Cardiac muscles are one of the unique and highly contractile muscle fibers which never stops contracting from birth till the death. Unlike other muscle types, there is no rest for cardiac muscles . Cardiac muscles are one of the 3 main types of muscles , and the other 2 includes striated muscles, non-striated muscles. In general, they are known as heart muscles exclusively found in the myocardium; the middle layer of coverings of heart. The myocardium is again surrounded by the outer epicardium and inner endocardium placed just below the myocardium. Heart muscles receive blood supply through coronary arteries that are branched out from the aortic channel. Electrical stimulation generated from the SA node and AV node send a series of impulses to the individual cardiac muscles to contract and relax. Cardiac muscles are elongated, fibrous, filamentous and extremely branched with intercalated discs in between the fibres. They stand out from others because of their very rich blood supply through the coronary arteries. Cardiac muscle tissues are involuntary in nature as we cannot stop as and when needed. Although they look-like striated muscle but they differ from them . Cardiac muscles are highly stronger and active than the striated muscles.
Gross anatomy of cardiac muscles
Anatomy of cardiac muscles
Even though cardiac muscles look like striated muscle tissue, they are shorter and thicker than them. The individual cardiac cells are called myocytes, each myocyte is a fibre with only one nucleus, found in the central region of the cell. Since cardiac muscles are extremely active, they demand a lot of energy to contract. Therefore, each fiber has many mitochondria and myoglobin, that help in the production of ATP by means of aerobic metabolism. Heart’s muscle fibres are highly branched and those are interconnected at their terminal ends by intercalated discs. Intercalated discs help in transferring the impulses generated by the nodes in a wavy fashion so that heart as a whole can contract systematically. When a specific cardiac muscle cell is triggered by the contraction movement, a gap junction between the fibers assists to transfer this trigger to the next cardiac cell. This allows the muscle to contract in a coordinated way. Desmosomes help in holding the individual cardiac muscles together as they are closely associated with the intercalated discs. Apart from the junctions, plasma membrane, transverse tubules, Z lines, the longitudinal sarcoplasmic reticulum, terminal cisternae, and the mitochondria also play important role in the muscular activity of heart. The thick myosin and thin components such as actin, troponin, and tropomyosin and the assistive protein filaments are arranged as contractile units. The striations in a cardiac muscle are a bit different from that of the skeletal muscle tissues as they are prominent and elastic fibres that make striations more vivid. The autonomic nervous system regulates the nature of contractions of heart, the force of the heartbeat, rate of heartbeat, stroke volume, and cardiac output. The right and left vagus nerves act as the primary neurological regulators that supply nerve tissues to the heart. Blood supply to the heart is from the aorta that exits from the left ventricle. Aorta is branched into 2 main coronary blood vessels and the 2 vessels again branched to produce left and right coronary arteries that supply blood to the cardiac muscles.
Steps of Cardiac Conduction
Cardiac conduction is the pictorial presentation of the electrical activity of the heart , its origin, direction of flow and the termination. As shown in the picture below, It begins at SA node, gradually progresses to AV node . From AV node, the impulses spreads across both the ventricles to reach each corner of the ventricle through Purkinje Fibers Impulse Conduction system. Therefore , cardiac excitation is divided as shown below.
- Step 1: Pacemaker Impulse Generation.
- Step 2: AV Node Impulse Conduction.
- Step 3: AV Bundle Impulse Conduction.
- Step 4: Purkinje Fibers Impulse Conduction.
Characteristics of cardiac muscles
1. Functional syncytium enables the wave-like contraction of the heart that allows the whole heart working as a unit. Functional syncytium begins with the pacemaker cells.
2. Heart rate is regulated by autorhythmicity that help in maintaining the heartbeat at set intervals. Autorhythmicity is carried out by specialized muscle fibres associated with gap junctions. These fibres work in conjunction with the pacemaker cells of the conduction system of the heart that produce contractions in a coordinated manner.
3. Cardiac muscles demonstrate relatively long action potentials in their fibres as they have a sustained depolarization “plateau.” The plateau is produced by Ca++ entry through voltage-gated calcium channels in the sarcolemma of cardiac muscle fibres. As a result of sustained depolarization (and Ca++ entry), there is a longer contraction potential in skeletal muscle.
4. Human heart works on all or none law (Contraction of a single fibre result in the contraction the whole heart)
Check your understanding
What is myocardium?
Explain the characteristics of cardiac muscles.
What is sclerolemma?
Describe the characteristics of cardiac muscles.
Explain the 4 phases of electrical conduction of heart.
What are actin and myosin?