Muscle Cells

Smooth Muscle Cells

Smooth muscle cells form the walls of hollow organs like blood vessels or the intestines. Like all muscle cells, these cells have characteristic oval, cigar-shaped nuclei that stain light blue. When smooth muscle in the walls of blood vessels contract, they can decrease the diameter of a blood vessel. This increases blood pressure. In people with a condition of high blood pressure, it can be helpful to give medicines that decrease the contractions of smooth muscle and diminish blood pressure. This eases the strain on the heart. In the intestines, contractions of smooth muscle help push food down the length of the intestines. These contractions are regulated by nerve cells found close to the smooth muscle cells.

Skeletal Muscle Cells

Skeletal muscle cells pull on tendons attached to bones. They allow us to walk, use our arms and hands, and adjust our facial expressions. Skeletal muscle cells are truly enormous, cylinder-shaped cells. They can be 0.1 millimeters in width and as much as 10 millimeters long, so they are the Godzillas of the cell world. In order to control so much cytoplasm, each skeletal muscle cell may contain as many as 1000 cell nuclei! Also, many small stripes, or striations, cross the cytoplasm of these cells. How are these special features created?

Skeletal muscle cells are created by the fusion of smaller cells called myoblasts (in the embryo) or satellite cells (in the adult). Numerous special proteins are required to accomplish this cell fusion and turn on the genes needed for muscle cell development.

The stripes in the cytoplasm are light-staining (I bands) or darker-staining (A bands). High magnification pictures show that these bands are composed of masses of highly organized filaments. We now know that these filaments are mainly composed of filaments of actin and filaments of myosin. When provided with energy by mitochondria, these filaments slide against each other. This sliding is what causes the muscle cell to contract. The actin and myosin proteins are forced into a highly regular, almost crystalline, pattern by other proteins that organize them (these proteins are called α-actinin, desmin, titin, and nebulin).

Smooth muscle cells also use actin and myosin to contract, but in these cells, the actin and myosin are less abundant and not so highly organized, so no stripes are visible in these cells.

Cardiac Muscle Cells

Cardiac muscle cells are found in the heart. They are much smaller than skeletal muscle cells, and are shaped like tiny shoeboxes. These cells also contain lots of actin and myosin and show striations similar to those of skeletal muscle cells. Unlike skeletal muscle cells, these cells never rest or take a break. From the moment they are formed until the death of a person, they contract 60 times a minute to cause the heart to beat.

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Written by John Young

John K. Young is a retired professor of Cell Biology. He worked in the Department of Anatomy at Howard University College of Medicine in Washington, DC for 35 years, teaching his students about cells. During his career, Dr. Young published scientific articles about a part of the brain called the hypothalamus and also wrote a number of books about cells and about the brain.

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