16.5 Musculoskeletal System

The skeletal system forms the body's framework, providing support, protection, and movement. It's divided into the axial skeleton, which includes the skull and spine, and the appendicular skeleton, comprising limbs and their attachments.

Joints connect bones, allowing for flexibility and movement. The muscular system works in tandem with the skeleton, using muscles attached to bones via tendons to create motion. This interaction enables our bodies to perform a wide range of movements.

Skeletal System

Axial vs appendicular skeleton

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• Axial skeleton forms the central axis of the body
  • Consists of the skull, vertebral column, and rib cage
  • Skull protects the brain and sensory organs (eyes, ears, nose)
  • Vertebral column provides support and flexibility to the trunk (cervical, thoracic, lumbar, sacral, and coccygeal vertebrae)
  • Rib cage protects vital organs in the chest cavity (heart, lungs)
• Appendicular skeleton consists of the bones of the limbs and their attachments to the axial skeleton
  • Pectoral girdle attaches the upper limbs to the axial skeleton (clavicles, scapulae)
  • Upper limbs include the humerus, radius, ulna, carpals, metacarpals, and phalanges (arms, wrists, hands)
  • Pelvic girdle attaches the lower limbs to the axial skeleton (hip bones: ilium, ischium, and pubis)
  • Lower limbs include the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges (legs, ankles, feet)

Joints for skeletal flexibility

• Joints are points where two or more bones meet, allowing for movement and flexibility in the skeleton
• Types of joints:
  1. Fibrous joints connected by fibrous connective tissue permit little to no movement (sutures between cranial bones)
  2. Cartilaginous joints connected by cartilage allow limited movement (intervertebral discs)
  3. Synovial joints are the most common and movable type, containing a synovial cavity filled with lubricating fluid
     • Examples: ball-and-socket (hip, shoulder), hinge (elbow, knee), pivot (atlantoaxial joint), and gliding (wrist, ankle) joints
• Joint structure includes:
  • Articular capsule surrounds the joint and contains synovial fluid
  • Articular cartilage covers the ends of bones, reducing friction and absorbing shock
  • Ligaments are fibrous connective tissue that connects bones and stabilizes joints
  • Bursae are fluid-filled sacs that reduce friction between bones, tendons, and muscles (knee, shoulder)

Bone remodeling and calcium homeostasis

• Bone remodeling is the continuous process of bone formation and resorption
  • Osteoblasts are cells responsible for bone formation, depositing new bone matrix
  • Osteoclasts are cells that break down and resorb old bone tissue
• This process helps maintain calcium homeostasis in the body, regulating calcium levels in the blood
• Bone acts as a reservoir for calcium, releasing or storing it as needed to maintain proper levels

Muscular System

Muscle-bone interaction for movement

• Muscles are attached to bones via tendons
  • Origin is the fixed attachment point of a muscle, typically proximal
  • Insertion is the movable attachment point of a muscle, typically distal
• Muscle contraction occurs through the sliding filament model
  • Myosin filaments pull on actin filaments, causing the sarcomere to shorten
  • Requires calcium ions ($Ca^{2+}$) and ATP for cross-bridge cycling
• Types of muscle contraction:
  1. Concentric: muscle shortens, generating force (lifting a weight)
  2. Eccentric: muscle lengthens under tension (lowering a weight)
  3. Isometric: muscle generates force without changing length (holding a weight)
• Bones act as levers, with joints serving as fulcrums and muscles providing the force to move the levers
  • Three classes of levers:
    1. First-class: fulcrum between effort and resistance (nodding the head)
    2. Second-class: resistance between effort and fulcrum (standing on tiptoes)
    3. Third-class: effort between fulcrum and resistance (lifting a weight with the elbow)
• Antagonistic muscle pairs work in opposition to each other, allowing for precise control and smooth movement
  • Examples: biceps (flexor) and triceps (extensor) in the upper arm, quadriceps (extensor) and hamstrings (flexor) in the thigh
• Skeletal muscle is composed of muscle fibers, which are long, cylindrical cells containing multiple nuclei and contractile proteins