Smooth Muscle

Smooth Muscle is the type of muscle tissue that is located in the walls of all the hollow organs of the body (except the heart).

The contraction of smooth muscle reduces the size of these structures it surrounds.

Functions of smooth muscle include:

Smooth muscle can, among other things:

• Regulate the flow of blood in the arteries,
• Move food through the gastrointestinal tract (also known as the ailmentary tract and the digestive tract),
• Expel urine from the urinary bladder
• Expel a foetus from the uterus, and
• Regulate flow of air through the lungs.

The Structure of smooth muscle:

Smooth muscle tissue is made up of thin-elongated muscle cells (muscle cells are also called 'muscle fibres'). Smooth muscle fibres are pointed at their ends and each has a single, large, oval nucleus. Each of these cells is filled with a special type of cytoplasm, called sarcoplasm, and is surrounded by a thin cell membrane called the sarcolemma. Each cell has many myofibrils which lie parallel to one another in the direction of the long axis of the cell.

The cells/fibres of smooth muscle are not arranged in a definite striped (striated) pattern, as in the case of skeletal muscles - hence the name smooth muscle.
Smooth muscle fibres interlace to form sheets or layers of muscle tissue rather than bundles.

Smooth muscle is involuntary tissue, that is - it is not under conscious control by the brain.

Other types of muscle tissue:

There are three main types of muscle tissue.
They are:

• Skeletal Muscle - muscles that move bones, under conscious control.
• Smooth Muscle (see above) - surrounding organs and other body structures, not under conscious control.
• Cardiac Muscle - the specialized muscle found only in the heart.