Human Anatomy - H. I. Koliadenko 2009

The Musculoskeletal System
General Overview
Phylogeny of the Human Skeleton

The human Musculoskeletal System comprises the Skeleton and striated skeletal Muscles. This system enables humans to adapt to environmental conditions, move through space, and perform a wide range of movements.

Throughout development (phylogeny and ontogeny), The Musculoskeletal System undergoes significant changes. Even in adults, it continuously improves through physical labor and occupational activities, steadily forming complex motor skills.

Conventionally, the musculoskeletal system is divided into passive and active parts: the skeleton represents the passive component, while muscles serve as the active one. The skeleton consists of bones connected either movably via joints or immovably (synostoses), as seen in the pelvis and Skull. Joints allow bones to move relative to one another (flexion, extension, abduction, adduction, etc.), thereby facilitating dynamic Muscle work. Bones connected immovably generally form cavities that house vital Internal Organs; for instance, the skull protects the Brain. The musculoskeletal system performs both dynamic and static Functions (such as standing and sitting). Furthermore, dynamic activity is predominantly performed against a backdrop of static muscle work—for example, walking is initiated from a standing posture. Both dynamic and static Functions of the musculoskeletal system are made possible by its active component: skeletal muscles, which attach to bones via tendons and contract in Response to nerve impulses originating from motor centers in the BRAIN AND SPINAL cord. Changes in muscle tension occur reflexively through the central and peripheral nervous systems (reflex arcs). Proprioceptors, as well as receptors in tendons, joints, and muscles, continuously inform nerve centers about the functional state of the muscles, allowing the body to fine-tune the musculoskeletal system's activity in response to environmental changes and shifts in organ functionality.

□ Phylogeny of the spine. An Axial Skeleton in the form of a notochord first appeared in a primitive chordate, amphioxus. The notochord is a resilient, strong, elastic cord running along the length of the body.

In Fishes, THE Vertebral Column is bony and divided into two regions: the trunk (with Ribs) and the tail (without ribs). In higher vertebrates—reptiles, birds, and mammals—the bony spine comprises five distinct regions: cervical, thoracic, lumbar, sacral, and caudal. Each region consists of vertebrae. While the number of vertebrae varies among different animal species, it remains constant within any given species. In fishes, the vertebrae are uniform, and the HEAD is joined immovably to the trunk. Amphibians are the first to develop an atlas, which articulates with The Skull and allows for some head mobility. Amphibians also exhibit the first true Sternum, and a pelvic bone attaches to a single sacral vertebra.

In reptiles, the spine is divided into cervical, thoracic, and lumbar regions, with ribs developing exclusively in the thoracic region.

In mammals and humans, ribs are preserved only in the thoracic region. In the cervical and lumbar regions, rudimentary ribs have fused with the transverse processes of the vertebrae, whereas in the sacral region, they have become incorporated into the lateral sacral crests.

□ Phylogeny of the skull. In cartilaginous fishes, the skull is likewise cartilaginous and divided into two parts: the neurocranium (braincase) and the viscerocranium (visceral skeleton). The neurocranium in sharks is continuous, featuring lateral depressions for the eyeballs and enclosing the brain, auditory organs, and olfactory organs. The visceral skeleton articulates movably with the braincase and consists of the jaw arch, hyoid arch, and five gill arches.

In chondrostean fishes (such as sturgeons), dermal bones—derived from the Connective Tissue of the head Skin—make their first appearance.

In teleost fishes, Cartilage ossification occurs in certain areas, leading to the Formation of secondary bones.

In amphibians, the extent of ossification increases, though a significant amount of cartilage still remains within the skull.

In reptiles, ossification is nearly complete. In birds, certain skull bones fuse together, resulting in a reduced overall bone count.

In mammals and humans, the skull is fully bony, with remnants of cartilage persisting only in the nasal septum.

□ Phylogeny of the limbs. Limbs first appeared in ancient amphibians (stegocephali) as modified paired fish fins. Such a limb possessed five digits—a trait preserved across major animal groups and humans. In certain vertebrates, this Structure adapted to their specific modes of life. For instance, in birds, the number of carpals and digits decreased as the forelimb transformed into a wing. Some mammals developed digitigrade locomotion. In other animals, the reduction of digits led to the evolution of protective adaptations, namely hooves: perissodactyls developed a single functional digit, whereas artiodactyls retained two digits, each encased in a hoof.

In terrestrial vertebrates, the skeleton comprises the pectoral and pelvic girdles along with the free forelimbs and hindlimbs. The forelimbs consist of the arm, forearm, and hand, while the hindlimbs comprise the thigh, leg (shank), and FOOT.

The Pectoral Girdle of fossil amphibians differs significantly in structure. It consists of the scapula, a procoracoid bone (which fused with the scapula at various stages of evolutionary development to form the coracoid process), and dermal clavicles. The Pelvic Girdle of a stegocephali consists of the ilium, ischium, and pubis, interconnected by a broad cartilaginous layer that houses the acetabulum for articulation with the free lower limb. In reptiles, these three bones fuse into a single innominate bone within the region of the acetabulum.

□ Bone tissue. Examining a cross-section of bone under a Microscope reveals that it is composed of orderly arranged bone lamellae formed by Collagen fibers, which are impregnated with an extracellular bone matrix and contain osteocytes.

Bone lamellae are arranged concentrically around a Haversian canal, which transmits a Blood capillary. Together, these lamellae, the canal, and the capillary form the fundamental structural unit of bone: the osteon. The central canal of the osteon facilitates the innervation and vascularization of the bone.

The number of bone lamellae varies within each individual osteon.

The spaces between osteons are filled with interstitial lamellae that interconnect the adjacent osteons. Interstitial and circumferential bone lamellae are located on the outer and inner surfaces of compact bone tissue, where they are appropriately termed outer circumferential and inner circumferential lamellae.