HEART PHYSIOLOGYRHYTHMICAL EXCITATION OF HEART

ByDr. Mudassar Ali Roomi (M.B;B.S, M.PHIL.)

Assistant Professor Physiology

CONDUCTION SYSTEM OF HEART

• SA node • Internodal pathways• AV node• AV bundle of His• Right and left bundle

branches• Purkinje fibers

Conduction System of the Heart• SA node: sinoatrial node. The

pacemaker. – Specialized cardiac muscle cells.– Generate spontaneous action

potentials (autorhythmic tissue). – Action potentials pass to atrial muscle

cells and to the AV node

Conduction System of the Heart

• AV node: atrioventricular node.– Action potentials conducted more

slowly here than in any other part of system.

• Function: Ensures ventricles receive signal to contract after atria have contracted

Conduction System of the Heart

• AV bundle: passes through hole in cardiac connective tissue skeleton to reach interventricular septum

Conduction System of the Heart

• Right and left bundle branches: extend beneath endocardium to apices of right and left ventricles

Conduction System of the Heart

• Purkinje fibers: – Large diameter cardiac muscle cells with

few myofibrils. – Many gap junctions. – Conduct action potential to ventricular

muscle cells (myocardium) very rapidly

Rate of impulse generation in heart:

• SA NODE: 70-80/min• AV NODE: 40-60/min• AV BUNDLE, BRANCHES & VENTRICLES: 15-40/min SA NODEnormal PACEMAKER OTHERS ECTOPIC FOCI

AV Node, and Delay of Impulse Conduction from the Atria to the Ventricles

• Location of the A-V node: in the posterior wall of the right atrium immediately behind the tricuspid valve

• there is a delay of 0.09 second in the A-V node

• A delay of another 0.04 second occurs mainly in A-V bundle

• Thus, the total delay in the A-V nodal and A-V bundle system is about 0.13 second.

AV Node, and Delay of Impulse Conduction from the Atria to the Ventricles

Cause of the Slow Conduction:• caused mainly by decreased

numbers of gap junctions b/w successive cells in the conducting pathways

Importance of AV nodal delay: • this delay allows time for

the atria to empty their blood into the ventricles before ventricular contraction begins.

Rapid Transmission in the Ventricular

Purkinje System

• They are very large fibers, even larger than the normal ventricular muscle fibers

• and they transmit action potentials at a velocity of 1.5 to 4.0 m/sec, a velocity about 6 times that in the usual ventricular muscle and 150 times that in some of the A-V nodal fibers.

Rapid Transmission in the Ventricular

Purkinje SystemCause of rapid transmission:• The rapid transmission of action

potentials by Purkinje fibers is believed to be caused by a very high level of permeability of the gap junctions at the intercalated discs between the successive cells that make up the Purkinje fibers.

Rapid Transmission in the Ventricular

Purkinje SystemFUNCTION: rapid transmission in Purkinje fibers is responsible for synchronous contraction of ventricular muscle.

Conduction velocities of heart tissues:

• ATRIAL MUSCLE= 0.3m/sec• INTERNODAL PATHWAYS=

1m/sec• AV NODE: slowest

0.05-0.1m/sec• AV BUNDLE &

BRANCHES/PURKINJE SYSTEM: Maximum velocity= 1.5-4m/sec

• VENTRICULAR MUSCLE= 0.5m/sec

SUMMARY OF SPREAD OF THE CARDIAC IMPULSE THROUGH THE HERAT

ECTOPIC PACEMAKER OF HEART

• Normal pacemaker of heart?

• Ectopic pacemaker?Causes of ectopic

pacemaker of heart?1. Excessive excitability of

some part of the heart other than SA node

2. Heart blocks