Electrical Activity of Heart & ECG

21.5.12

Electrical Properties of Various Body Cells

• Nerve cells:

• Skeletal muscle cells

• Cardiac 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

Electrical Properties of Various Body Cells

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

• 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

Contraction in Skeletal versus 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 P-Wave

• 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