Structure of smooth muscle

MUSCLE

Motion, as a reaction of multicellular organisms to changes in the internal and external environment, is mediated by muscle cells.

The basis for motion mediated by muscle cells is the conversion of chemical energy (ATP) into mechanical energy by the contractile apparatus of muscle cells. The proteins actin and myosin are part of the contractile apparatus. The interaction of these two proteins mediates the contraction of muscle cells. Actin and myosin filaments, each composed of many action and myosin molecules, form myofibrils arranged parallelto the direction of cellular contraction.

A further specialisation of muscle cells is an excitable cell membrane which propagates the stimuli which initiate cellular contraction.

Three structurally and functionally distinct types of muscle are found in vertebrates:

1. smooth muscle,
2. skeletal muscle and
3. cardiac muscle. 

Smooth Muscle

• Smooth muscle consists of spindle shaped cells of variable size.
  The largest smooth muscle cells occur in the uterus during pregnancy (12x600 µm). The smallest are found around small arterioles (1x10 µm).
• Smooth muscle cells contain one centrally placed nucleus.
  The chromatin is finely granular and the nucleus contains 2-5 nucleoli.
• The innervation of smooth muscle is provided by the autonomic nervous system.
• Smooth muscle makes up the visceral or involuntary muscle.

Structure of smooth muscle

In the cytoplasm, we find longitudinally oriented bundles of the myofilaments actin and myosin. Actin filaments insert into attachment plaques located on the cytoplasmic surface of the plasma membrane. From here, they extend into the cytoplasm and interact with myosin filaments. The myosin filaments interact with a second set of actin filaments which insert into intracytoplasmatic dense bodies. From these dense bodies further actin filaments extend to interact with yet another set of myosin filaments. This sequence is repeated until the last actin filaments of the bundle again insert into attachment plaques.

In principle, this organisation of bundles of myofilaments, or myofibrils, into repeating units corresponds to that in other muscle types. The repeating units of different myofibrils are however not aligned with each other, and myofibrils do not run exactly longitudinally or parallel to each other through the smooth muscle cells. Striations, which reflect the alignment of myofibrils in other muscle types, are therefore not visible in smooth muscle.

Smooth endoplasmatic reticulum is found close to the cytoplasmatic surface of the plasma membrane. Most of the other organelles tend to accumulate in the cytoplasmic regions around the poles of the nucleus. The plasma membrane, cytoplasm and endoplasmatic reticulum of muscle cells are often referred to as sarcolemma, sarcoplasm, and sarcoplasmatic reticulum.

During contraction, the tensile force generated by individual smooth muscle cells is conveyed to the surrounding connective tissue by the sheath of reticular fibres. These fibres are part of a basal lamina which surrounds muscle cells of all muscle types. Smooth muscle cells can remain in a state of contraction for long periods. Contraction is usually slow and may take minutes to develop.