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muscle tissue
According to histology and embryology, muscle tissue is mainly composed of muscle cells with contractile function and a small amount of connective tissue, blood vessels, lymphatic vessels and nerves.
Editado a las 2024-02-08 17:08:42,
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muscle tissue
concept
Muscle tissue is mainly composed of muscle cells with contractile function and a small amount of connective tissue, blood vessels, lymphatic vessels and nerves.
Muscle cells are in the shape of elongated fibers, so they are also called muscle fibers. The muscle cell membrane is also called sarcolemma. The muscle cytoplasm is called sarcoplasm or sarcoplasm.
Skeletal muscle and cardiac muscle are also called striated muscle; smooth muscle is also called non-striated muscle.
Skeletal muscles are voluntary muscles, cardiac muscle and smooth muscle are involuntary muscles
skeletal muscle
Generally attached to bones by tendons, its contraction characteristics are rapid, powerful and easy to fatigue. The connective tissue surrounding the entire muscle is called the epimysium; The connective tissue surrounding the muscle bundles is called the perimysium; The small amount of connective tissue distributed around each muscle fiber is called endomysium;
1. Light microscopic structure of skeletal muscle fibers
They are long cylindrical cells, multinucleated cells with multiple or even hundreds of nuclei, flat oval shape, light coloring, and located under the sarcolemma. In the sarcoplasm, there are thin filament-like myofibrils arranged parallel to the long axis of the muscle fiber, with horizontal striations.
The bright band is also called the I band. There is a darker and thin Z line in the center of the bright band. The dark band is also called the A band. There is a light narrow band in the center of the dark band, called the H band. There is a darker M in the center of the H band. Wire.
A section of myofibrils between two adjacent Z lines is called a sarcomere. Each sarcomere is composed of 1/2 light band, 1/2 dark band, and 1/2 light band. It is the basic unit of skeletal muscle fiber structure and function.
2. Ultrastructure of skeletal muscle fibers
(1) Myofibrils
Myofibrils are composed of two types of myofilaments: thick and thin. The two types of myofilaments are regularly alternated and arranged in parallel along the long axis of the muscle fiber.
Thick myofilament
It is located in the middle of the myotome, with both ends free and the center fixed by the M line.
Thin myofilaments
Located on both sides of the sarcomere, one end is attached to the Z line, and the other end is free and extends between the thick muscle filaments parallel to it, ending at the outside of the H band.
(2) Transverse tubule
Also known as T tubule, it is a tubular structure formed by the sarcolemma of skeletal muscle fibers indenting into the sarcoplasm. Its direction is perpendicular to the long axis of the muscle fiber and is located at the junction of the light band and the dark band. It can transmit the excitement of the sarcolemma to the interior of the muscle fiber.
(3) Sarcoplasmic reticulum
Also known as the sarcoplasmic reticulum, it is a specialized smooth endoplasmic reticulum in muscle fibers. The middle part of the sarcoplasmic reticulum surrounds each myofibril longitudinally and is called a longitudinal tubule; Both ends of the longitudinal tubule expand into flattened blind tubes, which are called terminal cisterns.
Each transverse tubule and the terminal cisternae on both sides form a triplet
Function: Store and release calcium ions
Function: Store and release calcium ions
3. Contraction principle of skeletal muscle fibers
Motor nerve endings transmit nerve impulses from the motor end plate to the sarcolemma
The excitement of the sarcolemma is transmitted to the sarcoplasmic reticulum through the transverse tubules, and a large amount of Ca flows into the sarcoplasm.
Ca binds to troponin, troponin and tropomyosin are allosteric, exposing the binding site on actin and the myosin head knot, and the two quickly combine
The ATPase of the myosin head is activated, ATP is broken down and energy is released, and the myosin head and rod flex, pulling actin toward the M line;
The thin myofilaments slide toward the M line between the thick myofilaments, the bright band narrows, the H band narrows or disappears, the sarcomeres shorten, and the muscle fibers contract.
After the contraction, the Ca in the sarcoplasm is pumped back to the sarcoplasmic reticulum, troponin, etc. return to their original state, and the muscle fibers relax.
4. Classification of skeletal muscle fibers
Red muscle fiber, white muscle fiber and intermediate fiber
myocardium
Distributed on the heart wall and the walls of large blood vessels adjacent to the heart, the contraction has the characteristics of automatic rhythm, slowness, persistence, and resistance to fatigue.
1. Light microscopic structure of myocardial fibers
Myocardial fibers are irregular short cylinders with branches and anastomoses. The junctions between adjacent myocardial fiber branches are deeply stained and are called intercalated discs. The striations of myocardial fibers are not as obvious as those of skeletal muscle. Most myocardial fibers have one nucleus, and a few have dual nuclei. The nucleus is oval and located in the center of the cell. The cytoplasm around the nucleus contains lipofuscin, which increases with age.
2. Ultrastructure of myocardial fibers
Myofibrils vary in thickness and have unclear boundaries, and there are extremely abundant mitochondria between myofibrils.
The transverse tubules are thicker and located at the level of the Z line
The sarcoplasmic reticulum is sparse, the longitudinal tubules are underdeveloped, and the terminal cisterns are few and small. The transverse tubules are often closely attached to the terminal cistern on one side to form a doublet.
The intercalated disk is located at the level of the Z line and is in the shape of a staircase. The transverse part has intermediate connections and desmosomes, which play a role of firm connection: there are gap connections in the longitudinal part, which are conducive to the exchange of chemical information and the conduction of electrical impulses between cells, respectively. Synchronization of overall contraction and relaxation of atrial and ventricular myocardium
smooth muscle
Distributed in the walls of hollow organs such as the digestive tract, respiratory tract, and blood vessels, it is an involuntary muscle. Its contraction is slow and long-lasting, and it is not easy to fatigue.
1. Light microscope structure of smooth muscle fibers
No striations, no myofibrils, long spindle shape, uneven size. There is a rod-shaped or oval-shaped nucleus in the center of the cell, which is often spiral-shaped when contracting. The smooth muscle cross-section is of different sizes, round or irregular.
2. Ultrastructure of smooth muscle fibers
There are relatively developed gap junctions between smooth muscle fibers, which can transmit information and electrical impulses, causing synchronous functional activities of adjacent muscle fibers or even the entire muscle bundle or muscle layer.