Skeletal System

Functions of the Skeletal System

• Support: Provides a structural framework for the attachment of soft tissues and organs.

• Mineral storage: Stores calcium and phosphate in the bone matrix.

• Blood cell production: Red bone marrow is the site of hematopoiesis (formation of blood cells).

• Protection: Protects vital organs such as the skull (brain), thorax (heart and lungs), and pelvis (reproductive organs).

• Movement: Levers that muscles pull on, generating body motion.

Structure of Long Bones

Long bones are characterized by a central shaft and expanded ends. They contain compact and spongy bone, and have specialized internal structures.

• Diaphysis: The central shaft, surrounds a marrow cavity (medullary cavity) filled with bone marrow.

• Epiphyses: Expanded portions at each end, covered with articular cartilage for joint movement.

• Periosteum: Outer surface covering of bone, a dense connective tissue layer.

• Endosteum: Inner surface lining the marrow cavity and spongy bone.

Example: The femur, humerus, and tibia are all long bones with these features.

Bone Cells and Homeostasis

Bone tissue contains three main cell types, each with distinct functions in bone growth and maintenance.

• Osteocytes: Mature bone cells located in lacunae. They maintain protein and mineral composition of bone.

• Osteoblasts: Bone-building cells that make and release proteins, increase mineral concentration, and become osteocytes when surrounded by matrix.

• Osteoclasts: Bone-resorbing cells, large and multinucleated, dissolve bone matrix (osteolysis) using acids and enzymes.

Bone homeostasis is regulated by two hormones:

• Calcitonin: Promotes calcium deposition in bone when blood calcium is high.

• Parathyroid hormone (PTH): Increases calcium absorption and resorption from bone when blood calcium is low.

Bone Growth and Development

Bone formation occurs through two main processes:

• Intramembranous ossification: Bone forms from stem cells in fibrous connective tissue (e.g., skull, lower jaw, clavicle, patella).

• Endochondral ossification: Most bones form by ossification of hyaline cartilage. The primary ossification center is in the diaphysis, and secondary centers are in the epiphyses.

• Long bones grow at epiphyseal cartilage (growth plate) between diaphysis and epiphysis. At puberty, ossification rate increases, closing the growth plate and forming the epiphyseal line.

Bone Disorders

• Osteoporosis: Reduced bone density, increased fracture risk.

• Gigantism: Overproduction of growth hormone before puberty.

• Acromegaly: Overproduction of growth hormone after puberty, thick bones and soft tissue changes.

• Pituitary growth failure: Reduced epiphyseal cartilage activity, short bones.

• Marfan's syndrome: Excessive cartilage at epiphyseal plates, long slender limbs, cardiovascular complications.

• Achondroplasia: Slow growth and early fusion of epiphyseal cartilage, short limbs.

• Fibrodysplasia ossificans progressiva: Abnormal ossification of fibrous tissue in muscles, tendons, and ligaments.

Muscular System

Organization of Skeletal Muscle Tissue

Skeletal muscle is organized into bundles wrapped in layers of connective tissue.

• Endomysium: Delicate connective tissue around each muscle fiber.

• Perimysium: Fibrous layer that bundles fibers into fascicles.

• Epimysium: Tough collagenous layer around the entire muscle organ.

• Tendon: Cord-like bundle attaching muscle to bone.

• Aponeurosis: Sheet of dense connective tissue.

Types of Muscle Tissue

• Cardiac muscle: Found in the heart, short branched striated cells, one or two nuclei, involuntary, connected by intercalated discs.

• Smooth muscle: Found in walls of hollow organs, spindle-shaped, non-striated, single nucleus, involuntary.

• Skeletal muscle: Attached to skeleton, long cylindrical striated multinucleate fibers, voluntary control.

Functions of Skeletal Muscle

• Skeletal movement: Pull on solid levers (bones).

• Controlling outer openings: Regulate entry/exit of materials.

• Maintaining temperature: Produce heat.

• Nutrient supply: Store and release nutrients.

Structure of a Skeletal Muscle Fiber

• Sarcolemma: Plasma membrane of a muscle fiber.

• Sarcoplasm: Cytoplasm of a muscle fiber.

• Sarcoplasmic reticulum (SR): Smooth ER that stores calcium, wrapped around myofibrils.

• Myofibril: Contractile organelle made of repeating sarcomeres (units of actin and myosin filaments).

• Transverse tubules (T-tubules): Invaginations of the sarcolemma that connect with SR and help transmit action potentials.

Filament Structure

• Thin filament: Composed of actin, tropomyosin, and troponin.

• Thick filament: Composed of myosin with heads that bind actin and ATP.

Excitation-Contraction Coupling

Describes the process by which a nerve impulse leads to muscle contraction.

1. Neurons release neurotransmitters (acetylcholine, ACh) at the neuromuscular junction.

2. ACh diffuses across the synaptic cleft and binds to receptors on the sarcolemma.

3. Binding opens sodium channels, causing depolarization and an action potential in the muscle fiber.

4. The enzyme acetylcholinesterase breaks down ACh to terminate the signal.

5. Action potential spreads across sarcolemma and down T-tubules to the SR.

6. SR releases Ca2+ ions, which bind to troponin on the thin filament.

7. Troponin changes shape, moving tropomyosin and exposing active sites on actin for myosin binding.

Example: Voluntary movement such as lifting an object involves this sequence of events.

Summary Table: Muscle Tissue Types

Additional info: The notes also reference the role of calcium and ATP in muscle contraction, and the importance of connective tissue layers in muscle structure and function.