Skeletal & Muscular System: Key Concepts and Structures

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.

• Energy Storage: Yellow bone marrow stores lipids.

Bone Classification

Bones are classified based on their shape and function.

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

• Short Bones: Nearly 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 in 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, mostly spongy bone.

• Metaphysis: Region between diaphysis and epiphysis; contains growth plate.

• Medullary Cavity: Central cavity containing bone marrow.

• Periosteum: Outer fibrous covering.

• Endosteum: Lining of the medullary cavity.

Bone Marrow

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

• Red Marrow: Produces red blood cells, white blood cells, and platelets.

• 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.

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

Bone Growth

Bones grow and remodel throughout life via two main processes.

• Intramembranous Ossification: Direct formation of bone from mesenchymal tissue (e.g., flat bones of skull).

• Endochondral Ossification: Bone develops by replacing hyaline cartilage (e.g., long bones).

• Epiphyseal Plate: Site of longitudinal growth 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; attaches upper limbs.

• Upper Limbs: Arms, forearms, hands.

• Pelvic Girdle: Hip bones; attaches lower limbs.

• Lower Limbs: Thighs, legs, feet.

Joints

Joints, or articulations, connect bones and allow movement.

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

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

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

Skeletal Muscle Structure

Skeletal muscle is organized into bundles of fibers surrounded by connective tissue.

• Muscle Fiber: Individual muscle cell.

• Myofibrils: Contractile elements within muscle fibers.

• Sarcomere: Functional unit of contraction.

• Connective Tissue Layers: Endomysium, perimysium, epimysium.

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 contraction.

Equation:

$V_m = V_{inside} - V_{outside}$

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.

Signal Transmission

Signal transmission involves the propagation of action potentials and neurotransmitter release.

• Depolarization: Sodium ions enter the cell, making it more positive.

• Repolarization: Potassium ions exit the cell, restoring negative charge.

• Synaptic Transmission: Release of neurotransmitters across the synaptic cleft.

Functions of Smooth Muscle

Smooth muscle controls involuntary movements in various organs.

• Regulates blood vessel diameter.

• Controls movement in digestive tract.

• Contracts in respiratory, urinary, and reproductive systems.

Origin vs. Insertion

Muscle attachment points are classified as origin and insertion.

• Origin: Fixed attachment, usually proximal.

• Insertion: Movable attachment, usually distal.

• Movement: Contraction pulls insertion toward origin.

Major Muscle Groups and Functions

Several muscle groups are responsible for key movements and functions.

• Muscles of Facial Expression: Control facial movements (e.g., orbicularis oculi, zygomaticus).

• Muscles of Mastication: Chewing muscles (e.g., masseter, temporalis).

• Diaphragm: Main muscle of respiration.

• Abdominal Muscles: Support trunk, aid in breathing and posture (e.g., rectus abdominis, obliques).

• Quadriceps Group: Extends the knee (e.g., rectus femoris, vastus lateralis).

• Hamstring Group: Flexes the knee (e.g., biceps femoris, semitendinosus).

Summary Table: Skeletal vs. Muscular System

Feature	Skeletal System	Muscular System
Main Function	Support, protection, movement, mineral storage, blood cell production	Movement, posture, heat production
Main Tissue Type	Bone	Muscle (skeletal, cardiac, smooth)
Cell Types	Osteoblasts, osteocytes, osteoclasts	Muscle fibers (myocytes)
Key Structures	Axial & appendicular skeleton, joints, ligaments	Muscle groups, tendons, neuromuscular junctions
Growth/Repair	Ossification, remodeling	Hypertrophy, regeneration (limited)

Additional info: Academic context and explanations have been expanded based on standard Anatomy & Physiology curriculum topics for the skeletal and muscular systems.