Musculoskeletal System Overview

Musculoskeletal System

• The musculoskeletal system provides structure and function to the human body.
• It allows for movement, support, protection of organs, and blood cell formation.

Bones

• Bones provide form and support to the body.
• They support surrounding tissues, give points of attachment for muscles, permitting movement.
• Bones protect vital organs.
• They act as a site for blood cell formation.
• Bones play a role in mineral and hormone homeostasis.

Bone Tissue Elements

• Bone tissue is a rigid connective tissue.
• Its components include cells, fibers, ground substance, and crystallized minerals.

Bone Tissue Elements (Continued)

• Bone cells are crucial for bone growth, repair, shape modification, and the synthesis/resorption of old tissue.
• Bone matrix and calcification are essential parts of bone formation.

Bone Cells: Osteoblasts

• Osteoblasts are derived from mesenchymal stem cells.
• They are the primary bone-producing cells.
• They respond to parathyroid hormone and produce osteocalcin.
• Osteoblasts synthesize the osteoid (nonmineralized bone matrix).
• They express RANKL, a cytokine receptor activator required for osteoclast formation.

Bone Tissue: Osteocytes

• Osteocytes are formed from transformed osteoblasts embedded in hardened osteoid.
• They synthesize matrix molecules.
• Osteocytes are key regulators of both bone formation and resorption.

Bone Tissue: Osteoclasts

• Osteoclasts are large, multinucleated cells.
• They are the major reabsorptive cells of bone.
• Their function is to break down bone tissue.

OPG/RANKL/RANK System

• Osteoprotegerin (OPG) inhibits bone remodeling/resorption and osteoclast formation.
• It plays a crucial role in the interaction between osteoblasts and osteoclasts.
• RANKL is an essential cytokine for osteoclast formation and activation.
• RANKL, binding to its receptor RANK on osteoclast precursor cells, triggers proliferation and increases bone resorption.

Bone Matrix

• Collagen fibers make up the majority of the bone matrix.
• Proteoglycans strengthen bone and also contribute to calcium deposition/calcification.
• Glycoproteins control collagen interactions for fibril formation.
• Bone mineralisation occurs in two phases: initial mineral deposit formation and the proliferation/accretion of additional mineral crystals.
• Crucial minerals include calcium and phosphate.

Bone Tissue: Compact Bone

• Compact bone accounts for 85% of the skeleton.
• It's highly organized, solid, and extremely strong.
• The Haversian system, including Haversian canals, lamellae, lacunae, osteocytes, and canaliculi, are key features.

Bone Tissue: Spongy Bone

• This type of bone lacks Haversian systems.
• It's made up of trabeculae (interconnecting bony spicules) and the periosteum.

Characteristics of Bone

• The human skeleton comprises 206 bones.
• The axial skeleton (80 bones) includes the skull, vertebral column, and thorax.
• The appendicular skeleton (126 bones) comprises the upper and lower extremities, shoulder girdle, and pelvic girdle.

Bone Parts of a Long Bone

• Long bones have a diaphysis, metaphysis, and epiphysis.
• The epiphyseal plate is the growth plate, and the medullary cavity houses the marrow.
• The endosteum lines the medullary cavity.

Types of Bone

• Flat bones (e.g., ribs, scapulae)
• Short (cuboidal) bones (e.g., carpals, tarsals)
• Irregular bones (e.g., vertebrae)
• Facial bones (part of the skull)

Bone Maintenance

• Bone integrity is maintained through remodeling, a process of basic multicellular units that repair microscopic bone injuries by resorbing existing bone and laying down new bone.
• The remodeling cycle includes three phases: activation, resorption, and formation of new bone.

Bone Repair

• Bone repair involves stages of inflammation/hematoma formation, procallus formation, callus formation, callus replacement, and remodeling.

Joints

• Joints, also called articulations, are where two or more bones meet.
• They provide stability and mobility to the skeleton.

Joint Classifications (Based on Movement)

• Synarthrosis: Immovable
• Amphiarthrosis: Slightly movable
• Diarthrosis: Freely movable.

Joint Classifications (Based on Structure)

• Fibrous joints: Connect bones with fibrous tissue.
• Cartilaginous joints: Connect bones with cartilage.
• Synovial joints: Contain a synovial cavity, synovial fluid, and articular cartilage.

Joint Components (Synovial Joints)

• Synovial membrane: Thin cellular membrane lining the synovial joint capsule.
• Synovial fluid: Lubricates the joint surfaces, nourishes cartilage.
• Articular cartilage: Covers the bone ends; reduces friction.

Muscle Structure

• Skeletal muscles are composed of millions of individual muscle fibers.
• They are approximately 75% water, 20% protein, and 5% organic and inorganic compounds.
• Muscles are categorized as fusiform (strap-like) and pennate (broad, flat, fan-shaped).
• Muscles consist of epimysium, perimysium, and endomysium.

Skeletal Muscle Components

• Myofibrils: Functional units of muscle contraction.
• Sarcomeres: Repeating units within myofibrils.
• Myofilaments: Protein filaments within sarcomeres (actin and myosin).

Muscle Mechanics

• All-or-nothing response: A motor unit either fully contracts or does not contract.
• Repetitive discharge: The stimulation of a motor unit multiple times.
• Physiological tetanus: Sustained muscle contraction due to rapid, repetitive stimulation.

Muscle Contractions & Movement

• Muscle contraction types include isometric (same length) and dynamic (changing length) with concentric (shortening) and eccentric (lengthening) components.
• Agonist and antagonist muscles work in opposition to produce movement.

Tendons & Ligaments

• Tendons attach muscles to bones and transfer forces.
• Ligaments attach bones to bones and stabilize joints.

Aging and the Musculoskeletal System

• Bone loss, decreased density, increased brittleness, and prolonged remodeling times occur with aging.
• Cartilage stiffens, making joints less flexible.
• Muscles experience a loss of mass and strength (sarcopenia).