Best note || 6 main properties of cardiac muscle:

Introduction:

The cardiac muscle fibres are striated and resemble quite a lot to skeletal muscle fibres in structure but the cardiac muscle fibres are involuntary (while the skeletal muscle fibres are voluntary) in nature.

The cardiac muscle fibres are ribbon like rather than cylindrical.

Cardiac Muscle, Image by-BruceBlaus,Source, License-CC BY-SA4.0

Length of each cardiac muscle fibre is about 80µm and it is about 15µm broad.

6 main Properties of cardiac muscle:

The basic properties of cardiac muscle include :

1.Excitability:

Excitability is defined as the ability of a living tissue to give respond to a stimulus. Like other muscle ,the cardiac muscle is also excitable by adequate stimulus.

In all the tissues initial response to a stimulus is electrical activity in the form of action potential. It is followed by mechanical activity in the form of contraction, excitation etc.

2. Contractility:

Contractility is the ability of the the cardiac muscle to actively generate force to shorten and thicken to do work when sufficient stimulus is applied.

Myofibril is the fundamental contractile unit of the cardiac muscle. Actin and myosin are the protein units present in the myofibril. When cardiac muscle contracts these two protein units make association with each other and thus the cardiac muscle fibre is shortened.

3. Conductivity:

Conductivity is the property of the cardiac muscle fibre by which impulses from the SA node (Sino Atrial ) to all other parts of the heart.

The conducting system of the heart consists of specialized fibres of the heart muscle present as the sinoatrial node , interatrial tract , internodal tracts,the atrioventricular node,the AV bundle of His and it's right and left terminal branches ,and the subendocardial plexuses of Purkinje fibres.

4.Rhythmicity:

Rhythmicity is one of the main characteristics features of the cardiac muscle fibres . This property describes the ability of the cardiac muscle fibres to initiate it's own impulse at constant rhythmical intervals.

It is also called autorhythmicity or self-excitation. This rhythmical property is present in all the tissues of the heart.

The normal rate of rhythmicity in the SA node is 70-80/min ,in the AV node is 40-60/min ,in atrium is 60/min ,in ventricle is 20-40/min.

The SA node has the the higher rhythmical property, that's why it controls the rest of the cardiac muscle and thus heart beats at rhythm of the SA node.

5. All or none response:

This property describes the response of the cardiac muscle to a stimulus is all or none in character,i.e. when a stimulus is applied either the heart does not contract at all ( none response) or contracts to its maximum ability(all response).

The cardiac muscle fibres shows this type property because of the syncytial arrangement of the cardiac muscle fibres.

6. Refractory period:

This is another characteristic property of the heart muscle . The refractory period of the heart is long ( 250-300ms in ventricles and about 150ms in atria) and can be divided into three parts- absolute refractory period, relative refractory period and supernormal phase or period.

Absolute refractory period:

Absolute refractory period extends throughout the whole period of contraction. In this period any stimulus,however strong ,will fail to elicit a response if it falls within this period.

Relative refractory period:

Relative refractory period starts immediately after the absolute refractory period and involves the first part of relaxation. Only a very strong stimulus will be effective.

Supernormal phase or period:

This starts after the relative refractory period . This period is limited from the point of termination of repolarization to the beginning of slow diastolic repolarization phase.

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