The Musculoskeletal System The structures that form the musculoskeletal system primarily provide support and mobility to the human body. To fulfill its function, it consists of a large number of bones that are connected to each other through joints and muscles, which develop the necessary force for body movement. The histological basis of the skeletal system is the bone tissue. Bones are made up of bone tissue and are arranged in a specific way, forming a complex and interconnected system. The joint system that includes every point of contact between the elements of the skeleton is highly variable and includes a large number of shapes, sizes, and functional possibilities. Lastly, the muscular system serves as a machine for both of the above systems, as it can convert chemical energy into mechanical energy for body movement.

Physiology of Bones The musculoskeletal system consists of a large number of bones that are connected to each other through joints and over 600 muscles. The bone structures are organized in such a way as to form the skeleton, which consists of a central axis and four limbs. The types of joints differ to ensure a wide range of movements. The muscles are arranged in layers and attached to the bones at different points to serve the mobility needs of the musculoskeletal system.

The adult skeleton consists of 206 different bones. It is divided into two parts: the trunk skeleton and the limb skeleton. The trunk skeleton includes 80 bones, while the limbs consist of 126 bones organized in extensions of the axis. The bones of the shoulder girdle, pelvis, as well as the upper and lower limbs, constitute the peripheral parts of the skeleton. On the other hand, the trunk skeleton includes all the bones of the skull, face, spinal column, sternum, and ribs.

The skeleton of the limbs is articulated with the skeleton of the trunk to provide support and mobility to the limbs. The clavicle and the scapula form the shoulder girdle, which is connected to the bones of the upper limbs through the articulation of the shoulder. The bones of the pelvis, including the sacrum and coccyx, form the pelvis. The pelvis has a stable round base that supports the trunk and serves as a point of support for the lower limbs. There are significant differences between the male and female skeleton. The male skeleton is usually larger and heavier than the female skeleton. The main differences are related to the shape of the pelvis, as the female pelvis is adapted for reproductive function.

Long bones are formed from two types of bone: compact and spongy. Compact bone consists of cylindrical anatomical units, the Haversian systems (or osteons), which are arranged parallel to their long axis in long bones. The spongy substance is located on the inner surface of the long bones, next to the medullary cavity. It is composed of thin bony lamellae, the trabeculae.

Osteoblasts are small cells that synthesize and secrete the organic components of the bone matrix, known as osteoid. Osteoid is a crucial component of the bone matrix, and upon it the mineralized bone is formed. Osteoblasts have high metabolic activity and are considered the cells responsible for bone formation. They have abundant endoplasmic reticulum, mitochondria, and secretory vesicles. Their activity increases during the stage of bone growth.

Osteocytes are mature bone cells that originate from osteoblasts. When osteoblasts complete the production of osteoid, most of them become inactive. However, others become trapped in the mineralized bone matrix and are enclosed within small cavities of the bone called lacunae. When surrounded by the matrix, these cells are called osteocytes and play a significant role in the solidification of bone mass.

Osteoclasts are large, multinucleated cells with abundant cytoplasm. It is believed that they originate from cells of the mononuclear phagocytic system, although it is still unknown whether they arise from the fusion of multiple mononuclear cells or repeated nuclear mitosis of mononuclear cells without cytokinesis. During bone resorption, they remain attached to the surface of bones, especially in small cavities created in bones by osteoclasts.

Bone is a living, dynamic tissue that undergoes continuous microscopic changes, all of which contribute to skeletal development, bone resorption, and remodeling. This activity is carried out by two types of cells: osteoblasts and osteoclasts. Osteoblasts are responsible for producing the components of the bone matrix, both by synthesizing organic material and by releasing inorganic ions involved in its mineralization. Bone resorption is carried out by osteoclasts, which secrete acids and lysosomal enzymes at the cellular level in contact with the bone surface. Enzymes break down the proteins of the matrix, and the low pH causes the breakdown of calcium crystals and the demineralization of bone tissue. The balance between bone formation and bone resorption depends on the type of stimuli received by the cells responsible for both functions.

Muscle Physiology

The body is composed of more than 600 skeletal muscles, which together make up about 50% of body weight. The shape of the body is determined by the skeleton, muscles, and subcutaneous fat. The way in which muscles are arranged and related to each other, as well as their relationship with the joints, such as their connection to the bones of the skeleton, determines the voluntary movements of the body. These are the result of the coordinated action of several muscles. As we move, some muscles contract while others relax.

Skeletal muscles are organized in an extremely specialized way. This organization allows muscles to contract when stimulated and relax as soon as the stimulus disappears. Their ability to contract or contract (shortening their belly) allows muscles to pull bones to produce movement. Structurally, the skeletal muscle is formed by contracting bundles of muscle fibers. These are made up of smaller fibers, known as muscle fibers, which are arranged in sarcomeres, the functional units of skeletal muscles. All of these formations are surrounded by membranes (epimysium, perimysium, and endomysium).

Muscle fibers are skeletal muscle cells, so called due to their elongated shape (ranging from 1 to 40 mm in length). Each muscle fiber is composed of a large number of myofibrils surrounded by a membrane called the sarcolemma. The cytoplasm of the muscle fiber is called sarcoplasm. They are multinucleated and contain a large number of mitochondria and a specialized endoplasmic reticulum called the sarcoplasmic reticulum. The myofibrils that run the length of the muscle tissue are composed of thinner subunits called myofilaments. The two main types are thick and thin filaments.

Each muscle fiber contains over a thousand subunits called myofibrils that are arranged in parallel. Myofibrils are composed of thousands of repeated sequences of thick and thin filaments, which are made up of the contractile proteins actin and myosin. The specialized arrangement of these filaments is critical to the mechanism of contraction: the sarcomere, the contracting unit of striated muscle. This arrangement allows for different zones and stripes to be identified, which change depending on whether the muscle is in a relaxed or contracted state.

When a muscle fiber is in a relaxed state, the thin filaments containing actin do not extend to the center of the sarcomere (the H zone). Instead, the thick filaments containing myosin do not attach to the Z lines and simply traverse the A bands of the sarcomere. When a muscle contracts, the thin filaments slide over the thick ones, causing the sarcomere to shorten and altering the length and arrangement of the zones and stripes mentioned earlier. The sliding of the muscle filaments causes the muscle fiber to shorten each time it contracts. As soon as the stimulus stops, the filaments return to their resting position and the sarcomere returns to its original length.

Each muscle fiber is innervated by a motor neuron. The motor neuron, together with the muscle fibers on which it acts as a motor unit, is the functional unit of skeletal muscle. A neuromuscular junction is a specialized synapse that promotes the transmission of the action potential of the motor neuron from the motor neuron to the final motor endplate of the muscle fiber. Acetylcholine is the neurotransmitter in all neuromuscular junctions. Its binding to the nicotinic receptors of the sarcolemma induces muscle contraction.

The molecular steps for muscle contraction require the interaction of four proteins: actin, myosin, tropomyosin, and troponin, as well as calcium ions and ATP molecules as an energy source. The cross-bridges of myosin move towards the resting position as soon as an ATP molecule binds to them and provides energy. Calcium released from the sarcoplasmic reticulum binds to the troponin of thin filaments, forcing tropomyosin to change position. In this way, myosin cross-bridges bind to the active sites of thin filaments, causing the breakdown of ATP into ADP and Pi. The release of stored energy generates the necessary force for the bridges to return to their initial position, pulling the actin. Each cross-bridge will remain bound to the actin until another ATP molecule binds to them and returns them to the resting phase.

Trunk and Spine

The posterior muscles of the trunk play a significant role in the mobility of the trunk. Thus, muscles such as the trapezius, latissimus dorsi, and the teres major work collaboratively to serve the coordinated movements of the trunk and upper limbs. Oblique abdominal muscles such as the rectus abdominis and the latissimus dorsi are responsible for the rotation of the trunk.

The muscles of the middle layer function similarly to the superficial muscles and are responsible for other functions as well. For example, the splenius muscles – the capitis and cervicis – and the semispinalis muscles are responsible for the rotational movements of the head in relation to the trunk. They also serve as auxiliary respiratory muscles.

All the muscles on the posterior surface of the trunk have common functions. However, there are functions of the muscles that are essentially specific to the deep level, and certainly, the most relevant function is the movement of the spine. Therefore, the range of movements of the spine is significant, although it is partly limited by the bones and ligaments that make up the spinal column.

Under conditions of normal rest, the prevertebral muscles are rarely activated. This is because, under normal circumstances, ventilation occurs without active participation of the thoracic cage. However, when there is high respiratory frequency, these muscles are vital and are considered the most important accessory respiratory muscles.

The spinal column forms the vertical axis of the skeleton. It is a flexible structure, as it is formed by different segments. It consists of 24 vertebrae, the sacrum, and the coccyx. The joints between the vertebral bodies allow movements of the spinal column forward, backward, and sideways.

The spinal column forms the vertical axis of the skeleton. Divided into sections, the spinal column is a flexible structure made up of 24 vertebrae which articulate with the sacrum and coccyx. The joints between the vertebral bodies allow the spinal column to move forwards, backwards, and laterally. The spinal column articulates with the head, ribs, and pelvic bones to form a single structure. In a lateral view, the spinal column forms four curves: the cervical, thoracic, lumbar, and sacral curves which are determined by the number and arrangement of structures that make up the spinal column.

Each vertebra articulates with its immediately superior and inferior through the middle of the intervertebral discs. The contact points are located in the vertebral bodies, including the intervertebral discs, the facet joints, and the transverse and spinous processes of the vertebrae. These joints hold the vertebrae tightly together, preventing dislocation and allowing the spinal column to be flexible as a whole. A strong complex of ligaments helps maintain the stability of these joints.

Among the ligaments that hold the vertebrae together and maintain the stability of the spinal column is the anterior longitudinal ligament, a strong fibrous band that runs along the anterior surface of the vertebral bodies, connecting them together from the atlas to the sacrum. The posterior surfaces of the vertebral bodies, on the other hand, are connected to each other by the posterior longitudinal ligament. The interspinous ligaments connect the spinous processes of two adjacent vertebrae, while the ligamentum flavum tightly connects the laminae and articular processes of adjacent vertebrae. These ligaments are connected to the supraspinous and infraspinous ligaments that connect the spinous and transverse processes of the vertebrae.

Beyond shaping the axis of the skeleton and protecting the spinal cord, vertebrae serve as articulation and contact points with other anatomical structures. They have a lateral half facet that belongs to the costovertebral joint and is surrounded by the radiate ligament. They also have a superior articular process that articulates with the inferior articular process corresponding to the underlying vertebra’s spinous process. Both the transverse and lateral transverse ligaments are carried between the transverse processes and the sides. However, the lateral ligament is carried posteriorly.

The cervical vertebrae are seven in number. They are characterized by small vertebral bodies and short, bifid spinous processes, except for the seventh vertebra whose spinous process is prominent. The vertebral foramina are large and triangular in shape, while the transverse processes serve for the passage of the vertebral artery, vein, and neural network.

The thoracic or spinal vertebrae are the twelve vertebrae below the seven cervical vertebrae. Twelve pairs of ribs articulate with the thoracic vertebrae. Typically, they are larger than the cervical vertebrae and have lateral demi-facets for articulation with the ribs. The lateral demi-facets are located at the ends of the transverse processes and on the vertebral bodies. The spinous processes of the upper thoracic vertebrae are thin and gradually slope downward.

The lumbar vertebrae are the five vertebrae below the twelve thoracic vertebrae and above the sacral vertebrae. They have a large vertebral body, as they bear the majority of the body’s weight. The vertebral joints are triangular in shape, while the spinous processes are flat and square. The transverse processes make up the lateral part of the lumbar vertebrae.

Upper limbs The biceps brachii and brachialis muscles are superficial muscles that cover the anterior surface of the arm, while the triceps brachii is the most important muscle that covers the posterior surface of the arm. These are typically bulky muscles that develop significant power. The forearm contains a large number of muscles, which can be distinguished into flexors and extensors, which work together to develop muscular strength and coordination.

The anterior brachialis is a deep muscle of the arm that occupies the anterior surface, while similarly the triceps brachii is also a deep muscle occupying the posterior surface of the arm. From a functional perspective, the muscles of the forearm are distinguished into anterior flexors and posterior extensors. The rotational movements of the forearm cause pronation or supination of the hand.

Several ligaments, tendons, and muscles, as well as the joint capsule that surrounds the joint, provide stability to the shoulder joint. There are several ligaments that connect the scapula to the humerus bone and the clavicle. The coracohumeral and the glenohumeral ligaments, which represent thickenings of the joint capsule, run from the scapula to the humerus bone. The coracoclavicular ligament, in turn, extends from the clavicle to the scapula.

The shoulder joint is the articulation between the head of the humerus bone and the glenoid of the scapula. The labrum is fibrous cartilage that attaches to the glenoid and deepens the articular cavity. The articular surfaces of the bones are surrounded by the articular capsule, while the membrane of the fibrous capsule extends from the lip of the glenoid to the margins of the articular cartilage of the humerus bone.

In the shoulder joint, four synovial cavities are distinguished. The most important is the subdeltoid cavity, which is interposed between the inferior surface of the deltoid muscle and the superior surface of the articular capsule. The muscles and tendons of the shoulder merge and surround the capsule of the joint to reinforce and hold the humerus bone in the glenoid cavity of the scapula. These muscles include the supraspinatus, the infraspinatus, the teres minor, and the subscapularis, which form the rotator cuff. Another important muscle of the shoulder is the deltoid.

The distal (or lower) end of the humerus bone articulates with the proximal ends of the ulna and radius at the elbow joint. The lateral ligaments of the joint are thickenings (capsular ligaments) of the fibrous capsule that attach to the lateral epicondyle and the annular ligament of the radius on one side and to the bones of the forearm and the collateral ligament of the ulna on the other side. These ligaments prevent lateral displacement of the elbow.

The articular capsule is reinforced on both sides by lateral ligaments. It attaches to the humerus bone from the supratrochlear to the supinator crest, to the anterior surface of the coronoid fossa of the ulna, and to the anterior surface of the annular ligament of the radius. In addition, the articular capsule is lined by a synovial membrane that extends downward to the proximal radioulnar joint.

The elbow joint, like all hinge or angular joints, is formed by the mutual concave/convex articular surfaces that allow flexion and extension movements. This joint is reinforced by the articular capsule, the ligaments, and the muscles that attach to its lip. When the elbow is bent at a 90° angle, the articular surfaces come into close contact.

In the elbow joint, the trochlea of the humerus bone articulates with the trochlear notch of the ulna, while the capitulum of the humerus bone articulates with the head of the radius. The articular membrane of the hinge (or angular) joint allows for flexion and extension movements of the elbow. Additionally, the head of the radius articulates with the radial notch of the ulna, which allows for rotation of the forearm and pronation or supination of the antebrachium and hand.

The wrist and hand joint include the distal radioulnar, radiocarpal (or wrist), midcarpal, and carpometacarpal articulations. The distal ends of the radius and ulna are connected by a strong interosseous membrane, while the bones of the wrist are connected to the radius and ulna through the palmar and dorsal radiocarpal ligaments and the ulnar and radial collateral ligaments. These structures are connected to the joint capsule, which is also part of the joint.

The main anatomical structure of the distal radioulnar joint is the articular disc. The wrist is composed of eight bones arranged in two rows of four. There are sliding joints between all the adjacent bones of the wrist, between the distal and proximal carpal bones, and between the proximal carpal bones and the radius and ulna. These bones are connected by midcarpal and intercarpal ligaments. Additionally, the bones of the distal row of the wrist articulate with the metacarpal bones in the carpometacarpal joints.

The bones of the metacarpus articulate with the proximal phalanges at the metacarpophalangeal joints, which consist of a loose articular capsule, collateral ligaments, and palmar ligaments. This type of joint allows for flexion, extension, abduction, adduction, and circumduction of the fingers. Additionally, the proximal, middle, and distal phalanges of the fingers articulate with each other at the interphalangeal joints, which also consist of a loose capsule and palmar and collateral ligaments that allow for flexion and extension.

Lower limbs

The superficial muscles of the lower limbs are primarily responsible for providing the power needed to move the joints of this area of the body. For example, the function of the quadriceps femoris muscle is to extend the lower limbs, which is why it is considered an anti-gravity muscle. Another important muscle of the lower limb is the gastrocnemius, which participates in flexion of the tibia towards the femur and plantar flexion. The adductor muscles are located on the inner surface of the thigh and constitute an important muscular group of three muscles (the adductor magnus, longus and brevis), the action of which brings the lower limb towards the midline (adduction).

The intermediate-level muscles are also large for the same reason as the deep muscles. In the gluteal and thigh area, other functionally important muscles are on the anterior surface: the adductors, the pectineus, and the iliacus, and on the posterior surface, the semimembranosus and the semitendinosus muscles. At the same level, other important muscles are the long plantar muscle (with the plantaris muscle) and the extensor digitorum longus muscle.

The muscles of the lower extremities should be strong enough to maintain the head and trunk in an upright position and allow for walking. These muscles are divided into planes. In the deep plane of the lower extremity muscles, the gluteus minimus muscle, the long and short heads of the biceps femoris muscle, the obturator muscle, the long flexor of the toes, and the long flexor of the big toe muscle are included.

The hip joint is characterized by great stability. The bones that form the joint and contribute to its stability are held together by a strong joint capsule and several ligaments. The iliofemoral ligament connects the iliac portion of the hip bone to the femur, while the ischiofemoral and pubofemoral ligaments connect the femur to the ischial and pubic portions of the hip bone, respectively. The iliofemoral ligament forms an inverted Y shape and is one of the strongest ligaments in the body.

In posterior view, it appears that the hip joint’s ligaments connect the hip to the femur. Some fibers of the ligament help form a circular band (circular fibers of the articular capsule that form a ring around the neck of the femur). The articular capsule covers the neck of the femur from behind and does not reach the intertrochanteric crest.

The hip joint belongs to the articulations and connects the femur with the acetabulum of the ilium bone. It allows flexion, extension, abduction, adduction, rotation, and circumduction of the lower limbs. This joint is formed by the head of the femur and the socket of the acetabulum, which is deepened by fibrous cartilage, the acetabular labrum. The articular surfaces are held together by the articular capsule, the ligaments that reinforce it, and the round ligament of the head of the femur, which is independent of the capsule.

The knee, one of the joints that bears the largest part of the body weight, mechanically connects the tibia with the femur. It is a hinge joint that allows movements of flexion and extension. Primarily, this joint is held together by the patellar ligament, the anterior and posterior cruciate ligaments, the medial and lateral collateral ligaments, and the medial and lateral menisci. The joint capsule and the synovial membrane are also important parts of the joint.

The knee joint is formed by the distal end of the femur, the patella, and the proximal end of the tibia. Ligaments, menisci, and the fat pad contribute to the correct anatomical alignment of the joint surfaces. The patella articulates with the articular surface of the distal end of the femur. The synovial membrane covers the articular capsule and the suprapatellar bursa of the joint.

Several structures that contribute to the stability of the knee joint can be distinguished on the anterior surface of the knee. The patellar tendon is a continuation of the tendon of the quadriceps femoris muscle. It enters the patella and continues along the knee to attach to the anterior tibial tuberosity. The medial and lateral menisci are two fibrocartilaginous discs that are interposed between the femoral condyles and the tibial surface, preventing bone damage from bone-to-bone contact.

The main ligament on the posterior surface of the knee is the posterior cruciate ligament which extends from the inner condyle of the femur to the posterior surface of the tibial plateau. Near its posterior attachment to the tibia, a strong band called the posterior meniscofemoral ligament (or the ligament of Wrisberg) arises from the lateral meniscus. This ligament attaches to the inner condyle of the femur immediately behind the attachment of the posterior cruciate ligament. Sometimes, it is directed anteriorly and blends with the posterior cruciate ligament.

The anterior cruciate ligament is attached from the lateral femoral condyle to the intercondylar area of the tibia. It serves to prevent anterior dislocation of the tibia relative to the femur. In addition, the posterior cruciate ligament is responsible for preventing posterior dislocation of the tibia relative to the femur. Both ligaments play a critical role in maintaining the stability of the knee. The horns of the lateral and medial menisci are connected via the transverse ligament of the knee.

The inner and outer menisci attach to the upper end of the tibia, deepening the joint surfaces to receive the condyles of the femur. The medial collateral ligament connects the inner condyle of the femur to the inner condyle of the tibia, while the lateral collateral ligament connects the outer condyle of the femur to the head of the fibula. The function of the collateral ligaments is to prevent hyperextension of the knee.

The ankle joint is a hinge or angular joint for flexion and extension. In flexion, the dorsal surface of the foot moves towards the anterior surface of the body, while in extension, it moves away from the body. Flexion is limited by the tension of the posterior ligaments, and under extreme conditions, by contact between the tibia and the talus. Extension is limited by the anterior ligaments of the lateral ligament complex.

The joints of the foot include the joints between the proximal bones of the tarsus (talocrural joint), the joints between the distal bones of the tarsus, the transverse tarsal joint (or Chopart’s joint), the tarsometatarsal joints (or Lisfranc’s joint), the metatarsophalangeal joints, and the interphalangeal joints.