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.