Chapter 6: Skeletal System

Functions of Skeletal System

The skeletal system provides the structural framework for the human body and serves several essential functions.

• Support: Bones support the body and maintain its shape.

• Protection: Vital organs such as the brain, heart, and lungs are protected by bones (e.g., skull, rib cage).

• Movement: Bones act as levers for muscles to produce movement.

• Mineral Storage: Bones store minerals, primarily calcium and phosphorus.

• Blood Cell Production: Hematopoiesis occurs in the bone marrow, producing red and white blood cells.

• Energy Storage: Yellow bone marrow stores fat as an energy reserve.

Bone Classification

Bones are classified based on their shape and function.

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Approximately equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and broad (e.g., skull, sternum).

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

• Sesamoid Bones: Embedded within tendons (e.g., patella).

Long Bone Structure

Long bones have a distinct anatomy that supports their function.

• Diaphysis: Shaft of the bone, composed of compact bone.

• Epiphysis: Ends of the bone, containing spongy bone and red marrow.

• Metaphysis: Region between diaphysis and epiphysis, includes the growth plate.

• Medullary Cavity: Central cavity containing yellow marrow.

• Periosteum: Dense connective tissue covering the bone's surface.

• Endosteum: Membrane lining the medullary cavity.

Bone Marrow

Bone marrow is a soft tissue found within bones, crucial for blood cell production.

• Red Marrow: Site of hematopoiesis; found in spongy bone of flat bones and epiphyses of long bones.

• Yellow Marrow: Stores fat; found in the medullary cavity of long bones.

Bone Cells

Bone tissue contains several specialized cell types.

• Osteoblasts: Build new bone matrix.

• Osteocytes: Mature bone cells maintaining bone tissue.

• Osteoclasts: Break down bone matrix for remodeling and calcium release.

Bone Growth

Bones grow and remodel throughout life via two main processes.

• Endochondral Ossification: Bone develops from cartilage models (most bones).

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (flat bones).

• Epiphyseal Plate: Growth plate where lengthwise growth occurs in children and adolescents.

Axial Skeleton

The axial skeleton forms the central axis of the body.

• Skull: Protects the brain.

• Vertebral Column: Supports the body and protects the spinal cord.

• Rib Cage: Protects the heart and lungs.

Appendicular Skeleton

The appendicular skeleton includes the limbs and girdles.

• Pectoral Girdle: Clavicle and scapula, supports the arms.

• Upper Limbs: Humerus, radius, ulna, carpals, metacarpals, phalanges.

• Pelvic Girdle: Hip bones, supports the legs.

• Lower Limbs: Femur, tibia, fibula, tarsals, metatarsals, phalanges.

Joints

Joints, or articulations, connect bones and allow movement.

• Fibrous Joints: Immovable (e.g., sutures in the skull).

• Cartilaginous Joints: Slightly movable (e.g., intervertebral discs).

• Synovial Joints: Freely movable (e.g., knee, shoulder).

Supportive Structures

Various structures support and stabilize the skeletal system.

• Ligaments: Connect bone to bone.

• Tendons: Connect muscle to bone.

• Cartilage: Provides cushioning and flexibility at joints.

Muscular System

Muscle Tissue Types

Muscle tissue is classified into three main types, each with distinct characteristics.

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

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

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

Muscle Properties

Muscle tissue exhibits several key physiological properties.

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten and generate force.

• Extensibility: Ability to be stretched.

• Elasticity: Ability to return to original shape after stretching.

Skeletal Muscle Structure

Skeletal muscle is organized into hierarchical structures.

• Muscle Fiber: Individual muscle cell.

• Fascicle: Bundle of muscle fibers.

• Myofibril: Contractile unit within muscle fibers, composed of sarcomeres.

• Sarcomere: Functional unit of contraction, defined by Z-lines.

Membrane Potential

Muscle cells maintain a membrane potential essential for contraction.

• Resting Membrane Potential: Difference in charge across the cell membrane, typically -70 mV.

• Action Potential: Rapid change in membrane potential that triggers muscle contraction.

Equation:

Neuromuscular Junction

The neuromuscular junction is the synapse between a motor neuron and a muscle fiber.

• Acetylcholine (ACh): Neurotransmitter released to stimulate muscle contraction.

• Motor End Plate: Specialized region of muscle membrane that receives neural signals.

Signal Transmission

Signal transmission involves the propagation of action potentials and muscle contraction.

• Depolarization: Sodium ions enter the muscle cell, reversing membrane potential.

• Repolarization: Potassium ions exit, restoring resting potential.

• Excitation-Contraction Coupling: Process linking action potential to muscle contraction.

Functions of Smooth Muscle

Smooth muscle performs involuntary movements in various organs.

• Regulates blood vessel diameter (controls blood pressure).

• Moves food through digestive tract (peristalsis).

• Controls pupil size in the eye.

Origin vs. Insertion

Muscle attachment points determine movement.

• Origin: Fixed attachment, usually proximal.

• Insertion: Movable attachment, usually distal.

• Movement occurs when muscle contracts, pulling insertion toward origin.

Muscles of Facial Expression

These muscles control facial movements and expressions.

• Orbicularis oculi: Closes eyelids.

• Orbicularis oris: Purses lips.

• Zygomaticus: Raises corners of mouth (smiling).

Muscles of Mastication

Muscles involved in chewing.

• Masseter: Elevates mandible.

• Temporalis: Assists in closing jaw.

• Medial and Lateral Pterygoids: Move jaw side to side.

Diaphragm

The diaphragm is the primary muscle of respiration.

• Contracts to enlarge thoracic cavity, drawing air into lungs.

• Separates thoracic and abdominal cavities.

Abdominal Muscles

Abdominal muscles support trunk movement and protect internal organs.

• Rectus abdominis: Flexes vertebral column.

• External and Internal Obliques: Rotate and laterally flex trunk.

• Transversus abdominis: Compresses abdominal contents.

Quadriceps Group

The quadriceps are a group of four muscles on the anterior thigh.

• Rectus femoris, vastus lateralis, vastus medialis, vastus intermedius.

• Extend the knee and flex the hip.

Hamstring Group

The hamstrings are located on the posterior thigh.

• Biceps femoris, semitendinosus, semimembranosus.

• Flex the knee and extend the hip.

Summary Table: Bone Classification

This table summarizes the main types of bones and examples.

Summary Table: Muscle Tissue Types

This table compares the three types of muscle tissue.

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for college-level Anatomy & Physiology students.