HEART PHYSIOLOGYRHYTHMICAL EXCITATION

OF HEARTBy

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

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

VentriclesCause 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.

SUMMARY OF SPREAD OF THE CARDIAC IMPULSE THROUGH THE HERAT

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

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