Properties of cardiac muscle
Properties of Cardiac Muscle
Excitability, rhythmicity, conductivity and contractility of cardiac muscle.
1 Excitability of Cardiac Muscle
Definition
In all tissues, initial response to a stimulus is the electrical activity in the form of action potential.
It is followed by mechanical activity in the form of contraction, secretion, etc.
Electrical Potentials in Cardiac Muscle
Resting Membrane Potential in Cardiac Muscle
Resting membrane potential in single cardiac muscle fiber is −85 to −95 mV.
Action Potential in Cardiac Muscle
Action potential in cardiac muscle is different from that of other tissues such as skeletal muscle, smooth muscle and nervous tissue.
Duration of action potential in cardiac muscle is 250 to 350 msec (0.25 to 0.35 second).
- Initial depolarization
- Initial repolarization
- Plateau: final depolarization
- Final repolarization
Initial depolarization is very rapid and lasts for about 2 msec.
Amplitude of depolarization is about +20 mV.
Immediately after depolarization, there is an initial rapid repolarization for a short period of about 2 msec.
End of this rapid repolarization is represented by a notch.
Afterwards, the muscle fiber remains in depolarized state for some time before further repolarization.
It forms the plateau (stable period) in action potential.
Plateau lasts for about 200 msec (0.2 second) in atrial muscle fibers and for about 300 msec (0.3 second) in ventricular muscle fibers.
Final repolarization occurs after the plateau.
It is a slow process and lasts for about 50 to 80 msec (0.05 to 0.08 second) before the re-establishment of resting membrane potential.
Ionic Basis of Action Potential
- Initial depolarization is due to opening of fast sodium channels and the rapid influx of sodium ions.
-
Initial repolarization is due to transient (short duration) opening
of potassium channels and efflux of potassium ions
in a small quantity from the muscle fiber.
Simultaneously, the fast sodium channels close suddenly and slow sodium channels open resulting in slow influx of sodium ions in a large quantity. - Plateau (final depolarization) is because of opening of calcium channels which are kept opened for a longer period. This causes influx of calcium ions in large numbers. Entry of both calcium and sodium ions is responsible for prolonged depolarization, i.e. plateau.
- Final repolarization is due to increase in efflux of potassium ions.
Spread of Action Potential Through Cardiac Muscle
Action potential spreads through the cardiac muscle very rapidly.
It is because of the presence of gap junctions between cardiac muscle fibers.
Gap junctions are permeable junctions and allow free movement of ions.
Due to this, the action potential spreads rapidly from one muscle fiber to another fiber.
Action potential is transmitted from atria to ventricles through the fibers of specialized conductive system.
2 Rhythmicity of Cardiac Muscle
Definition
It is more appropriately termed automaticity.
It is also called spontaneous excitability.
This property is present in all the tissues of heart.
However, heart has a specialized excitatory structure from which the discharge of impulses starts. This specialized structure is called pacemaker.
Pacemaker
Pacemaker is defined as a part of heart from where the impulses for heartbeat are produced normally.
Pacemaker is formed by pacemaker cells.
In mammalian heart, the pacemaker is the sinoatrial node (SA node).
SA Node
SA node is a small strip of modified cardiac muscle situated in superior part of lateral wall of right atrium, just below the opening of superior vena cava.
Fibers of this node do not have contractile elements.
Fibers of SA node are continuous with fibers of atrial muscle, so that the impulses from SA node spread rapidly through atria.
Other parts of heart like AV node, atria and ventricle also can produce the impulses and function as pacemaker.
Still SA node is called the pacemaker because the rate of spontaneous impulse (rhythmicity) is more in SA node than in other parts.
Spread of Impulses from SA Node
Mammalian heart has got a specialized conductive system by which the impulses from SA node spread to other parts of the heart.
Rate of Generation of Impulses by Different Parts of Human Heart
| Part of the Heart | Rate of Generation of Impulses (per minute) |
|---|---|
| SA node | 70 to 80 |
| AV node | 40 to 60 |
| Atrial muscle | 40 to 60 |
| Purkinje fibers | 35 to 40 |
| Ventricular muscle | 20 to 40 |
Electrical Potential in SA Node
Electrical potential in SA node is different from that of cardiac muscle fibers.
In SA node, each impulse triggers the next impulse.
It is mainly due to negative resting membrane potential.
Resting Membrane Potential in SA Node
Resting membrane potential in SA node cells exists only for a very short duration with negativity of −55 to −60 mV.
It is different from the −85 to −95 mV in other cardiac muscle fibers.
Action Potential in SA Node
Action potential in SA node has three phases:
- Pacemaker potential
- Rapid depolarization
- Rapid repolarization
Pacemaker potential is also called spontaneous depolarization or pacemaker current.
During this phase, the spontaneous depolarization occurs very slowly and reaches the threshold level of −40 mV very slowly.
Pacemaker potential triggers the rapid depolarization.
Depolarization reaches the voltage of +5 mV.
Rapid depolarization is followed by rapid repolarization.
Repolarization reaches a voltage of −55 to −60 mV resulting in hyperpolarization and ends in resting membrane potential.
Pacemaker current (pacemaker potential) is always referred as funny current because of its unusual properties.
Once again, the cycle is repeated with spontaneous depolarization.
3 Conductivity of Cardiac Muscle
Heart has a specialized conductive system through which impulses from SA node are transmitted to all other parts of the heart.
Conductive System in Heart
Conductive system of the heart is formed by modified cardiac muscle fibers.
Conductive tissues of the heart are also called the junctional tissues.
Components of Conductive System in Heart
- AV node
- Bundle of His
- Right and left bundle branches
- Purkinje fibers
SA node is situated in right atrium.
AV node is situated in right posterior portion of intra-atrial septum.
Impulses from SA node are conducted throughout right and left atria.
The impulses also reach AV node via some specialized fibers called internodal fibers.
From AV node, the bundle of His arises.
It divides into right and left bundle branches which run on either side of the interventricular septum.
From each branch of bundle of His, many Purkinje fibers arise and spread all over the ventricular myocardium.
4 Velocity of Impulses at Different Parts of the Conductive System
Velocity of electrical impulse through different parts of conductive system is given below.
| Part of Conductive System | Velocity (m/sec) |
|---|---|
| Atrial muscle | 0.3 |
| Internodal fibers | 1.0 |
| AV node | 0.05 |
| Bundle of His | 0.12 |
| Purkinje fibers | 4.0 |
| Ventricular muscle | 0.5 |
When electrical impulse travels from SA node down through the bundle of His, it is slowed down for a very short period before travelling down through the bundle of His.
This delay is called AV nodal delay.
5 Contractility of Cardiac Muscle
Following are contractile properties of cardiac muscle:
6 All-or-None Law
According to all-or-none law, when a stimulus is applied, whatever may be the strength, whole cardiac muscle gives maximum response or it does not give any response at all.
Below the threshold level, i.e. if strength of stimulus is not adequate, muscle does not give response.
In skeletal muscle, it is applicable only to a single muscle fiber.
7 Staircase Phenomenon
When ventricle is stimulated successively, at a short interval of 2 seconds without changing the strength, force of contraction increases gradually for first few contractions.
Then it remains constant.
Gradual increase in the force of contraction is called staircase phenomenon.
So, there is a gradual increase in force of contraction.
8 Summation of Subliminal Stimuli
When a stimulus with a subliminal strength is applied, the heart does not show any response.
When few stimuli with same subliminal strength are applied in succession, the heart shows response by contraction.
9 Refractory Period
Refractory period is of two types:
1. Absolute Refractory Period
Absolute refractory period is the period during which muscle does not show any response at all, whatever may be the strength of stimulus.
Depolarization occurs during this period.
So, a second depolarization is not possible.
2. Relative Refractory Period
Relative refractory period is the period during which muscle shows response if strength of stimulus is increased to maximum.
It is the stage at which the muscle is in repolarizing state.
Refractory Period in Cardiac Muscle
Cardiac muscle has a long refractory period compared to that of skeletal muscle.
Absolute refractory period extends throughout the contraction period of cardiac muscle.
It is for 0.27 second.
Relative refractory period extends during first half of relaxation period and is about 0.26 second.
Significance of Long Refractory Period in Cardiac Muscle
Long refractory period in cardiac muscle has three advantages:
- Summation of contractions does not occur.
- Fatigue does not occur.
- Tetanus does not occur.