Smooth muscle contraction

In the last article we have considered a variety of functions the smooth muscles serve in our bodies. They regulate diameter of blood vessels and thereby the blood pressures. Rings of smooth muscle (sphincters) control flow of material like urine or food. The detrusor muscle of the urinary bladder helps in voiding of urine. Peristaltic contractions (due to smooth muscle) push the food, waste material, urine, semen, ova etc. in one direction in a tubular organ. Smooth muscles attached to hair, giving us goose bumps. Uterine smooth muscles push a baby outside the womb into the world. Smooth muscles of the iris regulate diameter of the pupil and affect our vision. Let us now learn about the mechanism of their contraction.

Smooth muscle cells are smaller than cells of striated muscles. They are thick in the middle and tapering to the ends, whereas cells of striated muscles are cylindrical throughout. There is one nucleus per smooth muscle cell and it is in the center of the cell. Smooth muscle tissue has the contractile proteins - actin and myosin but they are not arranged in a regular repetitive pattern as a result, this tissue has no striations. The dense bodies scattered in cytoplasm and intermediate filaments anchor the thin actin filaments.

In smooth-muscle cells, intermediate filaments are interlaced through the cell much like the threads in a pair of "fish-net" stockings. The intermediate filaments anchor the thin filaments and correspond to the Z-disks of skeletal muscle. Unlike skeletal-muscle cells, smooth-muscle cells have no troponin, tropomyosin or organized sarcoplasmic reticulum. At molecular level the smooth muscle cells show special features like major structural protein component of intermediate filaments -desmin, and in vascular smooth muscle cells vimentin.

Though the smooth muscle cells lie parallel to each other like fibers of striated muscles, they are staggered. Thickest part of a cell is in contact with narrow tips of other cells. Smooth muscles have no tendons and have sparse blood supply.

Smooth muscle cells may spontaneously contract and remain contracted for a long time. Striated muscles contract up to 30% of their length whereas, smooth muscle cells contract up to 80%.

While smooth muscle is unique in several aspects, the basic principles of muscle contraction are similar in smooth and skeletal muscle.

The sliding filament mechanism is the most accepted theory of muscle contraction. According to this theory, muscle contraction results from the forming of cross-bridges and thin (actin) filaments sliding past thick filaments of myosin. These proteins do not change their length. The muscle shortens as myosin heads interact cyclically with active sites on actin. Adenosine Tri Phosphate (ATP) provides energy for this process.

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