Skeletal & Muscular System: Study Guide Notes (Module 3)

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, enabling 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 lipids as an energy reserve.

Bone Classification

Bones are classified based on their shape and structure.

• 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 distinct anatomical regions that contribute to 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 growth plate.

• Medullary Cavity: Central cavity containing yellow marrow.

• Periosteum: Outer fibrous membrane covering the bone.

• 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: Cells that build new bone matrix.

• Osteocytes: Mature bone cells maintaining bone tissue.

• Osteoclasts: Cells that break down bone matrix for remodeling.

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: Growth plate where lengthwise growth occurs in children.

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 are sites where two or more bones meet, allowing movement and flexibility.

• 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, providing joint stability.

• Tendons: Connect muscle to bone, enabling movement.

• Cartilage: Cushions joints and supports soft tissues.

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 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.

• Myofibril: Contractile unit within muscle fiber.

• Sarcomere: Functional unit of contraction, composed of actin and myosin filaments.

• Connective Tissue Layers: Endomysium (around fibers), perimysium (around fascicles), epimysium (around whole muscle).

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.

Neuromuscular Junction

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

• Synaptic Cleft: Gap between neuron and muscle fiber.

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

Signal Transmission

Signal transmission involves the propagation of action potentials leading to muscle contraction.

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

• Repolarization: Potassium ions exit, restoring resting potential.

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

Functions of Smooth Muscle

Smooth muscle performs involuntary movements in various organs.

• Regulates diameter of blood vessels.

• Controls movement in digestive tract.

• Contracts in respiratory, urinary, and reproductive systems.

Origin vs. Insertion

Muscle attachment sites are classified as origin and insertion.

• Origin: Fixed attachment, usually proximal.

• Insertion: Movable attachment, usually distal.

• Movement: Contraction pulls 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: Elevates and retracts mandible.

• Medial and Lateral Pterygoids: Side-to-side movement of jaw.

Diaphragm

The diaphragm is the primary muscle of respiration.

• Contracts: Increases thoracic volume, causing inhalation.

• Relaxes: Decreases thoracic volume, causing exhalation.

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 responsible for knee extension.

• Rectus femoris

• Vastus lateralis

• Vastus medialis

• Vastus intermedius

Hamstrings Group

The hamstrings are a group of three muscles on the posterior thigh responsible for knee flexion and hip extension.

• Biceps femoris

• Semitendinosus

• Semimembranosus

Table: Bone Classification

This table summarizes the main types of bones and examples.

Bone Type	Description	Example
Long	Longer than wide	Femur, Humerus
Short	Equal length and width	Carpals, Tarsals
Flat	Thin, broad	Skull, Sternum
Irregular	Complex shape	Vertebrae, Pelvis
Sesamoid	Embedded in tendons	Patella

Table: Muscle Tissue Types

This table compares the three types of muscle tissue.

Type	Location	Control	Striations
Skeletal	Bones	Voluntary	Present
Cardiac	Heart	Involuntary	Present
Smooth	Walls of organs	Involuntary	Absent

Key Equations

• Resting Membrane Potential:

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

• Force of Muscle Contraction:

$F = m imes a$

• Calcium Homeostasis (Bone):

$Ca^{2+}_{blood} \leftrightarrow Ca^{2+}_{bone}$

Additional info: Academic context and examples have been added to expand upon the brief study guide topics and make the notes self-contained for exam preparation.