Question

how does cardiac muscle regulate contractile force?

Answer 1

The heart is essentially a muscle that contracts and pumps blood. It consists of specialized muscle cells called cardiac myocetes,usually contain one elongated nucleus that lies in the centre. Cell organelles are concentrated in the cytoplasmic region around the nucleus. These include mitochondria,golgi appartus,lipofuscin filled granules and glycogen.The cytoplasm of cardiomycetes,called sacroplasm is eosinophillic and appears as a 3D network.

Due to the contractile force or high energy requirements,cardiac muscle tissue contains additional large and elongated mitochondria located between the myofibrils. They can run the full length of the sacromere and contain many internal cristae. In addition extra glycogen granules are also located between the myfibrils to store the energy. Threads of collagenous tissue fibers together with capilleries are also present between the muscle fibers to provide the tissue with support and a blood supply.

Contractile myocetes have a different set of ion channels. Their sacroplasmic reticulum, the SR stores a large amount of calcium. They also contain myofabrils. The contractile cells have a stable resting potential of -90mV and depolarize only when stimulated. When a cell is DE-polarized, positive ions leak through the gap junctions to the adjacent cell and bring the membrane voltage of this cell up to the thresthold of -70mV. Fast sodium channels open creating a rapid sodium influx and a sharp rise in voltage. This is the depolarizing phase. L-type, or slow,calcium channels also open at -40mV,causing a slow but steady influx. Sodium channels close quickly,voltage-gated potassium channels open and these result in a small decrease in membrane potential,known as early repolarized phase. The calcium channels remain open and the potassium efflux is eventually balanced by the calcium influx. This keeps the membrane potential relatively stable fro about 200msec resulting in the PLATEAU phase,is crucial in coupling electrical excitation to physical muscle contraction. The influx of calcium from the extracellular fluid triggers as much greater calcium release from the SR, in a process known as calium induced calcium release. Calcium sets off muscle contraction by sliding filament mechanism. Calcium channels close potassium efflux predominates and membrane voltage returns to its resting value.