Cardiovascular Module

Cardiovascular Physiology

Lect. one

Cardiac Action Potential

Prof. Dr. Najeeb Hassan Mohammed

Properties of the cardiac muscle:

1. Syncytium.

2. Automaticity and Rhythmicity.

3. Excitability and Conductivity.

4. Contractility.

Objectives

• List the major properties of cardiac muscle.

• Describe action potentials in cardiac muscle

cells.

• Explain how the SA node functions as the

pacemaker.

Syncytium

The myocardium

contracts as if it

is a single

Functional unit.

Physiologic anatomy of cardiac muscle:

adjacent myocardial cells are joined end to end at structures called intercalated discs, within the intercalated discs, there are gap junctions) that allow ions to flow from one cell directly into the next cell.

Therefore action potentials to move with ease from one cardiac myocyte to stimulate the neighbor cell.

Syncytium (single functional unit)

Autorhythmicity:

Automaticity: the property of self-excitation i.e. the

ability of spontaneously generating

action potentials independent of any

extrinsic stimuli.

Rhythmicity: the regular generation of action

potentials.

Excitability and conductivity: Specialized excitatory and conductive system:

1. Sinus node "SA" node:

generation of rhythmical

impulse (automaticity).

The pacemaker of heart.

2. Internodal pathways.

3. The AV node ( impulse

delay).

4. The AV bundle (bundle of

His).

5. The left and right bundles

of purkinje fibers.

The SA node as the pacemaker of the heart:

• SA node is the normal primary pacemaker of the heart (100-110 times per minute).

• The AV node is a secondary pacemaker (40 to 60 /minute).

• The Purkinje system is a tertiary pacemaker (15 and 40).

Intrinsic SA node rate:

• Although the SA node

discharges at an

intrinsic rhythmical

rate of 100-110 times

per minute but the

pulse rate averages 70

or 80 times per

minute, this is because

of the effect of vagal

tone.

Self-excitation of SA node: What causes the SA node to fire spontaneously?

• SA node does not have a

stable resting membrane

potential which starts at

about – 60 mV.

• This is due to the inherent

leakiness of the SA nodal

fibers to Na+ ions (Na+

influx). rising the

membrane potential up to

a threshed to fire an action

potential.

Atrioventricular node (AV node):

• The cardiac impulse will not travel from the atria into ventricles too rapidly.

• There is a delay of transmission of the cardiac impulse in the AV node(0.12 sec) to allow time for the atria to empty their blood into the ventricles before ventricular contraction begins.

Ionic basis of the action potential of the

cardiac ventricular muscle fiber cell:

• Phase 0 (upstroke): initial rapid depolarization (Na+ influx).

• Phase 1 (partial repolarization): K+ efflux (K+ outflow)

• Phase 2 (plateau): prolonged plateau is due to slower and prolonged opening of the voltage-gated Ca+2 channels with Ca+2 influx.

• Phase 3 (rapid repolarization): by (K+ efflux).

• Phase 4 (complete repolarization): by Na+-K+ pump.

Refractory period:

• Absolute refractory period (ARP): it is the interval during which no action potential can be produced, regardless of the stimulus intensity. It lasts the upstroke plus plateau and initial repolarization till mid-repolarization at about -50 to -60 mV. This period prevents waves summation and tetanus.

• Relative refractory period (RRP): it is the interval during which a second action potential can be produced but at higher stimulus intensity. It lasts from the end of ARP (midrepolarization) and ends shortly before complete repolarization

In summary

• The major properties of cardiac muscle.

• Action potentials in cardiac muscle cells.

• The SA node functions as the pacemaker.