CHAPTER I: CARDIAC EXCITATION
Asst. Prof. Dr. Emre Hamurtekin
EMU Faculty of Pharmacy
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.
Heart rate (HR) can be called as;
the rate at which action potentials are initiated in SA
node
HR is under control of autonomic nervous system
Sympathetic nervous system
HR
Parasympathetic nervous system
HR
Once the action potential is initiated in SA node,
the depolarization wave spreads outward in all
directions and covers all the atrial muscle cells.
• 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.
• The excitation wave
travels in the ventricles in
an order
• The order is,
a) Septum, b) Apex,
c) free walls, d) base of
ventricles.
• 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.
• 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)
SYMPATHETIC NS
NOREPINEPHRINE
β – 1 adrenergic receptors
cAMP
(+) chronotropy
Increased HR
PARASYMPATHETIC NS
ACETYLCHOLINE
M2 receptors
cAMP
(-) chronotropy
Decreased HR
• The term “dromotropy” is used to define the conduction speed in AV
node.
SYMPATHETIC NS
(+) dromotropy
Increased rate of conduction
through the AV node
PARASYMPATHETIC NS
(-) dromotropy
Decreased rate of conduction
through the AV node
•
•
•
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.
• 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.
HEIGHT
amount of muscle involved
INTERVALS
duration of electrical event