Subforms
The smooth musculature can also be divided into two subgroups, which differ in their excitation patterns (innervation), structure and consequently also in their function: the single-unit types and the multi-unit types, whereby mixed forms also exist (especially in the musculature of vessels). The single-unit type is characterized by the fact that the individual muscle cells are connected via so-called gap junctions, through which an exchange of ions and second messenger molecules is possible. This creates a functional unit and the cells are electrically coupled.
As a result, an electrical excitation is passed from one cell to the next so quickly that the entire cell group is excited practically synchronously and thus also contracts simultaneously. In this type, excitation is provided by pacemaker centers containing cells that can spontaneously discharge (depolarize). Smooth muscles of the single-unit type are found in the gastrointestinal tract, the ureter and the uterus, among others.
In the multi-unit type, on the other hand, each cell is excited individually and its condition is hardly or not at all dependent on its neighboring cells. In this case, excitation takes place through nerve fibers of the autonomic nervous system. The corresponding nerve endings are located near the muscle cells and release messenger substances (transmitters) here. This structure is also called “en-passant synapse”. This form of muscle is found, for example, in the inner eye muscles, hair muscles and the spermatic duct.
Function of the smooth musculature
In contrast to the striated muscles, the smooth muscles are not subject to our arbitrary control.As a result, the many vital processes in our body (to name just a few examples: the movements of the intestines during digestion, the pumping of the heart or even the erection of the fine hairs on the skin) run for the most part by themselves, without us even becoming aware of them or having to (or being able to) control them. Only the autonomic nervous system has an influence on the muscles of the hollow organs via the sympathetic nervous system (with the help of adrenaline) and the parasympathetic nervous system (with the help of acetylcholine), so that we can indirectly influence their activity. The special thing about the way the smooth muscles contract is that they can contract much more than the skeletal muscles, although it takes longer for this to happen.
On the other hand, the achieved state can then be maintained over a long period of time without showing signs of fatigue or having to expend a lot of energy. It is also known as real muscle tone or tonic permanent contraction. Although the heart is also a hollow organ, it is an exception in the classification of the musculature. Since it has characteristics of both smooth and striated muscles, the heart muscle is usually listed separately.