Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Musculoskeletal system
Muscular system
General information about muscles

Muscle Structure. Muscles are Organs of movement; they have a middle, active part—the belly, consisting mainly of Muscle tissue, and tendinous ends (tendons), formed of Cytology/practical/45.html">Dense Connective Tissue and serving for attachment. Tendons are characterized by a distinctive luster and a whitish-yellowish color. They possess significant strength: some of them can withstand loads of up to several hundred kilograms.

Usually, muscles attach by their tendinous ends to movably connected PARTS OF THE Skeleton—bones. However, some muscles can also attach to fasciae, to various organs (the Eyeball, laryngeal cartilages, etc.), to the Skin (on the face), and so on.

Each Muscle consists of many thousands of striated muscle fibers (Fig. 1.50) arranged in parallel and bound together into bundles by layers of loose connective tissue. The entire muscle is covered externally by a thin connective tissue sheath—the fascia.

Sometimes (for example, in the deltoid and gluteus maximus muscles), the bundles are so large that they can be distinguished with the naked eye, giving the muscle a coarse-fibered structure.

Muscles perform a great deal of work and, being active organs, are characterized by an intensive METABOLISM. Therefore, muscles are permeated by A large number of Blood Vessels, through which blood delivers nutrients and oxygen to them and removes Metabolic waste products. The Blood supply to muscles varies depending on the load. Those that work almost constantly, such as the Diaphragm, have a rich vascular network. Muscles that function only for short periods of time are poorer in blood vessels (biceps brachii, rectus abdominis, etc.). In addition to blood vessels, muscles also contain Lymphatic vessels through which Lymph drains.

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Fig. 1.50. Muscle fibers:

1 — smooth; 2 — striated

Muscle activity, like that of other organs, is regulated by The Nervous system. Nerve fibers terminate in muscles as receptors or effectors. Receptors, in the form of terminal ramifications of a sensory nerve or a complexly structured neuromuscular spindle (see Section 3.6.6), are located in both the muscle and the tendons. Receptors perceive the degree of Muscle contraction and stretch, giving rise to sensations in humans known as muscle sense. This sense allows, in particular, the Determination of the position of body parts. Effector nerve endings, or motor endplates, are specialized endings of a motor nerve on muscle fibers (Fig. 1.51). They transmit excitation to the muscle, which comes from The Nerve Center in response to Changes in the muscle's state perceived by the receptors.

Fig. 1.51. Neuromuscular Junction (scanning Electron Cell/15.html">Microscopy): 1 — nerve and 2 — muscle fibers

In addition, fibers of the Autonomic nervous system (sympathetic) terminate in muscles. The impulses they conduct increase the sensitivity of muscle tissue to excitations coming from the motor centers of the brain.

In each muscle, one of its ends is conventionally called the origin, and the other, the insertion. The origin is considered to be the proximal end of the muscle, which usually remains stationary during its contraction; this site on the bone is called the fixed point (punctum fixum). The insertion site, located on another bone set in motion by the contracting muscle, is called the mobile point (punctum mobile). However, THE CONCEPT OF fixed and mobile points is relative. Very often, their roles are mutually reversed. For example, when the biceps brachii contracts, it usually brings the forearm closer to the trunk, or rather, to the fixed point located on the scapula. But during pull-ups on a bar, the contraction of the same muscle brings the scapula and trunk closer to the forearm; at this time, the fixed point will be on the forearm, and the mobile point will shift to the trunk (more precisely, to the scapula).

The shape and size of a muscle, as well as the direction of its fibers, depend on the work it performs.

Muscles are classified as long, short, wide, and circular (Atl. Figs. 26, 27).

Long muscles are found where the range of motion is large, for example, on the limbs.

Short muscles are located where the range of motion is small, for example, between individual vertebrae.

Wide muscles are located mainly on the trunk, in the walls of Body Cavities, such as the Abdominal muscles and the SUPERFICIAL MUSCLES OF the back and chest. When wide muscles are arranged in multiple layers, their fibers usually run in different directions, and the muscles not only provide A wide variety of movements but also help strengthen the walls of body cavities. The tendons of wide muscles are flat, occupy a large surface area, and are called aponeuroses.

Circular muscles are located around body openings (for example, the orbicularis oris muscle) and constrict them by their contraction, which is why they are also called constrictors or sphincters.

In addition to simple muscles, there are complex ones (Atl. Fig. 28).

Fig. 1.53. Trunk and branchial musculature of a fish (diagram):

1 — Mouth; 2 — nostril; 3 — oblique and 4 — rectus Muscles of the eye; 5 — otic vesicle; 6 — musculature of the branchial sac; 7 — myotome, 8 — its dorsal and 9 — ventral

parts; 10 — gill slit; 11 — branchial arch

Muscle Development. In the lancelet and fish, the musculature of the entire body is clearly segmented, with each segment (myotome) divided into dorsal and ventral parts. In the lateral walls of the anterior end of the primary gut (HEAD gut), there are visceral and branchial arches and the striated muscles of the branchial sac (Fig. 1.53).

In terrestrial vertebrates, the ventral musculature differentiates into regions—cervical, thoracic, abdominal, and caudal. In phylogeny, especially in mammals, due to the Increasing complexity of body Functions and limb specialization, individual body myotomes migrate, fuse, or split, causing the primary segmentation to become obscured or disrupted. Nevertheless, primary segmentation is still partially evident in humans in the structure and arrangement of many trunk muscles (for example, the short muscles of the spine, intercostals, and rectus abdominis).

In terrestrial vertebrates, the mesoderm of the visceral and branchial arches differentiates into musculature with new functions through segregation and complex transformations. The masticatory muscles develop from the mesoderm of the mandibular arch, while the Muscles of facial expression and the platysma develop from the hyoid arch. Additionally, the Neck Muscles lying above the Hyoid bone develop from the mesoderm of these arches. The mesoderm of the first branchial arch gives rise to the muscles of the Pharynx and Larynx, while the mesoderm of the subsequent branchial arches forms muscles that originate on The Skull and extend to the shoulder girdle (trapezius and sternocleidomastoid). Corresponding to the primary Innervation of the metamerically arranged branchial pouches, all these muscles are supplied by nerve fibers running sequentially in Cranial Nerves V, VII, IX, X, and XI.

Trunk musculature in the human embryo develops from the mesoderm of the dorsolateral part of the somites, which forms the myotome (Fig. 1.54). As each myotome grows, it gives off a ventral process, from which the ventral musculature of the trunk (intercostal muscles, oblique and rectus abdominis muscles) subsequently develops. The dorsal musculature (deep back muscles) arises from the dorsal part of the myotome. Very early on, a nerve grows from the Spinal Cord into each myotome, which then, corresponding to the division of the myotome, also splits into dorsal and ventral branches. After the signs of muscle segmentation are obscured, the original neural connection of the myotomes with the corresponding segment of the spinal cord is preserved (with very few exceptions) and serves as an important indicator of muscle origin.

Complex muscles differ from simple ones in that their origin is not single but divided into two, three, or four parts—heads. Originating from different bony points, the heads then merge into a common belly. According to their structure, such muscles are called biceps, triceps, and quadriceps. The insertion end of the muscle can also be divided. In this case, the common belly divides and ends in several tendons that attach to different bones. Such muscles, for example, move the fingers and toes (extensor digitorum longus). The muscle belly can also be divided transversely by an intermediate tendon—a digastric muscle. Sometimes the belly is divided not by one, but by several tendons or tendinous intersections, as in the rectus abdominis.

The direction of fibers in a muscle can be parallel to its long axis or at an acute angle to it. In the first, more common case, the long fibers allow the muscle to shorten significantly during contraction, which provides a wide range of motion. In the second, the fibers located at an angle to the muscle axis are short but more numerous; therefore, when contracting, the muscle shortens only slightly but exerts great force. If the short fibers approach the tendon from one side, the muscle is called unipennate; if from both sides, bipennate. Some muscles (for example, the deltoid) represent a fusion of several unipennate muscles, giving their fiber direction a multipennate or spiral appearance. Such muscles are usually found around ball-and-socket joints; their fibers cross different axes of the joint, ensuring the greatest variety and strength of movements.

Accessory structures of muscles. The accessory structures of muscles include fasciae, synovial bursae, synovial sheaths, and sesamoid bones. All of them develop from the surrounding connective tissue under The Influence of muscle activity.

Fasciae are sheaths of dense Fibrous connective tissue. They cover individual muscles or muscle groups, as well as some other organs, such as neurovascular bundles and Kidneys. Surrounding a group of muscles, fasciae influence the direction of muscle pull during contraction and prevent muscles from shifting sideways. In different parts of the body, fasciae have varying density and strength, depending on the force of the muscles they enclose. In several areas, especially on the limbs, fasciae give off processes that penetrate between muscles to the periosteum, with which they fuse. Thus, intermuscular septa and channels arise from the fasciae—fibrous channels, formed exclusively by fascia, and osteofibrous channels, in The formation of which the periosteum participates along with the fascia. In areas where highly differentiated musculature is present but its potential attachment area to the skeleton is small, such as on the forearm and lower leg, bundles of muscle fibers originate from or attach to the heavily thickened fasciae in these regions. Therefore, fasciae also function as a so-called soft skeleton.

Synovial bursae are thin-walled connective tissue sacs filled with synovial-like fluid. They usually form where a tendon experiences significant friction against bone During muscle contraction, or where two tendons are in close contact with each other, as well as in areas of skin-on-bone friction (for example, in the elbow region). Thanks to the synovial bursa located between two moving structures, friction between them is reduced—that is, the walls of the bursa, lubricated by synovial fluid, slide more easily against each other. Synovial bursae mostly develop after birth, and their cavity increases with age.

Synovial sheaths develop inside fibrous or osteofibrous tunnels that surround long muscle tendons where they slide over bone (for example, in the carpal tunnel, beneath the flexor retinaculum) (Atl. Fig. 42). A synovial sheath consists of two layers: the inner layer covers the tendon on all sides, while the outer layer lines the walls of the fibrous tunnel. Both layers transition into one another along the entire length of the tendon, forming a double layer—the mesotendon, through which blood vessels reach the tendon. The facing surfaces of the layers secrete synovial fluid into the closed, slit-like cavity of the sheath. Synovial sheaths prevent friction between tendons and bone.

Fig. 1.54. Myotomes of the trunk and head at the seventh week of human Embryogenesis:

1 — eye; 2 — preotic myotomes; 3 — visceral arches I and II, branchial arch I; 4 — branchial arches II — IV; 5 — occipital, 6 — cervical, 7 — thoracic, 8 — lumbar, 9 — sacral, and 10 — coccygeal myotomes

Limb muscles arise from the ventral parts of certain trunk myotomes, the ends of which grow into the limb buds. Here, two muscle layers are initially formed—ventral (flexors) and dorsal (extensors). Subsequently, some limb muscles, expanding in a proximal direction, return to the trunk and attach there (latissimus dorsi, pectoral muscles); conversely, the primordia of some trunk muscles (rhomboid, serratus anterior), originating on the trunk, migrate with their other end to the limbs, where they attach. Such complex secondary transformations are particularly characteristic of upper limb development. Here, they provide a mobile yet strong connection of the shoulder girdle and the free limb segment to the trunk.

Muscles that migrate from their site of origin during development are called extrinsic muscles, in contrast to the intrinsic Muscles of the Trunk and limbs, which remain at their site of initial origin.

Most head muscles develop from the mesoderm of the visceral arches, and only a few develop from the myotomes retained here—preotic and occipital. The extraocular muscles form from the preotic myotomes (innervated by cranial nerves III, IV, and VI); the occipital myotomes form the muscles of the Tongue (innervated by cranial nerve XI).

During different age periods, muscles grow at varying rates. The weight of the entire musculature of a newborn is 24% of the total body weight, with tendons occupying a larger area than muscle fibers. In preschool age, the relative weight of muscles changes early, reaching 28% by age seven. However, in primary school age (from 7 to 12 years), There is a rapid increase in muscle mass, accompanied by the thickening of their fibers, changes in their chemical composition, and functional properties (gradual increase in strength, decreased fatigue). Then, slowing down, muscle growth continues until age 18, when its weight reaches an average of 42%. This figure is not the limit and can be increased through training. For example, in athletes, muscles account for 50% of body weight.

Individual muscle groups grow unevenly and at different times. In an infant, the abdominal muscles grow and strengthen faster, followed later by the masticatory muscles. By the end of the first year, the muscles of the back and limbs begin to grow noticeably. This is associated with the more intensive Development of the nervous system and is manifested in changes in the child's mobility, in their desire to crawl and then walk. Dynamic muscles are laid down earlier than static ones and develop faster During the first decade of life. Overall, muscle weight increases 35-fold over the entire growth period, more than any other organ.

The growth of the anatomical parts of most muscles is uneven. It has been established that tendons elongate particularly vigorously between the ages of 13 and 15, outpacing the longitudinal growth of the contracting part of the muscles. These growth characteristics allow the muscle as a whole to keep pace with the rapidly elongating long bones during school age. During this period, the muscles become long and thin, and the adolescents themselves appear long-legged and long-armed (Fig. 1.55).

In adults, muscles change little until age 50, but then atrophy of their fibers begins. The relative weight of muscles gradually decreases, sometimes falling to 30% of body weight in old age.

Sesamoid bones in most cases ossify within tendons or ligaments, reinforcing them and serving as a pulley over which the tendons run. This increases the lever arm of the muscle's force application and facilitates its work.

Muscle work. A muscle is an elastic, viscous body that can stretch under the Influence of External forces. When a muscle is stretched, excitation arises in its receptors. It reaches the Central nervous system via afferent nerve fibers and returns to the muscle via efferent pathways, causing tension that counteracts the stretching.

If a muscle attaches to bones, changes in its tension cause movement in the joint or, conversely, stabilize it. In rarer cases, when skeletal muscles attach to easily displaceable structures (skin, fascia, Joint Capsule), A change in muscle tension leads to the formation of skin folds, tension of the fascia, or tightening of the capsule, which prevents it from being pinched during joint movement. Muscle work is characterized by the force of muscle pull and the range of motion.

Force of pull is The amount of tension a muscle can develop when excited. The force of pull depends on the number and direction of the muscle fibers. A muscle is stronger the more muscle fibers it contains. However, counting them is practically very difficult. Therefore, force is determined by the physiological cross-sectional area of the muscle, which is defined as its cross-sectional area in a plane perpendicular to the length of all its fibers. If the fibers are parallel to the long axis of the muscle, its physiological cross-sectional area equals its anatomical cross-sectional area. With an oblique fiber arrangement, such as in a bipennate muscle, the physiological cross-sectional area is larger than the anatomical one. Each square centimeter of the physiological cross-sectional area of a muscle can withstand an average load of 10 kg.

The force of muscle pull is greater the closer the angle at which its fibers attach is to a right angle.

The degree of muscle excitation is of great importance for the manifestation of the force of pull. The stronger the stimulating effect of the nervous system, the greater the number of muscle fibers involved in excitation, and the greater the force of pull. The Influence of the nervous system depends on the general functional state of the body, the type of Higher Nervous Activity, etc.

In Setting a bone in motion, a muscle acts upon it as a lever. In mechanics, a lever is defined as a rigid body with a fulcrum about which it can rotate under the influence of opposing forces. Based on THE POSITION OF the point of muscle force application and the point of resistance relative to the fulcrum, levers are classified into first-class and second-class levers.

An example of a first-class, double-armed lever, or lever of equilibrium, in The Human Body is the head (Fig. 1.52, A). The movable support of the skull is located at the atlanto-occipital joint. The lever arms, which are unequal in length, lie anterior and posterior to this joint. The weight of the facial skeleton acts on the anterior arm, while the force of the Muscles Attached to the Occipital bone acts on the posterior arm. When the head is held upright, the forces of action and reaction acting on the lever arms are in equilibrium. The pelvis, balancing on the heads of the femurs, is also a first-class lever.

A second-class lever is single-armed. Here, the points of resistance and force application lie on the same side of the fulcrum. In the human body, it exists in two varieties. For example, consider the arm supported at the elbow joint. The weight of the forearm and hand acts on the lever arm. When the brachioradialis muscle, which attaches near the hand and thus near the point of weight application, contracts, highly favorable conditions for work are created, increasing its efficiency. This type of single-armed lever is called a lever of force. When the biceps muscle, which attaches near the fulcrum, contracts, it is less effective at overcoming weight, but the work is performed with greater speed. This variety of second-class lever is called a lever of speed (Fig. 1.52, B). Most muscles in the body operate on THE PRINCIPLE OF the second-class lever.

The range of motion depends on the length of the muscle belly and the lever arm. The greatest range of motion is found in the long BONES OF THE limbs, which describe an arc with a radius equal to their length. The range of motion is influenced by the degree of congruence between the articular surfaces, the presence of intra-articular Cartilage, the tension of the joint capsules, and the resistance offered by other muscles.

Fig. 1.55. Bodies of a one-year-old child, a young woman, and a young man. (Images are not to scale.)

The greater the congruence between the articular surfaces, the smaller the range of motion. For example, in the sacroiliac joint, the fit of the articular surfaces is complete, and the range of motion does not exceed 4—6°. In the shoulder joint, where there is a pronounced incongruence between the articular surfaces of the scapula and the head of the humerus, the range of motion reaches 70°. Intra-articular cartilages and glenoid/acetabular labra, by increasing the congruence of the articular surfaces, reduce the range of motion. Loose joint capsules, such as in the shoulder joint, do not restrict the range of motion, whereas tight ones, like those in the intervertebral joints, limit it. Muscles exert the primary influence on the range of motion. For example, the range of motion produced by the contraction of flexor muscles is limited by the tension of extensor muscles.

A muscle never works in isolation. The execution of diverse body movements is achieved through the coordinated action of many muscles. A distinction is made between synergist muscles, which perform a common action (for example, the radial and ulnar flexors of the wrist), and antagonist muscles, whose tension causes opposing actions. Thus, during wrist flexion, the radial and ulnar extensors act as antagonists to the ulnar and radial flexors. The antagonistic action of muscles is an essential adaptation in the function of the Locomotor System. With every movement, not only do the muscles performing it contract, but also their antagonists, which counteract the pull and give the movement precision and smoothness.

A muscle that moves a joint performs a specific type of work. The Nature of this work depends on the orientation of the joint axis and the position of the muscle relative to this axis. Accordingly, muscles are classified as: flexors and extensors (located anterior or posterior to the transverse axis of the joint), adductors and abductors (medial or lateral to the sagittal axis of the joint), and internal and external rotators (medial or lateral to the longitudinal axis of the joint).

Fig. 1.52. Levers of the human body:

A, B — levers of equilibrium; C, D — levers of speed; triangle — fulcrum; dark arrows indicate the direction of muscle pull; light arrows — direction of gravity; dashed arrow — movement

If all the muscle bundles within a muscle run in the same direction, the muscle's action is limited to one of the specified movements. However, if a muscle consists of bundles running in different directions, with separate groups crossing different axes of the joint, such a muscle can perform several movements, sometimes even antagonistic ones. The deltoid muscle serves as an example. Its anterior fibers cross the frontal axis of the shoulder joint anteriorly and, therefore, flex the arm, while the posterior fibers, crossing posteriorly, extend the arm. The middle fibers of the muscle cross the sagittal axis of the joint laterally; acting in isolation or together with the anterior and posterior fibers, they abduct the arm at the shoulder joint. The same muscles can produce opposite movements depending on the starting position of the limb. For instance, the brachioradialis muscle brings both a supinated and a pronated forearm into a neutral position. The same muscles can act as synergists or antagonists depending on the movement about a particular axis of a multiaxial joint. Thus, the flexors of the wrist joint are synergists during movements around the transverse axis and antagonists during movements around the sagittal axis. Consequently, the combinations of muscle actions are highly diverse.

Most muscles move adjacent parts of the body because they attach to neighboring bones that form a joint. Such muscles are called monoarticular. However, some muscles cross not just one, but two or even several joints; these are called biarticular and multiarticular. The action of these muscles is highly complex, as they set in motion not only the body parts to whose skeleton they attach, but also all the intermediate segments they bypass without attaching to them.

The amount of mechanical work performed by a contracting muscle is determined by the product of the mass of the load lifted and the height of the lift. Back in the 1880s, P.F. Lesgaft discovered that, based on the nature of their work, striated muscles can be divided into two groups: strong and agile. Strong muscles are better suited for performing static work. Such muscles, for example, the soleus, are characterized by an oblique arrangement of short (up to 5 cm) muscle fibers (i.e., they are pennate in shape), a large origin surface, and an insertion point located close to the point of weight application. Strong muscles are richer in blood vessels and muscle pigment (Myoglobin), making them darker in color, which is why they are called red muscles. During activity, they exert great force with minimal tension and do not fatigue for a long time. However, the speed and range of motion during their contractions are small. The work of these muscles, which counteract gravity, maintains the upright posture of the torso, enables standing, holds individual body parts in specific positions, and maintains a particular posture. This static muscle work reflects the supportive function of the musculature.

Agile muscles, according to Lesgaft, perform dynamic work more easily. For example, the biceps femoris is characterized by long, usually parallel fibers, a small area of origin and insertion, the latter being located far from the lever's fulcrum, and a smaller number of blood vessels, which is why they are called white muscles. These muscles are characterized by rapid contraction and, working under high tension, fatigue quickly. While inferior in strength, agile muscles are capable of producing fine, diverse movements. This capability is enhanced by the fact that they often have several heads that contract independently.

In higher animals and humans, each muscle typically contains both red fibers of the static type and white fibers of the dynamic type. A child's high mobility and relatively low strength are explained by the relatively large proportion of white fibers in their muscles. With age and depending on physical load, the ratio between white and red fibers changes.

In addition to mechanical work, muscles perform other functions: they participate in heat production and ensure the functioning of the speech-motor apparatus. The contraction of the muscles of the larynx, pharynx, tongue, and other parts of the vocal apparatus enables the pronunciation of words.

Muscle activity is based on complex Chemical transformations of organic substances. The breakdown of these substances in the muscle is accompanied by the release of energy, which is used not only for mechanical work; a significant portion of it is released as heat. This heat warms the body.

With any change in the state of a muscle, the receptors located within it are stimulated. These receptors represent the peripheral division of the sensorimotor system, which allows for the perception of the position of the body and its parts in space.

Muscle activity is an essential condition for their existence. Prolonged muscle inactivity leads to atrophy and loss of function. Training—that is, systematic, sufficiently intense, but not excessive muscle work—leads to an increase in their volume, strength, and performance, which promotes the physical development of the entire Organism.