Skeletal System Overview

Functions of the Skeletal System

The skeletal system provides the structural framework for the body and is essential for movement, protection, and other physiological functions.

• Support: Maintains body shape and posture.

• Protection: Shields vital organs (e.g., skull protects brain, rib cage protects heart and lungs).

• Movement: Acts as levers for muscles to produce movement.

• Mineral Storage: Stores calcium and phosphorus.

• Blood Cell Production: Houses red bone marrow for hematopoiesis.

Bone Structure

Bones are classified by their structure and composition, which relate to their function and location.

• Long Bone Parts: Diaphysis (shaft), epiphysis (ends), periosteum (outer membrane), marrow cavity (central cavity).

• Compact vs Spongy Bone: Compact bone is dense and forms the outer layer; spongy bone is porous and found at the ends of long bones.

• Microscopic Features: Osteons (structural units), lamellae (layers), canaliculi (small channels).

Bone Cells

Bone tissue contains several specialized cell types, each with distinct roles in bone formation and maintenance.

• Osteoprogenitor: Stem cells that differentiate into osteoblasts.

• Osteoblast: Cells responsible for bone formation.

• Osteocyte: Mature bone cells that maintain bone tissue.

• Osteoclast: Cells that resorb (break down) bone.

Ossification

Ossification is the process of bone formation, occurring via two main pathways.

• Intramembranous Ossification: Formation of flat bones (e.g., skull) directly from mesenchymal tissue.

• Endochondral Ossification: Formation of long bones from a cartilage model.

• Sequence: Cartilage model → primary and secondary ossification centers → epiphyseal closure.

Bone Remodeling

Bone remodeling maintains bone strength and mineral homeostasis.

• Balance of Formation and Resorption: Regulated by osteoblasts and osteoclasts.

• Influenced by: Mechanical stress, hormones (e.g., parathyroid hormone, calcitonin).

Key Terms to Know

• Red vs Yellow Marrow: Red marrow produces blood cells; yellow marrow stores fat.

• Periosteum vs Endosteum: Periosteum covers outer bone; endosteum lines inner surfaces.

Axial and Appendicular Skeleton

Axial Skeleton

The axial skeleton forms the central axis of the body and includes the skull, vertebral column, and thoracic cage.

• Skull Bones: Protect the brain and house sensory organs.

• Major Foramina: Openings for nerves and blood vessels.

• Vertebrae: Classified by region (cervical, thoracic, lumbar, sacral, coccygeal).

• Thoracic Cage: Includes sternum and ribs; protects thoracic organs.

Appendicular Skeleton

The appendicular skeleton consists of the limbs and girdles, enabling movement and manipulation of the environment.

• Pectoral Girdle: Connects upper limbs to the trunk; includes clavicle and scapula.

• Pelvic Girdle: Connects lower limbs; includes ilium, ischium, pubis.

• Major Landmarks: Humerus, radius, ulna, femur, tibia, fibula, scapula, pelvis.

• Articulations: Glenoid (shoulder) vs acetabulum (hip); differences in mobility and stability.

Joints and Articulations

Joint Classifications

Joints (articulations) connect bones and allow for movement. They are classified by structure and function.

• Structural Classification: Fibrous, cartilaginous, synovial.

• Functional Classification: Synarthrosis (immovable), amphiarthrosis (slightly movable), diarthrosis (freely movable).

• Synovial Joint Features: Articular cartilage, capsule, synovial fluid, ligaments, bursae, tendons.

Joint Movements

• Angular Movements: Flexion, extension, abduction, adduction.

• Rotational Movements: Circumduction, special movements (inversion, pronation, etc.).

Mobility vs Stability

Joints balance mobility and stability; increased mobility often means decreased stability.

• Shoulder vs Hip: Shoulder has a shallow glenoid cavity and loose capsule (high mobility, low stability); hip has a deep acetabulum and strong ligaments (lower mobility, higher stability).

Muscle Tissue and Contraction

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is classified into three types.

• Skeletal Muscle: Voluntary, striated, attached to bones.

• Cardiac Muscle: Involuntary, striated, found in the heart.

• Smooth Muscle: Involuntary, non-striated, found in walls of organs.

Muscle Structure Hierarchy

• Myofilament → Myofibril → Fiber → Fascicle → Muscle

• Sarcomere Structure: Z-line, A-band, I-band, H-zone, M-line.

Muscle Contraction Mechanism

• Sliding Filament Mechanism: Actin and myosin filaments slide past each other to shorten the muscle.

• Neuromuscular Junction: Motor neuron releases acetylcholine, triggering sarcolemma depolarization and contraction.

Functional Relationships

• Motor Unit: A motor neuron and all the muscle fibers it innervates.

• Recruitment: Increasing the number of active motor units to generate more force.

• Tone: Continuous and passive partial contraction of muscles.

• Types of Contraction: Isotonic (muscle changes length), isometric (muscle length remains constant).

Axial Muscles

Axial Muscle Groups and Functions

Axial muscles are named by location, action, shape, or attachments and play key roles in posture, breathing, and facial expression.

• Major Groups: Facial expression, mastication, neck, trunk, abdominal wall, back (erector spinae).

• Roles: Posture and breathing (e.g., diaphragm, intercostals, abdominals).

Appendicular Muscles

Major Appendicular Muscle Groups

Appendicular muscles move the limbs and are classified by their origin, insertion, and action.

• Shoulder/Arm: Deltoid, pectoralis major, latissimus dorsi, rotator cuff group, biceps, triceps.

• Hip/Thigh: Gluteals, iliopsoas, quadriceps, hamstrings, adductors.

• Leg/Foot: Gastrocnemius, soleus, tibialis anterior, fibularis longus.

Movement Analysis: Origins and insertions determine joint action.

Study Strategies and Practice

• Re-draw bones or muscles and label from memory.

• Practice matching muscle names to actions and regions.

• Compare shoulder vs hip for stability/mobility questions.

• Attend open-lab Fridays for hands-on review.

Key Table: Joint Classification

Key Equations and Concepts

• Force of Muscle Contraction:

• Sliding Filament Theory: Muscle contraction occurs as myosin heads bind to actin and pull, shortening the sarcomere.

• Ossification Sequence: