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Electrical Activity of
Heart & ECG
21.5.12
Electrical Properties of Various
Body Cells
• Nerve cells:
• Skeletal muscle cells
• Cardiac cells:
Electrical Properties of Various
Body Cells
• Action potentials in the heart differ considerably from action
potentials found in neural and skeletal muscle cells.
• One major difference is in the duration of the action potentials.
• In a typical nerve, the action potential duration is about 1 ms. In
skeletal muscle cells, the action potential duration is
approximately 2-5 ms. In contrast, the duration of cardiac action
potentials range from 200 to 400 ms
Cardiac Cells
• The heart consists of three special types of
cardiac cells
• Pacemaking cells: Have the properties of
automaticity and are capable of generating
electrical impulses. These cells are present in the
sinoatrial node and entire His-Purkinje system
• Conducting cells: Specialized for rapid
conduction of electrical impulses and are present
within the entire His-Purkinje system
• Muscle cells: Specialized for contraction and are
present in the atria and ventricles
Cardiac Action Potentials
• There are two general types of cardiac
action potentials.
• The
pacemaker
cells
generate
spontaneous action potentials that are
also termed "slow response" action
potentials because of their slower rate
of depolarization.
• Non-pacemaker action potentials, also
called "fast response" action potentials
because of their rapid depolarization,
are found throughout the heart except
for the pacemaker cells
Action Potentials of Cardiac
Cells
Action Potential of Autorythmic
Cells
• Pacemaker
potential
membrane slowly depolarizes
“drifts” to threshold, initiates
action potential, membrane
repolarizes to -60 mV.
• Pacemaker cells differs from
skeletal and nerve cells in that
Ca2+ influx rather than Na2+
influx causes the rising phase
of the action potential once the
threshold is reached
AP of Contractile Cardiac cells
• Phase 0: Depolarization by opening of
Na+ channels enabling entry of Na+ into
the cell
• Phase1:
Brief
repolarization
by
activation of fast K+ channels. K+
moves out of cell
• Phase 2: Slow inward diffusion of Ca2+
prolongs the duration of the action
potential and produces a characteristic
plateau phase.
• Phase 3: Inactivation of slow Ca2+
channels and delayed activation of slow
K+ channels causing rapid outward
diffusion of K+
Contraction in Skeletal versus
Cardiac Muscle
• Unlike skeletal muscles, cardiac contractile
muscles have special slow Ca2+ channels that
lie primarily in T-tubules
• These voltage gated channels open causing
the plateau phase of cardiac action potential
• Calcium entry from ECF in cardiac cells
induces a much larger Ca2+ release from the
sarcoplasmic reticulum. This is known as
“Ca2+ induced Ca2+ release”
• Results in much longer contraction compared
to a single skeletal muscle fiber (300msec
compared to 100msec)
Figure 20.15
Contraction in Skeletal versus
Cardiac Muscle
• In skeletal muscle, the refractory period is very
short compared with the duration of the resulting
contraction, so the fiber can be restimulated before
the first contraction is complete to produce
summation of contractions (tetanic contraction)
• A long refractory period prevents tetanus of
cardiac muscle
Electrocardiography (ECG or EKG)
• It is performed in clinical practices to
measure the heart function.
• ECG is the record of the overall spread of
the electrical activity of heart
Electrical conduction pathway
• SA node -> atrial
muscle -> AV node ->
bundle of His -> Left
and Right Bundle
Branches ->
Ventricular muscle
Activation of Sinoatrial Node
• Sinoatrial node is the origin of the
normal electrical impulse in heart.
Although there are other cells in the
heart that can also discharge
spontaneously, the sinus node has
the faster rate of discharge and is the
pacemaker of the heart
• Sinus impulse is not strong enough
to produce a deflection in ECG
Activation of the Atria: The PWave
• P-Wave is the first deflection in electrocardiogram and
is due to the activation of atria (atrial depolarization)
Activation of Atrioventricular
Node
• After depolarization of the atria,
the only pathway by which the
sinus impulse can reach the
ventricles
is
through
the
atrioventricular (AV) node.
• As the impulse traverses the AV
node on its way to the ventricles, it
does not generate any electrical
activity and a straight line is
recorded immediately after P-wave
Intraventricular Conduction
System
• After the impulse emerges from the AV node, it is
conducted through the His bundle, bundle branches and
terminating in a branching network of Purkinje fibers
• Causes no deflection in ECG
QRS Complex
• Impulse from Purkinje fibers to
the myocariudm. This results in
ventricular depolarization and
contraction
• The QRS complex represents
activation of the ventricles and is
the largest deflection in the ECG.
This is because the ventricles
contain the largest mass of muscle
cells in the heart, collectively
referred as “myocardium”
T-Wave and ST segment
• It represents rapid ventricular
repolarization. The action potential
abruptly returns to the resting
potential of -90mV
• ST segment is flat and represents the
time when all cells have just been
depolarized and ventricular muscle
cells are completely refractory