Smooth muscle is the muscle tissue of non striated muscle.It is distributed in human artery and vein wall, bladder, uterus, male and female reproductive tract, digestive tract, respiratory tract, ciliary muscle and iris of eyes.
Andskeletal muscleandmyocardiumThey are different in structure, function, coupling mechanism and contraction state.Controlled by autonomic nerve, it is involuntary muscle.It is also indirectly controlled by the endocrine system.The contraction of smooth muscle is caused by nerve or hormone stimulation.
Microstructure and shrinkage mechanism
Although smooth muscle also has a muscle filament structure similar to that of skeletal muscle, they do not have a parallel and orderly arrangement like that of skeletal muscle (the muscle filaments of smooth muscle have their own "orderly" arrangement). Its characteristic is that there is a cytoskeleton inside the cell, including some oval structures called compact bodies, which also appear at intervals on the inside of the cell membrane,It is called the compact area, and the latter is opposite to the similar structure of adjacent cells, and the two layers of cell membranes are also tightly connected here, so they together form a mechanical coupling to complete the transmission of tension between cells;There are also other types of connections between cells, such as gap junctions, which can realize electric and chemical coupling between cells.The protein composition similar to that in skeletal muscle Z band was found in the compact body and compact area, so it was thought that these two structures might be the connection part with the thin muscle filament.In addition, there is a filamentous substance between the dense body and the dense area with a diameter between the thick and thin muscle filaments, which is a polymer called desmin.In this way, the dense body connected by filaments and the dense area inside the membrane form a complete intracellular framework.
The fine filaments in smooth muscle cells have a molecular structure similar to that of skeletal muscle, but do not contain troponin;The amount of myofibrin in the same volume of smooth muscle is twice that of skeletal muscle. It is speculated that there are a large number of thin muscle filaments in the smooth muscle plasma, and their arrangement is roughly parallel to the cell long axis.In contrast, the amount of myosin in cytoplasm is only 1/4 of that in skeletal muscle.It is estimated that 3-5 thin filaments connected to the dense body will be surrounded by a small number of thick filaments, forming a staggered arrangement, which may be a functional unit similar to the sarcomere in skeletal muscle.
Generally, smooth muscle cells are spindle shaped, with a diameter of 2-5 μ m;Its length is highly variable, and about 400 μ m is the optimal length to generate tension.They do not have a well developed myotube system like skeletal muscle (and heart muscle).The muscle cell membrane has only some vertically arranged pocket like recesses, but its function is still unclear, but this makes the ratio of cell membrane surface area to cell volume larger, so it is not the transverse tube or sarcoplasmic reticulum system that is close to the muscle filament, but the muscle membrane.When cells are activated, extracellular Ca2+enters the membrane, but the sarcoplasmic reticulum near the membrane in smooth muscle cells also constitutes an intracellular Ca2+reservoir.When some excitatory neurotransmitters, hormones or drugs bind to muscle membrane receptors, they produce a second messenger in the cytoplasm through G-protein, causing the release of Ca2+from the Ca2+pool.Because there is no troponin in the thin filaments of smooth muscle, the mechanism of Ca2+causing the cross bridge circulation of thick and thin filaments sliding each other in smooth muscle cells is different from that of skeletal muscle.Academics believe that the activation of the transverse bridge starts from its phosphorylation, which in turn depends on the activation of a so-called myosin kinase;The process is that Ca2+first binds to a special protein called calmodulin in the cytoplasm, and the latter binds to four Ca2+before activating myosin kinase and decomposing ATP. The resulting phosphate group binds to the transverse bridge and keeps the transverse bridge in a high free state.Compared with skeletal muscle, the mechanism of smooth muscle transverse bridge activation takes a longer time, which is consistent with the slow contraction of smooth muscle.
Functional classification
Although the functional characteristics of smooth muscle contained in various organs in the body are very different, it can generally be divided into two categories: one is called multi unit smooth muscle, in which the smooth muscle cells contained are independent when they are active, similar to skeletal muscle cells, such as pilaris erectus, iris muscle, blink muscle (cat), and vascular smooth muscle,The activities of their cells are innervated by foreign nerves or affected by hormones that diffuse to their cells;The other is called single unit smooth muscle, which is similar to myocardial tissue, in which cells can carry out synchronous activities through electrical coupling between cells. Most of these smooth muscles are autonomic, and can also perform nearly normal contractive activities without external innervation (due to the autonomy of pacemaker cells and the role of the internal nerve plexus)The ureteral smooth muscle is representative.There are also some smooth muscles with two characteristics, which are difficult to be classified into any category. For example, smooth muscle of arterioles and venules is generally considered to belong to multi unit smooth muscle, but it has self-discipline;The bladder smooth muscle has no autonomy, but it can react as a whole when encountering stretch, so it is also included in the unit smooth muscle.
Characteristics of smooth muscle of digestive tube
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The smooth muscle of digestive tube has the common characteristics of muscle tissue, such as excitability, conductivity, contractility and extensibility, but these characteristics of smooth muscle of digestive tube have their own characteristics.
Low excitability and slow contraction: the electrical excitability of smooth muscle of digestive tube is lower than that of skeletal muscle, and the time to complete a contraction and relaxation is much longer than that of skeletal muscle, with large variation.
Autonomy: When the isolated digestive tube is placed in an appropriate environment, its smooth muscle can show rhythmic contraction, but its rhythm is not as good as
The heart muscle is so regular and contracts slowly.
Tension: the smooth muscle of the digestive tube still keeps in a slight continuous contraction state when resting, which is called tension.This tension keeps a certain basic pressure in the lumen of the digestive tract, and keeps various parts of the digestive tract in a certain shape and position.Various contractions of the smooth muscle of the digestive tract occur on the basis of tension.
Extensibility: Under the action of external force, the smooth muscle of digestive tube can stretch greatly to meet the actual needs.For example, the stomach can hold food several times its original size.
Sensitive to some physical and chemical stimuli;The smooth muscle of digestive tube is particularly sensitive to the stimulation of some biological tissue products.For example, a small amount of acetylcholine can make it contract, while adrenaline can make it relax.It is also highly sensitive to chemical, temperature and stretch stimuli.This is related to its environment. The food and digestive juice in the digestive tube are mechanical and chemical natural irritants that often act on the smooth muscle.
