Skeletal System

Functions of the Skeletal System

The skeletal system provides the framework for the body, protects vital organs, enables movement with the help of muscles, stores minerals, and produces blood cells. It is essential for both structural support and physiological functions.

• Support: Maintains body shape and posture.

• Protection: Shields organs such as the brain, heart, and lungs.

• Movement: Works with muscles to enable motion.

• Mineral Storage: Stores calcium and phosphorus.

• Blood Cell Production: Occurs in red bone marrow.

• Energy Storage: Yellow bone marrow stores fat.

Bone Structure: Inorganic and Organic Matrix

Bones are composed of both inorganic and organic components, each contributing to bone strength and flexibility.

• Inorganic Matrix (about 65%): Primarily calcium phosphate, which forms hydroxyapatite crystals. This provides hardness and resistance to compression.

• Organic Matrix (about 35%): Mainly collagen fibers and ground substance. Collagen gives bones flexibility and tensile strength, preventing easy breakage.

Together, these components make bone both strong and slightly flexible, essential for protection, movement, and support.

Bone Marrow and Age

Bone marrow changes with age, affecting blood cell production and fat storage.

• Infants and Young Children: Mostly red marrow, which produces blood cells.

• Adults: Red marrow is gradually replaced by yellow marrow (fat storage), but red marrow remains in certain bones (pelvis, sternum, vertebrae).

Bone Growth and Remodeling

Bone Cells and Their Functions

Bone growth and repair involve several specialized cells:

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoblasts: Cells that build new bone by producing matrix and starting calcification.

• Osteoclasts: Cells that break down old or damaged bone, essential for bone remodeling.

• Chondrocytes: Cartilage cells involved in bone growth and development.

Types of Bone Growth

Bones grow and repair through two main processes:

• Interstitial Growth: Growth from within, mainly in cartilage.

• Appositional Growth: Growth in width by adding new layers to the surface.

Epiphyseal Plate and Growth Closure

The epiphyseal plate (growth plate) is responsible for bone lengthening during childhood and adolescence. Growth plate closure occurs at the end of puberty, typically ages 14-18 for girls and 16-21 for boys.

• Hormones: Sex hormones (estrogen and testosterone) and growth hormone regulate growth plate activity.

• Closure: Rising hormone levels cause cartilage replacement by bone, ending lengthwise growth.

Hormonal Regulation of Bone Growth

Several hormones influence bone growth and remodeling:

• Growth Hormone (GH): Stimulates cartilage cell division and bone growth.

• Thyroid Hormone: Supports normal growth and metabolism.

• Sex Hormones: Increase bone growth during puberty and cause growth plate closure.

Pituitary Gigantism: Excess GH before growth plate closure causes abnormal height. Achondroplasia Dwarfism: Genetic disorder affecting cartilage growth, resulting in short limbs and normal-sized torso.

Bone Remodeling and Repair

Bone Remodeling

Bone remodeling is a continuous process involving bone resorption and deposition, essential for growth, repair, and calcium regulation.

• Osteoclasts: Resorb (break down) bone.

• Osteoblasts: Deposit new bone.

Remodeling rates vary with age and hormonal status. In adults, resorption and deposition are balanced; in older adults, resorption may exceed deposition, leading to bone loss.

Factors Affecting Bone Remodeling

• Hormones: Parathyroid hormone (PTH) increases resorption; sex hormones and growth hormone stimulate deposition.

• Diet: Adequate calcium, vitamin D, and protein support deposition.

• Age: Bone mass decreases with age.

• Genetics and Health Conditions: Affect bone strength and remodeling.

Calcium Homeostasis

Calcium levels are regulated by a negative feedback loop involving PTH and calcitonin.

• PTH: Increases blood calcium by stimulating bone resorption and activating vitamin D.

• Calcitonin: Lowers blood calcium by inhibiting bone resorption.

Equation:

Bone Fracture and Repair

Steps in Bone/Fracture Repair

The body repairs a fracture through a series of organized steps:

1. Hematoma Formation (Fracture Hematoma): Blood vessels rupture, forming a hematoma. Inflammation begins the healing process.

2. Fibrocartilaginous (Soft) Callus Formation: Fibroblasts produce collagen fibers, forming a soft callus to stabilize the bone.

3. Bony (Hard) Callus Formation: Osteoblasts replace the soft callus with spongy bone, forming a hard callus.

4. Bone Remodeling: Osteoclasts and osteoblasts reshape the area, restoring normal bone structure.

Example: A broken arm heals through hematoma formation, soft callus, hard callus, and remodeling, with different bone and cartilage cells working together.

Homeostatic Imbalances of Bone

Osteomalacia (Adults) & Rickets (Children)

Both conditions result from poor mineralization of bone, usually due to vitamin D or calcium deficiency.

• Osteomalacia: Softening of bones in adults.

• Rickets: Softening and deformity of bones in children.

Osteoporosis

A condition where bone resorption outpaces bone deposition, causing bones to become weak, porous, and easily fractured. Most common in postmenopausal women due to decreased estrogen.

Paget's Disease

Abnormal and excessive bone remodeling, where osteoclast and osteoblast activity is disorganized. Bones become thick but weak and deformed.

Joints (Articulations)

Classification of Joints

Joints are classified based on the type of tissue connecting the bones and their movement capabilities.

Joint Stabilization

Joints are stabilized by ligaments (connect bone to bone) and tendons (connect muscle to bone). Synovial joints also have a fluid-filled capsule that reduces friction and helps stabilize the joint.

Muscular System (Introductory)

Sarcomere Function

The sarcomere is the basic contractile unit of muscle fiber. Muscle contraction occurs when sarcomeres shorten due to the sliding of actin and myosin filaments.

• Shortening: Myosin heads pull actin filaments toward the center of the sarcomere.

• Lengthening: Occurs when the muscle relaxes and filaments slide apart.

Neuromuscular Junction

The neuromuscular junction is the site where a motor neuron communicates with a muscle fiber to initiate contraction.

• Acetylcholine (ACh): Neurotransmitter released from the neuron, binds to receptors on the muscle cell membrane, triggering contraction.

Role of Calcium Ions in Muscle Contraction

Calcium ions (Ca2+) are essential for muscle contraction. They bind to troponin, causing tropomyosin to move and expose binding sites on actin for myosin attachment.

Motor Units

A motor unit consists of a motor neuron and all the muscle fibers it controls. Small motor units allow fine control; large motor units generate powerful contractions.

• Small motor units: Precise movements (e.g., fingers).

• Large motor units: Powerful movements (e.g., legs).

Muscle Tone

Muscle tone is the continuous and passive partial contraction of muscles, helping maintain posture and readiness for action.

ATP and Muscle Contraction

Muscle contraction requires ATP for myosin head movement and detachment from actin. ATP is regenerated by cellular respiration and other metabolic pathways.

• Equation:

Additional info: Some explanations and examples have been expanded for clarity and completeness.