The Skeletal System: Bone Structure and Function

• The skeletal system provides support, protection, movement, mineral storage, and blood cell formation.
• It includes bones, cartilage, ligaments, and tendons.

Bone Functions

• Bones offer a supportive framework for the body and its soft organs.
• The skull protects the brain, while the rib cage protects the heart and lungs.
• Skeletal muscles attach to bones, using them as levers for movement.
• Bones act as a mineral reservoir, storing calcium and phosphorus.
• Hematopoiesis, or blood cell formation, occurs in the red marrow of bones.
• Fat is stored as triglycerides in the yellow marrow.
• Bones produce osteocalcin, a hormone that helps regulate insulin secretion, glucose homeostasis, and energy expenditure.

Bone Classification

• Long Bones: These are longer than they are wide; examples include the femur and humerus.
• Short Bones: Shaped like cubes, examples include carpals and tarsals.
• Flat Bones: These are thin, flattened, and often curved, such as skull bones and ribs.
• Irregular Bones: Bones with complex shapes, like vertebrae and hip bones.

Bone Structure

• Bones are organs that contains osseous tissue, nervous tissue, cartilage, connective tissue, and muscle tissue
• Compact Bone: A dense outer layer.
• Spongy Bone: An internal network of trabeculae.
• Periosteum: An outer membrane covering the bone surface, except at joints.
• Endosteum: A delicate membrane lining internal bone surfaces.

Microscopic Anatomy of Bone

• Osteon (Haversian System): The structural unit of compact bone.
• Lamellae: Concentric rings of bone matrix within an osteon.
• Central Canal (Haversian Canal): Contains blood vessels and nerves in the osteon's center.
• Lacunae: Small cavities housing osteocytes.
• Canaliculi: Tiny canals connecting lacunae to each other and the central canal.
• Perforating Canals (Volkmann's Canals): Connect the periosteum's and central canal's blood vessels and nerves.

Chemical Composition of Bone

• Organic Components: Include osteogenic cells, osteoblasts, osteocytes, osteoclasts, and osteoid (organic bone matrix).
• Inorganic Components: Primarily hydroxyapatites (mineral salts) such as calcium phosphate crystals.

Bone Development (Ossification)

• Intramembranous Ossification: Bone develops from a fibrous membrane, as seen in skull bones and clavicles.
• Endochondral Ossification: Bone develops by replacing hyaline cartilage, common in most bones.

Postnatal Bone Growth

• Interstitial Growth: Lengthens long bones.
• Appositional Growth: Increases bone thickness.

Bone Remodeling

• Bone deposit and resorption occur on the periosteum and endosteum surfaces.
• Remodeling Units: Consist of adjacent osteoblasts and osteoclasts.

Control of Remodeling

• Hormonal Controls: Parathyroid hormone (PTH) and calcitonin regulate blood calcium levels.
• Mechanical Stress: Wolff's law dictates that bones grow or remodel based on the stresses applied to them.

Bone Repair

• Hematoma Formation: Blood clots form at the fracture site.
• Fibrocartilaginous Callus Formation: Fibroblasts and chondrocytes create a soft callus.
• Bony Callus Formation: Osteoblasts replace the soft callus with spongy bone.
• Bone Remodeling: Osteoclasts remodel the bony callus.

Skeletal Divisions

• Axial Skeleton: Includes the skull, vertebral column, and rib cage.
• Appendicular Skeleton: Comprises the bones of the limbs and their girdles (shoulder and hip).

Skull

• Cranium: Encases and protects the brain.
• Facial Bones: Form the face, house special sense organs, and provide muscle attachment points.

Vertebral Column

• Cervical Vertebrae: The 7 vertebrae in the neck region.
• Thoracic Vertebrae: The 12 vertebrae in the upper back.
• Lumbar Vertebrae: The 5 vertebrae in the lower back.
• Sacrum: Formed by 5 fused vertebrae.
• Coccyx: The tailbone.

Rib Cage

• Ribs: 12 pairs of ribs.
• Sternum: The breastbone.
• Thoracic Vertebrae: Ribs attach to these vertebrae.

Appendicular Skeleton

• Pectoral Girdle: The clavicle and scapula, attaching the upper limbs to the axial skeleton.
• Upper Limb: Includes the humerus, radius, ulna, carpals, metacarpals, and phalanges.
• Pelvic Girdle: Hip bones (ilium, ischium, pubis), attaching the lower limbs to the axial skeleton.
• Lower Limb: Includes the femur, tibia, fibula, patella, tarsals, metatarsals, and phalanges.

Joints (Articulations)

• These are locations where two or more bones meet.
• Joints provide the skeleton with mobility and hold it together.

Classification of Joints

• Fibrous Joints: Immovable or only slightly movable; examples include skull sutures.
• Cartilaginous Joints: Slightly movable; examples include intervertebral discs.
• Synovial Joints: Freely movable, such as the knee and shoulder joints.

Synovial Joint Structure

• Articular Cartilage: Hyaline cartilage covers the ends of bones.
• Joint Cavity: Contains synovial fluid.
• Articular Capsule: An outer fibrous layer and an inner synovial membrane.
• Synovial Fluid: Lubricates and nourishes the articular cartilage.
• Reinforcing Ligaments: Strengthen the joint.

Types of Synovial Joints

• Plane Joint: Features flat articular surfaces, such as intercarpal joints.
• Hinge Joint: Allows flexion and extension, like the elbow joint.
• Pivot Joint: Permits rotation, such as the radioulnar joint.
• Condylar Joint: Allows flexion, extension, abduction, and adduction, as seen in metacarpophalangeal joints.
• Saddle Joint: Allows flexion, extension, abduction, adduction, and opposition, such as the carpometacarpal joint of the thumb.
• Ball-and-Socket Joint: Enables flexion, extension, abduction, adduction, and rotation, like the shoulder and hip joints.

Common Joint Injuries

• Sprains: Ligaments are stretched or torn.
• Dislocations: Bones are forced out of alignment.
• Bursitis: Inflammation of a bursa.
• Tendinitis: Inflammation of tendon sheaths.
• Arthritis: Inflammatory or degenerative diseases of joints.