Human Anatomy, Part 1 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2002

Special Part
Myology, myologia [the study of muscles] - General Overview
Structure of Muscles

Muscles have a complex Structure. They consist of Muscle and Connective Tissues, tendons, nerves, and Blood and Lymphatic vessels. Anatomically, a muscle is divided into a muscular part and a tendinous part.

Striated muscles are formed by bundles of muscle fibers separated by Connective Tissue layers. The length of striated muscle fibers can vary; for example, in the sartorius muscle, they reach up to 12 cm. The thickness of muscle fibers in adults is 38-40 мкм; whereas in individuals who regularly engage in sports, it reaches 100 мкм. A striated muscle fiber consists of sarcoplasm, numerous myofibrils (the contractile elements of the fiber) located within it, and a fibrillar sheath known as the sarcolemma that envelops the fiber. The diameter of muscle myofibrils is 1-2 мкм. A single muscle fiber can contain up to 2,000 myofibrils, which are built from thick and thin filaments.

¹ Lesgaft, P. F. Fundamentals of Theoretical Anatomy. - 1905. - Part I. - P. 249.

Thick filaments are formed by the protein Myosin. The basis of thin filaments is formed by the protein Actin, and they also include the Proteins troponin and Tropomyosin. During contraction, Different types of filaments slide relative to each other, resulting in a decrease in the length of the myofibril—that is, its contraction.

Based on The ratio of myofibrils, Cell/35.html">Mitochondria, and Myoglobin, muscle fibers are divided into two main types: red and white. The sarcoplasm of red muscle fibers has a high content of myoglobin, which gives them their red color. The sarcoplasm contains fewer myofibrils but more mitochondria. Red fibers contract slowly yet develop great force. Red fibers predominate in the Muscles of the back and Diaphragm.

In terms of histological structure, white muscle fibers are characterized by a high content of myofibrils with a small volume of sarcoplasm, as well as finer filaments. White fibers contract rapidly, but with less force. White fibers predominate in the muscles of the Tongue and facial expression.

A muscle fiber along with its sheath, nerve endings, and blood and lymphatic capillaries is called a muscle unit, or myon. Muscle fibers are grouped into bundles of the I-III orders. First-order bundles are bounded by thin connective tissue layers known as the endomysium, while second- and third-order bundles are externally covered by a connective tissue sheath called the perimysium. The connective tissue sheath covering the entire muscle is called the epimysium, or outer epimysium (epimysium externum), which continues onto the tendon and is termed the peritendineum. Thus, Skeletal Muscle contains from 3% to 30% connective tissue, depending on the type of muscle.

Most limb muscles have a body (or belly) and two tendons. When a muscle contracts, one end remains conditionally stationary and is considered the fixed point, or punctum fixum, while the other is movable, or punctum mobile. The concepts of fixed and movable points are relative and can interchange depending on the movement performed.

Blood supply to muscles is provided by branches of major Arteries and their ramifications. During physical activity, muscle blood supply increases by 50%. Typically, several arteries penetrate the muscle and branch out within the perimysium layers along the muscle bundles. Arterioles branch into a vast network of blood capillaries, which normally number about 2,000 per 1 mm³. Capillaries run along the muscle fibers and are interconnected by transverse vessels, forming a vascular net with irregular elongated meshes. Venules, which accompany the arterioles and drain into Veins, are formed from the inner muscle network. Lymphatic vessels run alongside the ramifications of the Blood Vessels.

Innervation of muscles. Along with blood vessels, one or more nerves enter the muscle. The nerves contain motor (efferent), sensory (afferent), and sympathetic fibers. Through sensory nerve fibers, information is transmitted to the Cerebral Cortex from specialized sensory nerve endings—proprioceptors (from Latin *proprius* meaning one's own and *capere* meaning to take or perceive)—which signal the state of a muscle or muscle group. Through motor nerve fibers, impulses travel from the cerebral cortex to the muscles, driving their contractility and tone. Via sympathetic nerve fibers, the Central Nervous system influences the trophic processes occurring within the muscles.

Thus, muscles are Organs that, together with The Nervous System, constitute an inseparable whole. I. M. Sechenov wrote: "Muscles are the engines of our body, but by themselves, without impulses from the nervous system, they cannot function. Therefore, the nervous system always participates in work alongside the muscles..."¹

The number of motor fibers entering a muscle and branching among the muscle fibers depends on its functional characteristics. If a muscle performs delicate work and must contract rapidly, a significant number of motor fibers predominate in it. For instance, in extraocular muscles, a single motor nerve fiber controls the contraction of 19 muscle fibers. The lower the contraction speed and precision of the work performed, the greater the number of muscle fibers innervated by a single fiber. For example, in the gastrocnemius muscle, a single motor nerve fiber branches across more than 430 muscle fibers.