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Content: Inside the cell they lie close to the cell's internal calcium store, the sarcoplasmic reticulum. Here, a single tubule pairs with part of the sarcoplasmic reticulum called a terminal cisterna in a combination known as a diad. [ 6]
The functions of T-tubules include rapidly transmitting electrical impulses known as action potentialsfrom the cell surface to the cell's core, and helping to regulate the concentration of calcium within the cell in a process known as excitation-contraction coupling. [ 5]
Intercalated discs[edit]
Main article: Intercalated disc
Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes. The cardiac syncytiumis a network of cardiomyocytes connected by intercalated discsthat enable the rapid transmission of electrical impulses through the network, enabling the syncytium to act in a coordinated contraction of the myocardium. There is an atrial syncytiumand a ventricular syncytiumthat are connected by cardiac connection fibres.[7] Electrical resistance through intercalated discs is very low, thus allowing free diffusion of ions. The ease of ion movement along cardiac muscle fibers axes is such that action potentials are able to travel from one cardiac muscle cell to the next, facing only slight resistance. Each syncytium obeys the all or none law. [ 8]
Intercalated discs are complex adhering structures that connect the single cardiomyocytes to an electrochemical syncytium(in contrast to the skeletal muscle, which becomes a multicellular syncytium during mammalian embryonic development).