Smooth muscle tissue
The smooth muscle is the contractile tissue of many hollow organs, blood and lymph vessels and other structures in animals and thus also in humans. The term " smooth " refers to the lack of striation in the microscopic image. As opposed to striated muscle is not subjected to the arbitrary control.
An exception to this system forms the heart muscle, which is not composed of smooth muscle, although it is an inner hollow organ in the heart.
- 4.1 flow
- 4.2 relaxation
As well as the skeletal muscle cells are smooth muscle cells of mesodermal origin.
The cells are 30 to 200 microns ( in the womb of a pregnant woman up to 500 microns ) long, two to ten microns wide and by a basal lamina ( protein layer ) surrounded. The elongated nucleus is located in the center of the cell. In the nuclear ends cell organelles such as mitochondria, ribosomes and the rough endoplasmic reticulum and glycogen retention focus.
In contrast to striated muscle, which owes its name to the regular arrangement of myofibrils, such an arrangement of filaments in the smooth muscle cell does not exist. The light microscope, the cytoplasm appears homogeneous. Main filaments are actin, myosin, and intermediate filaments in addition from the group of desmins ( desmin, vimentin ). A single myosin filament is doing 13-14 actin filaments grouped into a kind of " bundle ". The actin and intermediate filaments attach themselves to so-called " dense bodies " (also called compression zone ), which are comparable to the Z- discs of striated muscle, and Anheftungsplaques on the edge of the cell. This arrangement of the contractile elements a greater capacity for contraction of the muscle cell, as this is possible due to the striated muscle. A network of reticular fibrils connecting the individual cells to a contractile tissue.
A characteristic of this muscle type represent the so-called caveolae, invaginations of the cell membrane ( sarcolemma ), which are associated with the extracellular matrix in contact and are probably involved in the electromechanical coupling.
Due to structural differences and the resulting functional differences, a subdivision of the smooth muscle in the single -unit and multi -unit type is possible. Mixed forms are often esp. in the muscle.
Single -unit type
The single -unit- type cells by gap junctions ( nexus ) electrically coupled cell clusters. About the gap junctions, an exchange of ions and second messenger molecules takes place, allowing rapid spread of excitation and thus a nearly synchronous contraction of the cell assembly ( "functional syncytium "). The muscle cells are not excited by neural structures, but by the spontaneous depolarization morphologically definable pacemaker cells ( myogenener tone ). However, a modulation by the fibers of the autonomic nervous system is also possible.
The single -unit type preference occurs in the muscles of the intestines, the uterus (womb ), and the ureters ( ureter ), but partly also in the muscular wall of larger vessels.
From the neighboring cells dependent contractions in multi -unit type not found or only very limited place. Each muscle cell ( "en passant synapse " ) separately by nerve fibers of the vegetative nervous system which release from varicosities transmitter in the immediate environment of the cell, innervated ( neurogenic tone ).
This cell type is including in the arrector pili muscle of hair, the internal eye muscles, the vas deferens ( vas deferens ), in the bronchial vessels and in front.
Contraction of smooth muscle mechanism corresponds substantially to the muscle contraction of the striated muscle. However, the Gleitfilament mechanism is much slower, but also consumes less oxygen and less energy in the form of ATP.
Smooth muscle contractions may be caused by three mechanisms leading to the increase in intracellular calcium concentration.
- Excitation-contraction coupling: by action potentials or prolonged depolarization of the cell membrane to open voltage-dependent calcium channels.
- Pharmakomechanische coupling: receptor -controlled calcium channels respond to neurotransmitters such as norepinephrine, acetylcholine, angiotensin, histamine or drugs and open. In addition, receptor-mediated additional calcium ions can be released from the sarcoplasmic reticulum.
- Mechanical strain causes the opening stretch -dependent calcium channels.
By triggering events result in an increase in Ca2 concentration in the cytosol due to inflowing ions from the extracellular space and at a significantly lower proportion of the sarcoplasmic reticulum. Up to four Ca2 ions bind to the protein calmodulin. The resulting Ca2 - calmodulin complex results in activation of the enzyme myosin light chain kinase ( MLCK ). This enzyme cleaves the activated form of ATP and phosphorylates the light chain of Myosinmoleküls. Phosphorylation of the cross- bridge cycle can proceed.
The latency time, comprising the time interval between the activating signal and contractile response of the muscle, is in this type of muscle around 300 ms and is largely concluded by the diffusion of calcium to the cell and the mechanism of activation described above.
Overall, in the smooth muscle shortening reaches a higher than in the striated, this is due to the different arrangement of the myosin heads about.
For relaxation (relaxation) occurs through decreasing Ca2 levels, triggered by the absence of nerve impulses and other excitatory processes. Calcium is by Na / Ca2 antiporter and Ca2 - ATPases ( SERCA ) transported from the interior of the cell back into the extracellular space and the sarcoplasmic reticulum. The Ca2 - calmodulin complex is dissociated and the enzyme myosin light chain phosphatase ( MLCP ) dephosphorylated the light chains of the Myosinmoleküls.
About the activation of the endothelial nitric oxide synthase of the neurotransmitter nitric oxide ( NO) is released a Gasotransmitter in the corresponding vessel section. Nitric oxide ( NO), which is constitutively formed from the vascular endothelium, can diffuse into the adjacent smooth muscle cells and activate soluble guanylate cyclase there. The consecutive increase in cGMP leads to activation of protein kinase G, which enables the Myosinphosphatase through phosphorylation and thus leads to the relaxation of the smooth muscle cells.
Smooth muscle can maintain a long-lasting tone ( tonic contraction duration ) due to their structure and the operations described in the contraction. Both the peristalsis in the stomach, intestines and urinary tract as well as the regulation of blood pressure in the inner walls of the arteries based on the effect of smooth muscle. During childbirth, it allows the rhythmic contraction ( phasic - rhythmic contraction ) of the uterus ( contractions ).
Smooth muscle cells for the synthesis of collagen and other components of the extracellular matrix, such as proteoglycans, elastin, and laminin, capable.