Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Muscular System
General Myology

Myology is the scientific Study of the development, Structure, and function of skeletal Muscles. A thorough understanding of skeletal muscles is essential for nursing and allied health professionals—for instance, when properly administering massages, performing intramuscular and intravenous injections, or placing electrodes for diagnostic and physiotherapeutic Procedures.

Skeletal muscles are composed of Cytology/practical/58.html">Striated Skeletal Muscle tissue. They are voluntary muscles, meaning their contraction is under conscious control and depends on our will. The Human Body contains a total of 639 muscles, of which 317 are paired and 5 are unpaired. In men, skeletal muscle mass accounts for approximately 40% of total body weight, compared to 35% in women. In newborns, muscle mass does not exceed 20%. Sustained physical training and load lead to an increase in relative muscle mass; for instance, in weightlifters, muscle mass can reach 50 — 60% of total body weight. Conversely, due to reduced physical activity, the muscles of elderly individuals tend to weaken and typically constitute 25 — 30% of total body weight.

A skeletal muscle is an organ with a characteristic shape and structure, a typical angionervous architecture (Vessels and nerves), built primarily of Striated Muscle tissue, enclosed by its own deep fascia, and possessing The ability to contract.

Principles of muscle Classification. The Classification of Skeletal muscles in the human body is based on several criteria: body region, origin and shape, function, anatomical and topographical relationships, direction of muscle fibers, and their relation to joints.

Depending on the body region, muscles are categorized into those of the trunk, HEAD, neck, and limbs. Muscles of the Trunk are further divided into back, chest, and Abdominal muscles. Muscles of the Upper Limb correspond to the skeletal segments and are grouped into muscles of the shoulder girdle, arm, forearm, and hand. Homologous divisions characterize the lower limb: Muscles of the Pelvic Girdle (hip muscles), thigh, leg, and FOOT (Fig. 6.1, 6.2).

Based on their embryological origin, muscles are distinguished into cranial-derived muscles—such as those of the head, and PARTS OF THE neck and back (innervated by Cranial Nerves)—and spinal-derived muscles—such as those of the trunk, limbs, and parts of the neck (innervated by Spinal Nerves). During development, some spinal-derived muscles may remain at their primary site of origin, and these are referred to as autochthonous muscles.

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Fig. 6.1. Human muscles (anterior view):

1 — palmar aponeurosis; 2 — flexor digitorum superficialis; 3 — flexor carpi ulnaris; 4, 28 — brachioradialis; 5 — brachialis; 6 — triceps brachii; 7 — coracobrachialis; 8 — axillary fossa; 9 — latissimus dorsi; 10 — serratus anterior; 11 — external oblique; 12 — rectus abdominis (contours); 13 — inguinal ligament; 14 — sartorius; 15 — vastus lateralis; 16 — vastus medialis; 17 — dorsal muscles of the foot; 18 — tibialis posterior; 19 — triceps surae; 20 — gracilis; 21 — extensor digitorum brevis; 22 — inferior extensor retinaculum; 23 — superior extensor retinaculum; 24 — extensor digitorum longus; 25 — tibialis anterior; 26 — flexor carpi radialis; 27 — palmaris longus; 29 — biceps brachii; 30 — deltoid; 31 — pectoralis major; 32 — sternocleidomastoid; 33 — Infrahyoid muscles; 34 — Muscles of facial expression

Fig. 6.2. Human muscles (posterior view):

1 — sternocleidomastoid; 2 — trapezius; 3 — deltoid; 4 — triceps brachii; 5 — biceps brachii; 6 — brachioradialis; 7 — extensor carpi radialis longus; 8 — abductor pollicis longus; 9 — extensor digitorum; 10 — gluteus maximus; 11 — semimembranosus; 12 — gastrocnemius; 13 — soleus; 14 — Lateral Muscle Group of the leg; 15 — Achilles tendon; 16 — dorsal muscles of the foot; 17 — fibularis (peroneus) longus; 18 — biceps femoris; 19 — iliotibial tract; 20 — semitendinosus; 21 — external oblique; 22 — latissimus dorsi; 23 — rhomboid muscles; 24 — teres major; 25 — infraspinatus; 26 — brachialis; 27 — extensor carpi ulnaris; 28 — flexor carpi ulnaris

According to their shape, muscles can be simple or complex. Simple muscles include long, short, and broad muscles, which typically have a fusiform (Fig. 6.3) or rectangular shape. Complex muscles include multi-headed (biceps, triceps, quadriceps), multi-tendoned, and digastric muscles. Muscles with distinct geometric forms—such as circular, square, deltoid, trapezoid, and rhomboid—are also classified as complex.

Some muscles can alter their anatomical Location by migrating from the trunk to the limbs, known as truncofugal muscles. Conversely, muscles that form on the limbs and subsequently migrate...

Based on their function, muscles are categorized into flexors and extensors; adductors and abductors; rotators; sphincters (constrictors) and dilators. Depending on the direction of movement, rotators are further subdivided into pronators and supinators (inward and outward rotators).

In addition to specific types of movement, the Functional Classification of muscles includes synergists and antagonists. Synergists are Muscles that perform the same function and thereby reinforce each other's action, such as the brachialis and biceps brachii. Antagonists are muscles that perform opposing Functions, thus producing movements in opposite directions; for example, the biceps brachii flexes the elbow joint, whereas the triceps brachii extends it.

Fig. 6.3. Muscle shapes:

a — fusiform; b — biceps; c — digastric; d — polygastric; e — bipennate; f — unipennate; 1 — head; 2 — belly; 3 — tendon; 4 — intermediate tendon; 5 — tendinous inscription

Depending on their location and topographical relationships, muscles are divided into superficial and deep; external and internal; medial and lateral.

Based on the direction of their muscle fibers, muscles are classified into those with parallel, oblique, circular, and transverse fiber arrangements. Unipennate and bipennate muscles are also included among muscles with an oblique fiber direction.

Regarding their relation to joints, muscles can be monoarticular (acting on a single joint), biarticular, or multiarticular. Biarticular and multiarticular muscles have more complex actions because they not only move the specific skeletal segment to which they attach, but can also alter the overall position of a limb or part of the trunk.

Muscle Structure. As an organ, a skeletal muscle comprises the muscle proper (flesh) and tendinous parts, a system of Connective Tissue sheaths, as well as its own Blood Vessels and nerves. The middle, thickened portion of the muscle is called the belly (see Fig. 6.3). In most cases, both ends of the muscle feature tendons that attach it to bones. A broad, flat tendon is referred to as an aponeurosis.

Fig. 6.4. SCHEMATIC STRUCTURE OF striated muscle fibers:

1 — striated muscle fiber; 2 — blood capillary; 3 — myofibrils; 4 — Nucleus; 5 — vegetative nerve fiber; 6 — neuromuscular synapse; 7 — motor nerve fiber; 8 — endomysium; 9 — tendon filament

The Structural and functional unit of the actual muscular tissue is the striated muscle fiber. Externally, it is covered by a membrane called the sarcolemma, and internally it contains nuclei and specialized contractile elements known as myofibrils (Fig. 6.4). A single fiber contains from 100 to 1,000 myofibrils oriented along its longitudinal axis. Each myofibril, in turn, consists of 1,500 to 2,000 protofibrils. The latter are built from macromolecules of specialized Muscle Proteins—Myosin and Actin—which appear under light Cell/15.html">Microscopy as alternating dark and light bands. Thicker myosin molecules correspond to the dark bands (exhibiting optical birefringence), whereas thinner actin molecules correspond to the light discs. During Muscle contraction, actin filaments slide into the gaps between myosin filaments, alter their configuration, and bind to each other. These processes are fueled by the Cleavage of ATP molecules within the Mitochondria.

The functional unit of a muscle—the myon—is a group of striated muscle fibers innervated by a single motor nerve fiber. A muscle comprising A large number of myons does not necessarily contract as a whole; rather, it can contract in separate bundles.

Striated muscle fibers, positioned in parallel and interconnected by loose connective tissue, form a primary bundle (first-order bundle) surrounded by the endomysium (see Fig. 6.4). Three to five primary bundles unite to form secondary bundles, which are covered by the perimysium. The latter combine into larger bundles (third-order bundles) that constitute the muscle itself. The connective tissue layer enveloping the third-order bundles from the outside is termed the epimysium.

Accessory apparatus of muscles. The Accessory structures of skeletal muscles include fasciae, fibrous and osteofibrous canals, synovial sheaths, synovial bursae, muscle pulleys, and sesamoid bones.

Fasciae are connective tissue sheaths that delimit the subcutaneous adipose tissue, enveloping muscles and certain Internal Organs. Depending on their location, they are classified into superficial, deep (proper), and internal fasciae.

The superficial fascia lies directly beneath the subcutaneous adipose tissue. Through connective tissue septa, it is firmly attached to the Skin, compartmentalizing the subcutaneous fat into small Cells.

The deep fascia (fascia propria) covers the muscles of various body regions. Like the superficial fascia, it is named after the respective region: deep Fascia of the back, chest, abdomen, neck, head, arm, forearm, hand, etc. It forms fascial compartments for individual muscles or groups of muscles (Fig. 6.5).

The deep fascia forms enclosed enclosures for muscles, which may take the form of fibrous or osteofibrous sheaths. Fibrous sheaths are bounded entirely on all sides by fasciae. Osteofibrous sheaths are formed on one side by the deep fascia covering the muscles and on the other by the periosteum of the adjacent bone. The closed nature of fibrous and osteofibrous sheaths creates optimal conditions for the independent function of each individual muscle.

Fig. 6.5. Fasciae of the arm:

1 — biceps brachii muscle; 2 — brachialis muscle; 3 — medial intermuscular septum; 4 — triceps brachii muscle; 5, 10 — superficial fascia; 6 — skin; 7 — lateral intermuscular septum; 8 — humerus; 9 — deep fascia; 11 — subcutaneous adipose tissue

In 1840, N.I. Pirogov noted that fibrous and osteofibrous sheaths function as hermetically sealed compartments. Consequently, knowing their anatomical layout and structure enables clinicians to predict the pathways of Hemorrhage and purulent spread during injuries and suppurative processes. Muscle sheaths are also utilized for administering local anesthetics (fascial sheath anesthesia according to Vishnevsky).

The internal fascia lines the body cavity from the inside. Body Cavities are present in the cervical, thoracic, and Abdominal Regions. Accordingly, the endocervical, endothoracic, and endoabdominal fasciae are distinguished.

Fibrous and osteofibrous canals are receptacles for muscle tendons, blood vessels, or nerves in the wrist and ankle joints, as well as the Phalanges of the fingers and toes, formed by the thickening of the deep fascia. The gliding of tendons relative to the canal walls is facilitated by specialized structures known as synovial sheaths—tubular sleeves surrounding the tendon. Structurally, they resemble a double-walled cylinder positioned around the tendon and anchored to the canal walls. The outer wall, fused with the canal walls, is called the parietal layer; the inner wall, fused with the tendon, is called the visceral layer. Synovial fluid resides between the layers, acting as a lubricant to reduce friction. Under excessive loads or infection, these synovial sheaths can become inflamed, leading to tenosynovitis.

The accumulation of a large volume of serous fluid or pus within them can compress the blood vessels supplying the tendon and even lead to necrosis. In chronic tenosynovitis, the parietal and visceral layers fuse, rendering tendon movement during muscle contraction impossible.

Synovial bursae are fluid-filled cavities lined by a synovial membrane situated between fascial layers and containing synovial fluid. They are typically located near the attachment points of tendons to bones, reducing friction during muscle contraction. Excessive accumulation of synovial fluid or infection within the bursa is referred to as bursitis.

Sesamoid bones develop within the substance of tendons, close to their insertion sites. They are most commonly found in the region of the hand and foot digits. The largest sesamoid bone is the Patella.

Factors determining muscle strength. The strength of a skeletal muscle is determined by the following factors:

1) the physiological cross-sectional area of the muscle, which is defined as the sum of the cross-sectional areas of all striated muscle fibers. It is worth noting that the physiological cross-sectional area does not coincide with the anatomical cross-sectional area. The latter includes the cross-sectional area of not only the muscle fibers, but also blood vessels, nerves, and connective tissue;

2) the surface area of attachment on bones, cartilages, or fasciae;

3) the degree of neural excitation;

4) the adequacy of blood supply;

5) the condition of the skin and subcutaneous adipose tissue.

Work and functions of muscles. Like every individual striated muscle fiber, a muscle shortens and thickens upon contraction. In doing so, it brings the points of origin and insertion closer together, driving the movement of the body and its parts in space. Upon maximal contraction, a muscle can shorten by up to 50% of its initial length. Skeletal muscles attach on opposite sides of a joint and induce movement within it when they contract.

Different muscle groups function in a coordinated manner: for instance, when flexor muscles contract, the corresponding extensor muscles relax at the same time. The Nervous system plays the primary role in coordinating these movements.

Muscles operate reflexively, meaning they contract under METABOLISM/18.html">The Influence of nerve impulses originating from the Central Nervous System. The cortical area of the motor analyzer is located in the precentral gyrus of the Cerebral Cortex. However, muscles directly receive impulses from motor Neurons whose cell bodies lie in the anterior horns of the Gray matter of the Spinal Cord and the Brainstem.

Excitation is transmitted from a nerve to a muscle via the Neuromuscular Junction. Acetylcholine serves as the neurotransmitter; it is stored in vesicles located at the terminals of motor nerve fibers. Triggered by a Nerve Impulse, Acetylcholine is released into the synaptic cleft, binds to receptors on the postsynaptic membrane of the muscle fiber, and depolarizes it. The resulting electrical impulse propagates along the membrane, increasing the permeability of the muscle fiber's Endoplasmic reticulum to Ca²⁺ ions. These ions enter the Cytoplasm, activate contractile proteins, and catalyze the cleavage of one phosphate group from ATP. Consequently, the energy required for contraction is released.

The Nature of skeletal muscle contraction depends on the frequency of nerve impulses arriving at the muscle. Under natural conditions, a train of impulses reaches the muscle from the CNS, to which the muscle responds with a sustained tetanic contraction. At a frequency of 10–20 impulses per second, the muscle enters a state of muscle tone, which is essential for maintaining posture. Tetanus occurs due to the summation of individual muscle twitches at a frequency of 40–50 impulses per second. Accordingly, tonic and dynamic types of muscle contractions are distinguished. Tonic contractions are maintained by so-called red muscle fibers, which are fatigue-resistant. They are characterized by a high rate of oxidative processes and consist of relatively thin myofibrils. Muscles composed of red muscle fibers are responsible for posture maintenance, such as the muscles of the back. Dynamic contractions are produced by white muscle fibers, which have a larger diameter, larger and stronger myofibrils, and a low rate of oxidative processes. They predominate in muscles that perform rapid movements, such as the limb muscles.

Intense muscular exertion can lead to fatigue, which is a temporary decrease in the performance capacity of a cell, organ, or the entire Organism resulting from work and disappearing after rest. In experimental settings, the reduction in muscle performance during prolonged stimulation is associated with the accumulation of Metabolic waste products (phosphoric and lactic acids) that affect the excitability of The Cell membrane, as well as with the depletion of energy reserves. Prolonged muscle work decreases Glycogen reserves within the muscle, thereby disrupting the synthesis of ATP required for contraction.

Under normal conditions, fatigue affects the central nervous system first, then the neuromuscular junction, and lastly the muscle itself. I. M. Sechenov proved that the temporary restoration of work capacity in a fatigued arm can be achieved by engaging the muscles of the other arm or the lower limbs in work. He viewed these findings as evidence that fatigue develops primarily within the nerve centers.

Muscle training increases their performance capacity, thickens muscle fibers, raises their glycogen content, increases the oxygen utilization coefficient, and allows recovery processes after muscular work to proceed faster than in untrained individuals.

The primary purpose of muscles stems from their contractile function and consists in executing various motor acts. This underpins human locomotor and labor activity. To perform this function, skeletal muscles convert chemical energy into mechanical energy while releasing a large amount of heat. In the vivid expression of I. P. Pavlow, skeletal muscles act as a "stove" that warms the body, meaning muscles perform a heat-producing function.

Muscles play a colossal role in human cognitive activity. They contain a vast number of proprioceptors that determine body position in space, muscle tone, and the degree of muscle contraction. The Significance of muscle proprioceptors increases substantially in individuals with impaired Vision or Hearing.

Skeletal muscles assist Heart function by acting as a pump. They have a very rich blood supply, and during activity, BLOOD FLOW IN muscle vessels increases 20- to 30-fold. During contraction, muscles facilitate the suction of blood into the venous vessels (the suction effect), thereby promoting the Circulation of BLOOD AND Lymph.

The configuration of the human body depends on the arrangement of Introduction/39.html">Muscles and their development. Consequently, skeletal muscles perform a shaping or form-forming role.

Finally, Muscles Attached to the skin give the face a specific expression and thereby reflect a person's psycho-emotional state, serving as an outward expression of their inner world. This function is particularly important in medical practice for diagnosing and assessing a patient's psycho-emotional state.