The Skeletal System: Structure, Function, and Integration with the Muscular System

Skeletal System

Objectives

• Identify structures of the skeletal system.

• Explain the functions of the skeletal system.

• Describe the relationship between the skeletal system and the muscular system.

Main Functions of the Skeletal System

The skeletal system has three main functions: movement, protection, and storage of minerals and fat. It also supports the body and produces blood cells.

• Movement: Bones act as levers and points of attachment for muscles, enabling body movement.

• Protection: Bones protect delicate organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Storage: Bones store minerals (calcium, phosphate) and fat (yellow marrow).

• Blood Cell Production: Red marrow produces red blood cells, white blood cells, and platelets.

Structure and Classification of Bones

• Long Bones: Longer than they are wide (e.g., femur, humerus). Contain a shaft (diaphysis) and ends (epiphyses). Growth plates (epiphyseal plates) allow for lengthening during growth.

• Short Bones: Roughly equal in length and width (e.g., wrist and ankle bones).

• Flat Bones: Thin and flat, protect organs (e.g., skull, ribs, sternum).

• Irregular Bones: Complex shapes (e.g., vertebrae, hip bones).

Bone Structure

• Compact Bone: Dense outer layer providing strength.

• Spongy Bone: Lighter, porous inner structure containing red marrow.

• Diaphysis: Shaft of a long bone, contains yellow marrow (fat storage).

• Epiphysis: Ends of long bones, contain red marrow (blood cell production).

• Periosteum: Tough outer membrane covering bone surface.

Microscopic Structure of Bone

• Osteons (Haversian Systems): Cylindrical structures containing a central canal with blood vessels and nerves, surrounded by concentric layers of bone matrix.

• Osteocytes: Mature bone cells embedded in the matrix.

• Osteoblasts: Cells that build new bone by depositing minerals.

• Osteoclasts: Cells that break down bone, releasing minerals into the blood.

Joints

Joints are places where two or more bones meet. They are classified by their structure and function:

• Immovable Joints: Bones are fused together (e.g., skull sutures).

• Slightly Movable Joints: Bones are joined by cartilage (e.g., vertebrae).

• Freely Movable (Synovial) Joints: Allow a wide range of movement (e.g., ball-and-socket joints in the hip and shoulder, hinge joints in the elbow and knee).

Bone Growth and Remodeling

• Bones grow in length at the epiphyseal plates during childhood and adolescence.

• Bone remodeling is a continuous process where osteoclasts break down bone and osteoblasts build new bone.

Diseases of Bone

• Osteoporosis: Excessive bone loss due to increased osteoclast activity or decreased osteoblast activity, leading to weak and brittle bones.

• Osteogenesis Imperfecta: Genetic defect in collagen matrix, resulting in brittle bones.

• Arthritis: Inflammation of joints, often due to cartilage damage.

The Skeletal and Muscular Systems

The skeletal muscles attach to bones and work together to produce movement. Muscles contract to move bones at joints, and the nervous system coordinates these actions.

• Origin: The fixed attachment point of a muscle.

• Insertion: The movable attachment point of a muscle.

• Antagonistic Pairs: Muscles work in pairs; when one contracts (agonist), the other relaxes (antagonist). Example: biceps and triceps in the arm.

Summary Table: Types of Bones

Type of Bone	Shape	Example	Main Function
Long	Longer than wide	Femur, humerus	Movement, support
Short	Equal length and width	Carpals, tarsals	Stability, support
Flat	Thin, flat	Sternum, ribs, skull	Protection, muscle attachment
Irregular	Complex shapes	Vertebrae, hip bones	Protection, support

Key Equations

• Bone Remodeling:

$\text{Bone Mass} = \text{Bone Formation (by osteoblasts)} - \text{Bone Resorption (by osteoclasts)}$

Additional info: Bone health depends on a balance between osteoblast and osteoclast activity. Hormones such as parathyroid hormone (PTH) and calcitonin regulate this balance. Vitamin D and calcium intake are also critical for bone strength.