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.

(1) Myofibrils

(2) Transverse tubule

(3) Sarcoplasmic reticulum

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.

Red muscle fiber, white muscle fiber and intermediate fiber

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.

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

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.

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.