous system system
Sympathetic nervous systemParasympathetic nervous system
HR HR
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Once the action potential is initiated in SA node, the depolarization wave spreads outward in all directions and covers all the atrial muscle cells.
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• 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.
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• The excitation wave travels in the ventricles in an order
• The order is, a) Septum, b) Apex, c) free walls, d) base
of ventricles.
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• 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.
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• 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)
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SYMPATHETIC NS
NOREPINEPHRINE
ββ – 1 – 1 adrenergic receptors
cAMP
(+) chronotropy(+) chronotropyIncreased HR Increased HR
PARASYMPATHETIC NS
ACETYLCHOLINE
M2 M2 receptors
cAMP
(-) chronotropy(-) chronotropyDecreased HR Decreased HR
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• The term “dromotropy” is used to define the conduction speed in AV node.
SYMPATHETIC NS
PARASYMPATHETIC NS
(+) dromotropy(+) dromotropyIncreased rate of conductionIncreased rate of conduction
through the AV nodethrough the AV node
(-) dromotropy(-) dromotropyDecreased rate of conductionDecreased rate of conduction
through the AV nodethrough the AV node
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• 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 timing, direction and the mass of tissues involved.
• Waves in the ECG recording are created by excitation and recovery of different regions of the heart.
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• P wave: 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 quiet brief period follows the P wave.
• QRS complex: Ventricular depolarization produces the QRS complex
• QRS complex refers the ventricular excitation. • It is composed of three components:
a) Q wave: Excitation of interventricular septum, b) R wave: 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.
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HEIGHT amount of muscle involved
INTERVALS duration of electrical event
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