What Is a Smooth Muscle Contraction?

Smooth muscle is a non-striated muscle tissue. It is distributed in the walls of human arteries and veins, bladder, uterus, male and female reproductive tract, digestive tract, respiratory tract, ciliary muscle and iris of the eye.

Smooth muscle is a non-striated muscle tissue. It is distributed in the walls of human arteries and veins, bladder, uterus, male and female reproductive tract, digestive tract, respiratory tract, ciliary muscle and iris of the eye.

Chinese name

smooth muscle

Foreign name

smooth muscle

Department

Anatomy

Smooth Muscle Overview

It differs from skeletal muscle and myocardium in structure, function, coupling mechanism, and contraction state. Dominated by autonomic nerves, are involuntary muscles. It is also indirectly controlled by the endocrine system. The expansion and contraction of smooth muscle comes from nerve or hormone stimulation.

Smooth muscle microstructure and contraction mechanism

Although smooth muscles also have a structure similar to skeletal muscle, but because they do not have a parallel and orderly arrangement like skeletal muscles (the smooth filaments have their own "ordered" arrangement), it is characterized by cells There is a cytoskeleton inside, which contains some oval structures called dense bodies. They also appear on the inner side of the cell membrane, called the dense area, and the latter is opposite to the similar structure of adjacent cells. It is also tightly connected here, so they form a mechanical coupling to complete the transmission of tension between cells. There are also other forms of connection between cells, such as gap connections, which can achieve electrical coupling between cells and Chemical coupling. Protein components similar to those in the skeletal muscle Z-band were found in the dense bodies and dense areas, so it is thought that these two structures may be the sites connected with the thin muscle filaments. In addition, there is a filament between the dense body and the dense area between thick and thin muscle filaments. They are a kind of aggregate called desmin. In this way, the dense body connected by the filament and the dense area inside the membrane form a complete intracellular framework.

The thin muscle 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 smooth muscle of the same volume is twice that of skeletal muscle. It is speculated that there are a large number of fine muscles in smooth muscle sarcoplasm Filaments exist, and their arrangement is approximately parallel to the long axis of the cell. In contrast, the amount of myosin in the cytoplasm is only one-fourth that of skeletal muscle. It is estimated that 3 to 5 thin muscle filaments connected to the dense body will be surrounded by a small number of thick muscle filaments, forming an interlaced arrangement, which may be a functional unit similar to the sarcomere in the skeletal muscle.

Generally, smooth muscle cells are spindle-shaped, with a diameter of 2 to 5 m; their lengths are highly variable, and the optimal length for tension is about 400 m. They do not have a well-developed myotube system like skeletal muscle (and myocardium). The muscle cell membrane has only some longitudinal pocket-shaped recesses, but its function is not clear, but this makes the ratio of the surface area of the cell membrane to the volume of the cell larger. Therefore, it is not the transverse tube or sarcoplasmic reticulum system that is close to the myofilament. It's sarcolemma. When the 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 transmitters, hormones, or drugs bind to the sarcolemmal receptor, a second messenger is generated in the cytoplasm through G-proteins, causing Ca2 + release from the Ca2 + pool. Because troponin does not exist in the thin muscle filaments of smooth muscle, Ca2 + causes a mechanism of transverse bridge circulation in which smooth and thick muscle filaments slide in each other in smooth muscle cells, unlike skeletal muscle. The academic community believes that the activation of the transverse bridge begins with its phosphorylation, which in turn depends on the activation of a protein called myosin kinase; the process is that Ca2 + first binds to a cytoplasm called calmodulin A special protein, which activates myosin kinase after binding to four Ca2 +, which decomposes ATP. The resulting phosphate group binds to the bridge and leaves the bridge in a high free state. The mechanism of smooth muscle transverse bridge activation takes longer than skeletal muscle, which is consistent with the slow contraction of smooth muscle.

Classification of smooth muscle function

Although the smooth muscles contained in various organs in the body are highly distinguished in terms of their functional characteristics, they can generally be divided into two categories: one is called multi-unit smooth muscle, and each smooth muscle cell contained in it is independent of each other during movement, similar to Skeletal muscle cells, such as erector muscle, iris muscle, transient membrane muscle (cat), and large blood vessel smooth muscle, etc., the activities of their cells are dominated by external nerves or affected by hormones that spread to each cell; another type is called Single-unit smooth muscle, similar to myocardial tissue, in which cells can synchronize with each other through electrical coupling between the cells. Most of these smooth muscles are self-disciplined and can perform near normal when there is no external innervation. Contractile activity (due to the autonomic nature of pacemaker cells and the role of the inner plexus) is represented by gastrointestinal, uterine, and ureteral smooth muscles. There are some smooth muscles that have both characteristics. It is difficult to classify them. For example, arteriolar and venous smooth muscles are generally considered to belong to multi-unit smooth muscles, but they are also self-disciplined. Bladder smooth muscles are not self-disciplined, but when they are struggling, It can respond as a whole when pulled, so it is also included in the unit smooth muscle.

Characteristics of smooth muscle digestive tract smooth muscle

Gastrointestinal smooth muscle has common characteristics of muscle tissue, such as excitability, conductivity, contractility and stretchability, but these characteristics of digestive smooth muscle have their own characteristics.

Low excitability and slow contraction: The electrical excitability of digestive smooth muscle is lower than that of skeletal muscle. It takes much longer to complete a contraction and relaxation than skeletal muscle, and it has a larger variation.

Self-discipline: Put the isolated digestive tract in a suitable environment, its smooth muscle can show rhythmic contraction, but its rhythm is not as good

Myocardium is as regular as it is, and shrinks slowly.

Tension: The smooth muscles of the digestive tract remain at a mild, continuous contraction state at rest. This is called tension. This tension often keeps a certain base pressure in the lumen of the digestive tract, and keeps the shape and position of the various parts of the digestive tract. Various contractions of the smooth muscle of the digestive tract occur on the basis of tension.

Rich in extensibility: under the action of external forces, the smooth muscles of the digestive tract can make a large stretch to meet actual needs. For example, the stomach can hold food several times its original volume.

It is sensitive to certain physical and chemical stimuli; smooth muscle of digestive tract is particularly sensitive to the stimulation of some biological tissue products. For example, a small amount of acetylcholine can cause it to contract, while adrenaline can cause it to relax. It is also highly sensitive to chemical, temperature and stretch stimuli. This is related to the environment in which it is located. The food and digestive fluid in the digestive tract are mechanical and chemical natural stimuli that often act on smooth muscle.