Definition
Smooth muscle is the type of muscle that is found in most of the human hollow organs and, due to its special structure, is able to work very effectively and independently without high energy expenditure.
Features
The smooth musculature got its name from the fact that it is clearly different from the other type of musculature, namely the musculature which is mainly found in the skeletal muscles and which is called striated musculature. This is because under polarizing light, these muscles are striated due to the regular arrangement of the proteins actin and myosin. Since this regular order is missing in smooth muscles, the muscle cells appear homogeneous here even under polarizing light.
Structure of the smooth musculature
Typically, the cells of smooth muscles, also called myocytes, are spindle-shaped and have a diameter of about 5 to 8 μm. However, this of course depends on the state the cell is in: in a contracted muscle the cells are slightly thicker than in a flaccid muscle. The length of the muscle cells varies particularly greatly, not only because of the contracted state, but also depending on the location of the cell.
In blood vessels, for example, the cells are on average only 15 to 20 μm long, in other organs they can reach a length of up to 200 or 300 μm, and in the uterus of a pregnant woman muscle cells can even become up to 600 μm long through special adaptation procedures. Smooth muscle nuclei are usually also somewhat elongated and are typically located in the middle of the cell, just like most other cell organelles (endoplasmic reticulum, mitochondria, ribosomes, etc.). The filaments actin and myosin, as mentioned above, are also present in high concentrations in the cytoplasm of these muscle cells, but are not subject to such a strict structure as in the striated muscle cells.
Here, they move more or less disorderly and more or less criss-cross through the muscle cell, whereby they are attached to the so-called dense bodies within the cytoplasm and to the anchoring plaques at the edge of the cell. This arrangement means that a single cell, and thus the entire muscle, can contract more strongly during contraction than is the case with a striated muscle. Each individual cell is surrounded by a thin skin, the basal lamina. Usually, several cells arrange themselves into small groups, either very densely packed or in the form of smaller bundles.
