Smooth muscle
From Free net encyclopedia
Smooth muscle is a type of non-striated muscle, found within the "walls" of hollow organs; such as blood vessels, the bladder, the uterus, and the gastrointestinal tract. Smooth muscle is fundamentally different to skeletal muscle and cardiac muscle in terms of structure and function.
Contents |
Structure
Smooth muscle can, like any muscle, contract and relax. In order to do this it contains intracellular contractile proteins called actin and myosin. While the fibres are essentially the same in smooth muscle as they are in skeletal and cardiac muscle, the way the are arranged is different. As non-striated muscle, the actin and myosin is not arranged into distinct sarcomeres that form ordely bands throughout the muscle cell. The cells themselves are generally arranged in sheets or bundles and connected by gap junctions. In relaxed state, each cell is spindle-shaped, 25-50 um long and 5 um wide.
Function
The contractile function of this muscle, to a large extent, determines function of the organ. For example, contractile function of vascular smooth muscle contributes to setting the level of blood pressure. Smooth muscle tissue serves to guide medium transport, such as blood, urine, sperm, bile by means of controlled contractions inducing peristaltic movements.
Contraction
Smooth muscle contraction is caused by the sliding of myosin and actin fibres over each other. The energy for this to happen is provided by hydrolysis of ATP. Movement of the fibres over each other happens when heads on the myosin fibres form crossbridges with the actin fibre. These heads tilt and drag the actin fibre a small distance. The heads then release the actin fibre and adopt their original conformation. They can then re-bind to another part of the actin molecule and drag it along further. This process is called crossbridge cycling and is the same for all muscle.
Crossbridge cycling, though, cannot occur until the myosin heads have been activated to allow crossbridges to form. The myosin heads are made up of a "light" protein chain. When this is phosphorylated it becomes active and will allow contraction to occur. The enzyme that phosphorylates the light chain is called myosin light chain kinase (MLCK). In order to control contraction, MLCK will only work when the muscle is stimulated to contract. Stimulation will increase the intracellular concentration of calcium ions. These bind to a molecule called calmodulin and form a calcium-calmodulin complex. It is this complex that will bind to MLCK to activate it, allowing the chain of reactions for contraction to occur.
MLCK can be inhibited to prevent crossbridge cycling: this causes relaxation of the muscle. It is by causing inhibition of MLCK that vasodilators such as endothelium-derived relaxing factor, or nitric oxide(NO), work to dilate blood vessels. Production of cAMP or cGMP is stimulated. This binds to MLCK and inhibits it, preventing it from activating the myosin light chains.
Control
Smooth muscle cells can be stimulated to contract or relax in many different ways. They may be directly stimulated by the autonomic nervous system ("involuntarily" control), but can also react on stimuli from neighbouring cells and on hormones (vasodilators or vasoconstrictor) within the medium that it carries.
Growth and Rearrangement
The mechanism in which external factors stimulate growth and rearrangement is not yet fully understood. The cells are able to produce their own extracellular matrix. When cultured outside the body, the cells tend to differentiate into a synthetic phenotype, which is not able to contract.
Related Diseases
"Smooth muscle condition" is a condition in which the body of a developing embryo does not create enough smooth muscle for the gastrointestinal system. This condition is fatal.
Anti-smooth muscle antibodies (ASMA) can be a symptom of an auto-immune disorder, such as hepatitis, cirrhosis, or lupus.
External links
See also: skeletal muscle, cardiac muscle, vascular smooth muscle
| Muscular system - edit |
|---|
| Muscular tissue | Muscle contraction | Muscles of the human body |
| Muscular types |
| Cardiac muscle | Skeletal muscle | Smooth muscle |
es:Músculo liso eo:Glatmuskolo fr:Muscle lisse nl:Glad spierweefsel ja:平滑筋 sk:Hladká svalovina vi:Cơ trơn