Chapter i CARDIAC EXCITATION

CARDIAC EXCITATION

By Tayyab khankmu

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 HR is under control of autonomic nervous is under control of autonomic nervous system system

Sympathetic nervous systemParasympathetic nervous system

HR 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

ββ – 1 – 1 adrenergic receptors

cAMP

(+) chronotropy(+) chronotropyIncreased HR Increased HR

PARASYMPATHETIC NS

ACETYLCHOLINE

M2 M2 receptors

cAMP

(-) chronotropy(-) chronotropyDecreased HR Decreased HR

• 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

• 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