Human Anatomy - H. I. Koliadenko 2009

The Doctrine of Muscles (Myology)
General Information

Muscle Structure. Depending on The structure of the Muscle tissue and the work performed, Muscles are classified into striated and smooth. Cardiac muscle occupies an intermediate position between them, combining the Structure and properties of both groups.

Smooth muscles are part of Internal Organs and Blood and Lymphatic vessels. In the walls of internal organs and vessels, they are arranged in two layers: an outer longitudinal layer and an inner circular layer. The structural unit of smooth muscles is the myocyte (Fig. 63). Myocytes range from 60 to 100 microns in length, and each Cell contains a single Nucleus. Myocytes contain thin Actin and thick Myosin filaments, as well as Intermediate filaments (protofibrils). In addition, smooth muscle myocytes contain an Endoplasmic reticulum, Ribosomes, Mitochondria, and other Organelles. Smooth muscles are characterized by high plasticity—after stretching, they can maintain their original length acquired through stretching for a long time. Unlike striated muscles, smooth muscles contract involuntarily, very slowly, and most often rhythmically (for example, pendulum-like and peristaltic Movements of the intestines).

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Fig. 63. Middle part of a smooth muscle cell

Striated muscles (Fig. 64) are also called skeletal muscles because they move the Skeletal System and constitute the active part of The Musculoskeletal System. Muscle contractions drive various movements associated with human labor processes.

A Skeletal Muscle fiber has a cylindrical shape, ranging from several microns to 12.5 cm or more in length, and a thickness of 10–20 microns.

The sarcoplasm of a skeletal fiber contains mitochondria, which provide energy processes for active muscles. In addition, skeletal fibers, like all Cells, contain ribosomes, Lysosomes, the sarcoplasmic reticulum, and other organelles. However, skeletal fibers contain specific formations—thin contractile filaments called myofibrils. Under a Microscope, dark and light bands (discs) are visible in each myofibril. In all fibers of a muscle, both dark and light discs are located at the same level, which gives the muscle fiber a striated appearance. Dark discs refract light twice, whereas light discs refract it once.

The dark disc, also called the anisotropic (A) disc, is divided by a light band (M), while a dark line (7), known as the telophragma (Z-line), runs across the center of the light, isotropic (I) disc.

As it turned out, myofibrils are not continuous along the length of the skeletal fiber; they consist of individual protofibrils (myofilaments) 1–2 µm long. Furthermore, protofibrils are heterogeneous, comprising thick (16 nm) and thin (5–7 nm) filaments. A thick protofibril contains 180–360 longitudinally oriented molecules of the protein myosin, while a thin protofibril is formed by tiny filaments of another protein, actin, which has the appearance of a double helix.

Between the thick and thin protofibrils are tiny protoplasmic bridges formed by the meromyosin protein of the thick protofibril.

Upon muscle fiber excitation, these protoplasmic bridges push the thin protofibrils between the thick ones, causing the sarcomere and the entire muscle fiber to shorten. Since a Muscle consists of many fibers, it contracts As a result, moving the bone to which it is attached.

Fig. 64. Striated muscle fibers:

1 — Cytoplasm; 2 — nuclei

Thin protofibrils penetrate through the telophragma, attach to it, and continue to pass between the thick myofibrils.

At the boundary between the A and I discs, the sarcolemma folds to form T-tubules, which branch inside the fiber. All striated muscle fibers feature a well-developed agranular endoplasmic reticulum that surrounds each sarcomere. These networks interconnect, and their channels at the boundaries between sarcomeres form terminal cisternae that lie parallel to and in contact with the T-tubules.

The sarcoplasm of skeletal fibers is rich in the protein Myoglobin, which, like blood Hemoglobin, binds oxygen. Depending on the myoglobin content, muscle fibers are classified as red or white. Red fibers contain significantly more myoglobin and mitochondria than white ones. White fibers are thicker and contract faster than red ones, which is why they fatigue more quickly. Human skeletal muscles contain both types of fibers, whereas bird muscles are richer in red fibers. In chickens, conversely, white fibers predominate in the muscles.

Muscle fibers contain up to 75% Water and 25% dry matter, which includes Proteins, fats, CARBOHYDRATES, mineral salts, and extractives. Each fiber has a two-layered membrane (sarcolemma), beneath which the sarcoplasm contains numerous nuclei (from several to hundreds). Because of this structure, the fiber does not conform to typical cellular features and is referred to as a syncytium (symplast).

Skeletal Muscle contraction is accompanied by heat production, which helps maintain a constant body Temperature. Skeletal muscle contractions also facilitate the movement of BLOOD AND Lymph through blood and Lymphatic vessels, the Absorption of food in the digestive tract, and overall bodily Functions.

The mass of skeletal muscles in an adult accounts for 30–35% of body weight, in newborns 20–25%, in elderly and senile individuals it decreases to 25–30%, and in well-trained athletes it increases to 40% or more.

Humans have about 400 skeletal muscles.

The functional unit of striated muscles is the muscle fiber.

The muscle as an organ. The word "muscle" comes from the Latin *musculus* (meaning "little mouse"), because when contracting, a muscle resembles a mouse with a HEAD and a tail.

A skeletal muscle is built of bundles of striated muscle fibers enveloped in loose Connective Tissue that connects them to one another. Each bundle contains numerous muscle fibers that form independent motor units. Motor units can contain anywhere from 5–8 to 2,000 or more muscle fibers. Each motor unit is innervated by a single motoneuron.

A muscle consists of a belly and two tendon ends.

The proximal end of a muscle is called the head (or origin), and the distal end is called the tail (or insertion). Tendons extend from both the head and the tail, anchoring the muscle to bones or fasciae.

Tendons are composed of Cytology/practical/45.html">Dense connective tissue fibers. Encompassing the ends of the muscle fibers, tendential fibrils merge with the muscle sarcoplasm, establishing a robust connection between the tendon and the muscle. Unlike the muscle belly, head, and tail, tendons are white in color; they lack Blood Vessels but are rich in nerve endings and Collagen fibers, while containing very few elastic fibers. Consequently, tendons exhibit exceptional resistance to stretching.

A muscle consists of fiber bundles of various orders. First-order bundles combine to form second-, third-, and higher-order bundles. The internal connective tissue enclosing these bundles is called the endomysium, whereas the connective tissue wrapping the muscle externally is termed the perimysium (Fig. 65).

In addition to the primary muscle structures described above, a muscle possesses accessory apparatuses: fasciae, bursae, tendon sheaths, fascial sheaths, intermuscular septa, fascial nodes, tendon and muscle retinacula, osteofibrous or fibrous sheaths, and pulleys.

Fasciae. Every individual muscle or muscle group (e.g., the flexor and extensor groups of the arm or forearm) is enveloped in a thin layer of connective (fibrous) tissue known as fascia. Fascia separates one muscle group from another. Certain layers of the branching fascia penetrate deep between muscles and are therefore referred to as deep fascia. In the limbs, extensions of the fasciae project between muscle groups and attach to the periosteum of bones. Fasciae can serve as sites for muscle attachment and act as a barrier preventing the spread of infection or inflammatory processes from one muscle group to another. Fasciae are surrounded by the nerves, blood vessels, and lymphatic vessels that supply the muscles.

Fig. 65. Structure of a striated muscle:

a — muscle between two bones; b — isolated muscle fiber (enlarged); 1 — periosteum; 2 — fiber cross-section; 3 — myofibrils; 4 — muscle fiber nuclei; 5 — connective tissue nucleus; 6 — sarcolemma; 7 — perimysium of an individual muscle fiber; 8 — muscle fiber originating and terminating on bone; 9 — external perimysium; 10 — synovial bursa; 11 — periosteum; 12 — tendon fibers; 13 — muscle fiber originating from bone as a tendon and terminating within the muscle; 14 — muscle fiber terminating at both ends within the muscle; 15 — muscle fiber originating from bone and terminating within the muscle

The structure of fasciae depends on Muscle Function and strength. They are best developed in individuals engaged in physical labor and in athletes.

Synovial bursae are located at the sites where muscle tendons attach to bones and predominantly around major joints to reduce friction between bone and tendon. They are formed from connective tissue containing smooth-walled cavities filled with a small amount of synovial fluid. They develop immediately after birth.

Synovial tendon sheaths are cylindrical sac-like structures formed from connective tissue. A sheath consists of two layers: one adheres directly to the tendon, while the other envelops the tendon externally. Synovial fluid is contained between these layers. During muscle contraction, the tendon glides alongside the attached layer of the sheath, and the synovial fluid minimizes friction. Such sheaths are found surrounding the Phalanges of the hand and FOOT.

Ligaments represent thickenings of connective tissue or fascia. They are strong, glistening fibrous bands situated over muscle tendons, forming a cuff attached to bony prominences to hold tendons securely in place during muscle contraction.

Thickenings formed by fasciae between individual muscle groups are called intermuscular septa.

Fascial nodes are thickenings of fascia situated at junctions where two fascial sheets meet. They reinforce the fascial sheaths of blood Vessels and nerves. All of these fibrous structures merge with the bones and Complement the Skeleton.

In the region of certain joints (such as the wrist and ankle), fasciae become thickened to form tendon or muscle retinacula. Retinacula prevent the displacement of tendons and muscles during contraction. Muscle tendons pass through the underlying cavities, known as osteofibrous or fibrous sheaths. In some cases, a single fibrous sheath is shared by multiple tendons.

Pulleys are located on the epiphyses of bones as Cartilage-covered ridges with grooves. A tendon glides smoothly over these pulleys without shifting or altering its direction. Furthermore, pulleys increase the leverage of the muscle fiber, thereby enhancing muscle strength.

Sesamoid bones, which are dermal derivatives, act as pulleys. They form when the direction of a tendon shifts due to various movements not accounted for by the standard morphological layout of the existing structures. The largest sesamoid bone is the Patella, with significantly smaller ones including the pisiform bone and others.

Muscle contraction. Each motor unit is innervated by a single motor neuron. Where the nerve ending meets the muscle fiber, a neuromuscular synapse forms, presenting as a motor end plate.

Between the nerve ending and The surface of the muscle fiber lies a synaptic cleft 20–60 nm wide, across which the Nerve Impulse (excitation) from the motor nerve fiber is transmitted to the muscle fiber via the neurotransmitter of the motor end plate. Subsequently, the excitation spreads in both directions along the fiber's sarcolemma, causing Calcium Ions to be released from its sarcoplasmic reticulum and enter the myofibrils. During this phase, actin acquires The ability to interact with the protein myosin, leading to the contraction of the muscle fiber and the entire muscle, as a significant portion—if not all—of the muscle's motor units are typically excited in the process.

Skeletal bones move only when a muscle spans two or more joints, actuating the bones connected by them. During muscle contraction, one of its attachments always remains stationary and fixed, known as the punctum fixum, while the other end is termed the punctum mobile. When contracting, a muscle shortens in length by about 1/3, drawing the movable point toward the fixed point. The concepts of movable and stationary points are relative; during certain movements, these roles may interchange. For instance, when the biceps brachii contracts, it brings the forearm closer to the trunk, with the fixed point located in the scapular region. Conversely, during a pull-up on a pull-up bar, contraction of the biceps draws the scapula and trunk toward the forearm, placing the fixed point on the forearm while the movable point shifts to the scapula.

By contracting, a muscle performs a great deal of work and, as an active organ, is characterized by an intensive METABOLISM sustained by blood vessels that deliver oxygen and nutrients while removing Metabolic waste products. The density of blood vessels in a muscle depends on the work it performs; they are abundant, for example, in the Diaphragm and significantly sparser in the biceps brachii. Muscle activity is regulated by The Nervous system. The nerve fibers present within muscles are known as proprioceptors. Muscle tissue contains specialized receptors in the form of muscle spindles, which perceive and reflect the degree of muscle contraction and stretch. Excitation from the proprioceptors of muscles, ligaments, and tendons travels via sensory (afferent) fibers to the Central Nervous System's motor centers, and from there back to the muscles, modulating muscle tone and correcting their activity. This gives rise to the so-called muscle sense, which enables the perception of the body's position or that of its individual parts in space. Muscles also terminate sympathetic nerve fibers, which regulate metabolism by inducing vasodilation or vasoconstriction.

Based on morphological characteristics, every muscle can exist in three states: resting (initial), elongated, and shortened; functionally, it can be either tense or relaxed. Consequently, during contraction, a muscle may exhibit five distinct states.

1. The muscle is elongated and tense; it is stretched and firm, with its origin and insertion situated far apart.

2. The muscle is in its initial position and tense. In this state, the muscle is firm, and the distance between its origin and insertion remains unchanged.

3. The muscle is shortened and tense. In this state, it is firm, and its origin and insertion are brought closer together.

4. The muscle in its initial position is relaxed. In this state, it is unstressed, soft, and its points of attachment remain unchanged.

5. The muscle is contracted and relaxed. In this state, it is soft, sags under its own weight, and its points of origin and insertion are brought very close together.

Muscles are capable not only of contraction and relaxation, but also of continuously maintaining muscle tone, which allows them to resist stretching. Muscle tone is assessed by its firmness: the higher the tone, the greater the muscle's working capacity. Tone is regulated by the central nervous system and is reflex in nature, meaning it arises from nerve impulses generated in the muscle proprioceptors, especially during stretching. When nerves are severed, muscles lose their tone and become extremely soft.

Muscle shape and nomenclature. For the most part, the shape of a muscle depends on the work it performs. The main muscle groups are considered to be: long, short, broad, and circular.

Long spindle-shaped muscles feature a central belly and tendons at both ends. The proximal tendon is called the head, and the distal tendon is the tail. Such muscles are found in the limbs, where a wide range of motion is required.

Short muscles are found where the range of motion is minimal, such as between the vertebrae of the spinal Column, because the length and width of these bones are small.

Broad and flat muscles are located primarily on the trunk—specifically the Muscles of the abdomen, back, and chest. When arranged in multiple layers, they form strong walls of Body Cavities to protect internal organs, while the multidirectional arrangement of their muscle fibers facilitates various functions. Broad muscles have correspondingly broad tendons covering large surface areas, which are known as aponeuroses.

Circular muscles are located around the openings of the body (e.g., the orbicularis oculi and orbicularis oris). Also known as sphincters, they contract to close openings, whereas dilators do the opposite by opening them.

Other recognized muscle shapes include deltoid, rhomboid, quadrate, trapezoid, serratus, soleus, piriform, arytenoid, pyramidal, round, and triceps (Fig. 66). Depending on how many joints a muscle acts upon, they are also classified as monoarticular, biarticular, or multiarticular. An example of a monoarticular muscle is the deltoid, which crosses only the shoulder joint and therefore performs abduction, flexion, and extension at that joint. The quadriceps femoris is a biarticular muscle because it runs down the anterior thigh across two joints: the hip and the knee. At the hip joint, this muscle flexes the leg, and at the knee, it extends it. Most forearm muscles are classified as multiarticular because they cross several JOINTS OF THE wrist and hand.

Muscles whose bellies are divided by tendinous inscriptions are called digastric or multicipital accordingly. Some broad muscles contain several tendinous layers (e.g., the rectus abdominis). Depending on the number of tendinous heads by which muscles originate on bones, they are referred to as bicipital, tricipital, or quadriceps (Fig. 67).

According to the direction of their fibers, muscles are subdivided into straight (parallel), transverse, and oblique (Fig. 68).

Based on their function, muscles are termed: flexors, extensors, abductors, adductors, those that rotate inward around a vertical axis are pronators, and those that rotate outward are supinators.

Muscles that perform the same movements are called synergists (e.g., the flexor carpi radialis and flexor carpi ulnaris), while those that perform movements in opposite directions are antagonists. For example, the pronator teres and pronator quadratus of the forearm are antagonists to the forearm supinator.

P. F. Lesgaft divided all muscles into two types: strong and agile. He wrote: "Muscles that are predominantly strong originate and insert onto large surfaces; as they extend away from the fulcrum of the lever upon which they act due to their expanded attachment surface, they are capable of developing considerable force with minimal tension, and thus do not fatigue as quickly. Strong muscles act predominantly with their entire mass and cannot execute fine nuances of movement; they manifest their strength with relatively low speed and more frequently consist of short muscle fibers. Muscles of the second type are characterized by agility in their actions; they originate and insert onto small surfaces, close to the fulcrum of the lever they act upon. They work under high tension, fatigue more quickly, predominantly consist of long fibers, and are capable of acting in separate parts, exhibiting various nuances of movement. These muscles primarily perform agile, rapid movements." Examples of strong muscles include the erector spinae and the quadriceps femoris, whereas agile muscles include, for instance, the facial expression muscles.

Fig. 66. Muscle shapes:

1 — deltoid; 2 — rhomboid; 3 — quadrate; 4 — trapezoid; 5 — serratus; 6 — soleus; 7 — piriform; 8 — lumbrical (vermiform); 9 — circular; 10 — pyramidal; 11 — round; 12 — triangular

Fig. 67. Muscle names related to their structural features:

1 — digastric; 2 — semimembranosus; 3 — semitendinosus; 4 — biceps; 5 — triceps; 6 — quadriceps

Strong muscles contain a rich supply of blood vessels and muscle pigment (myoglobin), giving them a dark red color, which is why they are referred to as red muscles. During activity, they exhibit great strength with minimal tension and can work for extended periods without fatigue. The speed and range of motion during their contraction are minimal. These muscles perform static work, maintaining the body in a required posture (e.g., upright standing).

Agile muscles, according to Lesgaft, are characterized by long, parallel-arranged fibers. They have a sparser Blood supply and are poorer in hemoglobin, which is why they are also called white muscles. These muscles contract actively, work under considerable tension, fatigue rapidly, but are capable of performing a series of fine, intricate operations that strong muscles are incapable of.

Fig. 68. Muscles with different fiber directions:

1 — fusiform; 2 — unipennate; 3 — bipennate; 4 — multipennate

Muscle strength depends on the number of their fibers: the more fibers there are, the stronger the muscle. In addition, muscle strength is influenced by the direction of these fibers. Specifically, it has been established that muscles with oblique and pennate fiber arrangements are stronger than muscles with parallel fibers, i.e., fusiform ones. In this case, muscle strength is determined by the physiological cross-section of the muscle, which refers to the plane of its cross-section perpendicular to the length of the fibers. If the fibers are parallel to the long axis of the muscle, its physiological cross-section will equal its anatomical cross-section.

The anatomical cross-section is the plane of a muscle's cross-section perpendicular to its long axis. When muscle fibers are arranged obliquely, the physiological cross-section is significantly larger than the anatomical one. Therefore, muscles with an oblique (pennate, bipennate) fiber arrangement are much stronger.

Each square centimeter of a muscle's physiological cross-section can support 8—10 kg. Consequently, the muscle strength for the forearm flexors is approximately 160 kg, and for the leg flexors, 480 kg. At first glance, these data may seem exaggerated because the loads a person can lift when flexing the forearm and leg are significantly smaller. It must be remembered that realizing a muscle's natural potential depends on several factors, with the leading role belonging to the point of load application and its distance from the joint axis, which determines the torque, among other things. The torque will be greater the further the point of load application is from the joint and the muscle lifting it.