Corpus: Smooth muscle

Smooth muscle is a contractile type of tissue. It is a non-voluntarily controllable type of musculature that influences the function, tension and shape of internal organs, among other things, through its action.

Smooth muscle is characterised by the elongated, thin muscle cell (myocyte), which has no transverse striation. Smooth muscle is primarily found in the walls of all hollow organs (e.g. intestines, respiratory tract, blood vessels, urinary tract and genital organs).

Compared to striated skeletal muscle, smooth muscle shortens much more slowly, but to a greater extent (up to a third of its original length). It is generally not controllable by the will and can remain in a contracted state for a long time without great expenditure of energy, fatigue or action currents (true muscle tone).

Morphology[Bearbeiten]

Smooth muscle cells (SMCs) have a spindle-shaped basic form, which is due to the fact that their cell body tapers at both poles. Their length varies depending on where they are used and the degree of contraction. In blood vessels, the smooth muscle cells have a length of 15-20 µm. In the uterus of pregnant women, the length of a muscle cell can have increased to up to 800 µm by the end of pregnancy due to functional adaptation. Their average diameter is 5-8 µm, although the degree of contraction is also decisive here. Cells in a contracted state are thicker than cells in a relaxed state.

Like most other cell organelles, the elongated, rod-shaped cell nucleus is located in the centre of the cytoplasm. It has a round cross-section and relatively little chromatin. The nucleus changes with the functional state of the muscles and takes on a slightly folded appearance during contraction. However, the relationship to the volume of the cytoplasm remains largely unchanged.

Each cell is surrounded by a basal lamina, which, however, is absent in the area of the cell contact points. Furthermore, a stocking of reticular fibre lattices covers the cell.

Smooth muscle cells usually occur in groups - densely packed or in the form of small bundles. Due to their spindle shape, the cell clusters show a characteristic image in histological sections with round to ellipsoidal, strongly varying cell cross-sections. Small, medium-sized and large cell sections can be seen side by side.

...according to innervation pattern[Bearbeiten]

Smooth muscle tissue can be differentiated into two types based on its innervation or functional pattern:

• Single-unit type
• Multi-unit type

Both types can also occur in mixed forms, e.g. in the vascular musculature.

Single-Unit-Typ[Bearbeiten]

The single-unit type occurs primarily in the intestinal wall, the uterus and the ureters. In this type, the smooth muscle cells are electrically coupled by so-called gap junctions. These cell connections enable the rapid exchange of ions and second messenger molecules, which results in a rapid spread of excitation. As a result, the cell assembly contracts synchronously, so to speak. The excitation is triggered by pacemaker cells whose activity is modified by fibres of the autonomic nervous system.

Multi-Unit-Typ[Bearbeiten]

The multi-unit type is found in the hair muscles (arrector pili muscle), in the inner vegetative eye muscles, in the ductus deferens and in the respiratory tract and blood vessels. In this type, contraction is only dependent on neighbouring cells to a limited extent or not at all. Instead, the cells are innervated by nerve fibres of the autonomic nervous system. They release transmitter substances to the smooth muscle cell via special nerve endings, the so-called "synapse par distance".

...by contraction type[Bearbeiten]

Based on their contraction behaviour, smooth muscles can be divided into two types:

• tonic -> permanently contracted
• phasic -> rhythmic alternation of contraction and relaxation

Both types of contraction occur in the gastrointestinal tract. Phasic contraction is the basis of peristalsis. Tonic contracting sphincters are the basis for the directed transport of chyme from oral to aboral. Tonic contracting muscles are also found primarily in the walls of blood vessels and the bronchial tree.

At the highest magnification, a very fine, delicate striation can still be seen in the longitudinal section under the light microscope. These myofibrils are embedded as filaments in the sarcoplasm. In contrast to the striated fibrils of the heart and skeletal muscles, they are anisotropic (luminescent) under the polarisation microscope.

The contraction of smooth muscle cells, as with striated muscle cells, is due to the sliding of myofilaments (sliding filament theory). Myofilaments, consisting of actin and myosin, form one component of the myofibrils. The second component is the cytoskeleton, which is made up of intermediate filaments and non-muscular actin and runs through the cell like a net. It serves as an attachment for the actin and myosin filaments during contraction. Smooth muscle cells come together in bundles in the connective tissue. Tensile forces can thus be transmitted without any problems.

Smooth muscle cells are controlled by the autonomic nervous system. Noradrenaline is the neurotransmitter of the sympathetic nervous system, acetylcholine that of the parasympathetic nervous system.

Smooth muscle and its dysfunctions are encountered more frequently in medical practice than skeletal or cardiac muscle. It regenerates very poorly, usually by forming a connective tissue scar. Defective smooth muscle is the cause of many clinical pictures (e.g. labour weakness of the uterus or bronchial asthma).

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