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cardiac muscles properties - morphology

Cardiac muscle is striated same as a typical skeletal muscle. The muscle fibrils are surrounded by numerous and elongated mitochondria since the heart needs energy supply continously. The muscle fibres branch and interdigitate. They lie parallel to one another but at the end of each muscle fibres there are extensive folds of the cell membrane that are called as intercalated disks/discs.

The intercalated discs act as gap junctions that is very permeable and allow almost free diffusion of ions. They always occur at the Z lines i.e. the dark middle section of the light (I) band of the muscle. They provide a strong union between fibers, maintaining cell-to-cell cohesion, so that the pull of one contractile cell can be transmitted along its axis to the next.

The heart muscle thus acts as a syncytium. i.e. a multinucleated mass. Therefore when one of the cells is excited, the action potential spreads from cell to cell through the latticework interconnections fast and the syncytium as a whole contracts.

The heart is divided into the atrial and ventricular syncytium separated by the fibrous annulus at the A-V juntion. The action potential has to travel through the specialised conducting system that pierces the fibrous tissue. This division of the heart into 2 functional syncytiums allow the atria to contract a short time before the ventricles, which is important for the effectiveness of the heart as a pump.

The T system in cardiac muscle is located at the Z lines rather than at the A–I junction, where it is located in mammalian skeletal muscle. The T system of transverse tubules, which is continuous with the sarcolemma of the muscle fiber, forms a grid perforated by the individual muscle fibrils. It provides a path for the rapid transmission of the action potential from the cell membrane to all the fibrils in the muscle. The sarcoplasmic reticulum is an important store of Ca2+ and also participates in muscle metabolism.

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