Muscles

Skeletal muscles, being attached to bones, set them in motion, participate in the formation of the walls of body cavities: oral, thoracic, abdominal, pelvic, are part of the walls of some internal organs (pharynx, upper part of the esophagus, larynx), are among the auxiliary organs of the eye (oculomotor muscles), affect the auditory ossicles in the tympanic cavity. With the help of skeletal muscles, the human body is kept in balance, moves in space, respiratory, chewing and swallowing movements are carried out, facial expressions are formed. The total mass of skeletal muscles is equal to an average of 28 kg in men, 17 kg in women. In an adult, muscle mass is about 30% in men, about 20% in women (in newborns - 20-22%). In elderly and old people, the mass of muscle tissue is slightly reduced compared to younger people.

There are about 400 muscles in the human body, consisting of striated (cross-striated, skeletal) muscle tissue, contracting in accordance with our will. Under the influence of impulses coming through the nerves from the central nervous system, skeletal muscles contract, set bone levers in motion, and actively change the position of the human body.

Muscle structure

Each muscle (museums) consists of bundles of striated (cross-striated) muscle fibers, each of which has a thin connective tissue sheath - endomysium (endomysium). Between the bundles of muscle fibers there are connective tissue layers that form the sheaths of these bundles - the internal perimysium (perimysium internum). The sheath of the entire muscle is the external perimysium, or epimysium (perimysium externum, s.epimysium), which continues onto the tendon called peritendinium (peritendineum). Muscle bundles form the fleshy part of the organ - the muscle belly (venter), which passes into the tendon (tendo). With the help of muscle bundles or the proximal tendon, which is called the head of the muscle (caput), the muscle originates on the bone. The distal end of the muscle or its distal tendon, which is also called the "tail", attaches the muscle to another bone. It is generally accepted that the origin of the muscle is closer to the midline axis of the body (more proximal) than the attachment point, which is located distally. Tendons in different muscles vary in shape, thickness, and length.

Classification of muscles

Skeletal muscles are divided according to their position in the human body, shape, direction of muscle bundles, function, and relation to joints.

According to their location, muscles are classified as superficial and deep, medial and lateral, external and internal.

Accessory apparatus of muscles

Muscles, when contracting, perform their function with the participation and assistance of anatomical structures, which should be considered as auxiliary apparatus of muscles. These include fascia, tendon sheaths, synovial bags and muscle blocks.

Fascia (fascia) is a connective tissue covering of a muscle. By forming sheaths for muscles, fascia separates them from each other, creates support for the muscle belly during its contraction, eliminates friction between muscles. Having a sheath-like structure, fascia in pathology limits the spread of pus, blood during hemorrhage, and makes it possible to carry out "sheath" local anesthesia. Between the surface of the muscle, its membrane (epimisium) and fascia there is a thin layer of loose cellulose. In some places (on the shin, forearm), fascia serve as the place where muscles begin, and then it is difficult to separate the muscle from the fascia.

Muscle work and strength

The main property of muscle tissue that forms skeletal muscles is contractility, which changes the length of the muscle under the influence of nerve impulses. Muscles act on the bones of the levers that are connected by joints. In this case, each muscle acts on the joint in only one direction. In a uniaxial joint (cylindrical, block-shaped), the movement of the bone levers occurs only around one axis, so the muscles are located in relation to such a joint on both sides and act on it in two directions (flexion - extension; adduction - abduction, rotation). For example, in the elbow joint, some muscles are flexors, others are extensors.

Muscle work. Since the ends of the muscle are attached to the bones, the points of its origin and attachment come closer to each other during contraction, and the muscles themselves perform a certain amount of work. Thus, the human body or its parts change their position when the corresponding muscles contract, move, overcome the resistance of gravity or, conversely, yield to this force. In other cases, when the muscles contract, the body is held in a certain position without performing a movement. Based on this, a distinction is made between overcoming, yielding, and holding muscle work.

Muscle development

The source of origin of all skeletal, striated muscles of the body in humans, as in animals, is the middle germ layer - the mesoderm. However, the development of muscles within the trunk, head and limbs has a number of features that are easier to understand by tracing the initial stages of embryogenesis. The muscles of the trunk develop mainly from the dorsal, paraximal (near-axial) section of the mesoderm, which forms the primary segments of the body - the somites. The somites are located on the sides of the axial organs of the embryo - the neural tube and the dorsal cord. In the 4th week of development, there are about 40 pairs of somites: from 3 to 5 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 4-5 caudal. Then each somite is subdivided into 3 parts: sclerotome, dermatome and myotome; the muscles of the trunk develop from the latter.

[ 1 ]