Cardiac muscles are one of the unique and highly contractile. Cardiac muscles never stop contracting from birth till the death. They are the only muscles that act continuously unlike the striated and non-striated muscles. In general, they are known as heart muscles exclusively found in the myocardium. Cardiac muscles are elongated, fibrous, filamentous and highly branched with intercalated discs. They have a very rich blood supply through coronary circulation. Cardiac muscle tissues are involuntary in nature hence we can`t control the cardiac muscles. They look like striated muscle but are stronger than the striated muscles. Cardiac muscles are present in the myocardium-middle muscular layer of the heart. The myocardium is again surrounded by the outer epicardium and there is endocardium just below the myocardium. Heart muscles receive blood supply through coronary arteries branch out from the aortic channel. Electrical stimulation generated from the SA node and AV node which in turn trigger the individual muscles to contract and relax.
Table of Content
2. Gross anatomy of cardiac muscles
3. Characteristics of cardiac muscles
Gross anatomy of cardiac muscles
Anatomy of cardiac muscles
Although cardiac muscles look like striated muscle tissue, they are shorter than skeletal muscle fibres. The term used to describe the individual cardiac cells is myocytes. Most of the cardiac fibres have only one nucleus, found in the central region of the cell. Since cardiac muscles are highly active, they need a lot of energy to contract, so there are many mitochondrion and myoglobin, resulting in the production of ATP by means of aerobic metabolism. Heart muscle fibres are highly branched and the branches are interconnected to one another at their ends by intercalated discs.The intercalated discs help in making the heartbeat in a wavy fashion that ejects the blood out of the chambers. There gap junctions located just beneath the intercalated discs. When a specific cardiac muscle cell triggers a contraction movement, a gap junction 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. They are closely associated with the intercalated discs. plasma membrane and transverse tubules in registration with the Z lines, the longitudinal sarcoplasmic reticulum and terminal cisternae, and the mitochondria are the most vital components in excitation and metabolic recovery processes of the 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 are a bit different than that of the skeletal muscle tissues because cardiac muscles have prominant 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.
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)