Smooth Muscles

Smooth muscles are found in the walls of hollow organs and their contractions cause involuntary movements that are triggered by impulses from the autonomic nervous system. These unstriated muscles appear because their myofilaments are less organised.

The cells in smooth muscles are arranged such that the actin and myosin filaments appear as stacked sheets. The actin filaments stretch from one side of the cell to the opposing side and connect dense bodies in the cytoplasm and at the cell membrane, while myosin filaments lie between the actin filaments. This ensures that nervous impulses can spread to adjacent cells faster.

In comparison to skeletal muscles, smooth muscles contact in smaller but stronger waves that require less energy. Smooth muscles are triggered by a nerve impulse that releases calcium which in turn causes the constant binding of actin and myosin filaments. In fact, this complex pathway is only disrupted when calcium levels fall. Because calcium controls the amount of adenosine triphosphate that is available, when the amount of calcium required falls, binding of actin and myosin filaments are disrupted.
As such, even when the nervous impulse is removed, myosin and actin filaments continue to bind until calcium levels fall.

In order to maintain normal function, many hollow organs in the body are lined with smooth muscles as they can continuously contract for extended periods.

1. Closing our eyes - smooth muscles line sphincters that seal orifices.

2. Digesting dinner - moving masticated food from the esophagus down to the stomach and later chyme through the digestive system. Most of the organs of the gastrointestinal system are lined by smooth cells.

3. Child birth - the walls of the uterus are lined with smooth muscles to aid delivery of the baby.