Osseous Tissue and the Skeletal System

Primary Functions of the Skeletal System

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

• Support: Bones support the body and cradle soft organs.

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

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

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

• Blood Cell Formation: Hematopoiesis occurs in red bone marrow.

• Triglyceride Storage: Yellow bone marrow stores fat.

Bone Parts and Functions

Bones have distinct regions, each with specific functions.

• Compact Bone: Dense outer layer providing strength.

• Spongy Bone: Porous inner layer housing marrow.

• Epiphysis: Ends of long bones, site of red marrow.

• Diaphysis: Shaft of long bones, contains yellow marrow.

• Medullary Cavity: Central cavity in diaphysis, stores fat.

• Endosteum: Membrane lining the medullary cavity.

• Articular Cartilage: Covers joint surfaces, reduces friction.

Bone Classification and Matrix Composition

Bones are classified by shape and matrix composition.

• Shapes: Long, short, flat, irregular, sesamoid.

• Matrix: Collagen fibers (flexibility) and calcium salts (hardness).

• Matrix Spaces: Lacunae (house osteocytes), lamellae (layers), canaliculi (channels).

Cells of Osseous Tissue

Bone tissue contains specialized cells with distinct functions.

• Osteoblasts: Bone-forming cells; secrete matrix.

• Osteocytes: Mature bone cells; maintain matrix.

• Osteoclasts: Bone-resorbing cells; break down matrix.

Compact vs. Spongy Bone

Compact and spongy bone differ in structure and function.

• Compact Bone: Dense, organized in osteons (Haversian systems).

• Spongy Bone: Porous, organized in trabeculae; contains red marrow.

• Location: Compact bone forms outer layer; spongy bone found at ends of long bones and inside flat bones.

Bone Development and Ossification

Bones develop through two main processes: endochondral and intramembranous ossification.

• Endochondral Ossification: Bone replaces cartilage (most bones).

• Intramembranous Ossification: Bone develops from fibrous membrane (flat bones).

Bone Growth and Remodeling

Bones grow in length and width through specific mechanisms.

• Longitudinal Growth: Occurs at epiphyseal plates.

• Appositional Growth: Increases bone thickness.

• Remodeling: Continuous process of bone resorption and formation.

Hormonal Regulation of Bone

Several hormones regulate bone growth and remodeling.

• Growth Hormone: Stimulates bone growth.

• Thyroxine: Regulates metabolism and bone development.

• Sex Hormones: Promote growth and closure of epiphyseal plates.

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts.

Bone Disorders

Common bone disorders include:

• Rickets: Softening of bones due to vitamin D deficiency.

• Osteomalacia: Adult form of rickets.

• Osteoporosis: Decreased bone mass and increased fracture risk.

Joints (Articulations)

Joint Classification

Joints are classified by function and structure.

• Functional Classification:

  • Synarthrosis: Immovable joints (e.g., sutures in skull).

  • Amphiarthrosis: Slightly movable joints (e.g., intervertebral discs).

  • Diarthrosis: Freely movable joints (e.g., shoulder, knee).

• Structural Classification:

  • Fibrous: Bones joined by fibrous tissue.

  • Cartilaginous: Bones joined by cartilage.

  • Synovial: Bones separated by a fluid-filled cavity.

Synovial Joint Structure

Synovial joints are the most movable type of joint.

• Articular Cartilage: Covers bone ends, reduces friction.

• Joint (Synovial) Cavity: Space containing synovial fluid.

• Synovial Membrane: Produces synovial fluid.

• Articular Capsule: Encloses joint cavity.

• Accessory Structures: Ligaments, bursae, tendons.

Types of Synovial Joints

Synovial joints are classified by the shapes of their articulating surfaces and movements allowed.

• Ball-and-Socket: Shoulder, hip; allows multiaxial movement.

• Hinge: Elbow, knee; allows flexion and extension.

• Pivot: Atlas/axis; allows rotation.

• Condyloid: Wrist; allows flexion, extension, abduction, adduction.

• Saddle: Thumb; allows similar movements to condyloid.

• Plane: Intercarpal joints; allows gliding.

Angular Movements

Angular movements change the angle between bones.

• Flexion vs. Extension: Decreases vs. increases joint angle.

• Abduction vs. Adduction: Moves limb away from vs. toward midline.

• Pronation vs. Supination: Rotation of forearm (palm down vs. up).

• Elevation vs. Depression: Lifting vs. lowering body part.

Muscular System

Functions of Skeletal Muscle

Skeletal muscle is responsible for voluntary movements and posture.

• Movement: Locomotion and manipulation of environment.

• Posture: Maintains body position.

• Joint Stability: Stabilizes joints.

• Heat Production: Generates heat during contraction.

Types of Muscle Tissue

There are three types of muscle tissue, each with unique characteristics.

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

• Cardiac Muscle: Involuntary, striated, single nucleus, found in heart.

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

Muscle Fiber Structure

Muscle fibers contain specialized structures for contraction.

• Sarcolemma: Muscle cell membrane.

• Sarcoplasm: Cytoplasm of muscle cell.

• Myofibril: Contractile organelle composed of sarcomeres.

• Sarcoplasmic Reticulum: Stores calcium ions.

• Transverse Tubules (T-tubules): Conduct action potentials.

Muscle Organization

Muscle tissue is organized into bundles surrounded by connective tissue.

• Endomysium: Surrounds individual muscle fibers.

• Perimysium: Surrounds fascicles (bundles of fibers).

• Epimysium: Surrounds entire muscle.

Sarcomere Structure

The sarcomere is the functional unit of muscle contraction.

• A Band: Contains thick filaments (myosin).

• I Band: Contains thin filaments (actin).

• Z Line: Defines boundaries of sarcomere.

• H Zone: Center of A band; only thick filaments.

• M Line: Center of sarcomere.

• Titin: Elastic protein stabilizing filaments.

• Actin, Tropomyosin, Troponin: Components of thin filaments.

Muscle Contraction Mechanism

Muscle contraction is initiated by nerve impulses and involves several steps.

• Motor Neuron: Transmits signal to muscle fiber.

• Neuromuscular Junction: Site of communication between neuron and muscle.

• Acetylcholine: Neurotransmitter that triggers contraction.

• Events: Action potential, calcium release, cross-bridge formation, contraction, relaxation.

Energy for Muscle Contraction

Muscle contraction requires ATP, produced by several metabolic pathways.

• Phosphorylation: Direct transfer of phosphate to ADP.

• Glycolysis: Anaerobic breakdown of glucose.

• Aerobic Respiration: Oxidative metabolism in mitochondria.

• Electron Transport Chain (ETC): Produces most ATP.

Nervous System and Nervous Tissue

Central vs. Peripheral Nervous System

The nervous system is divided into central and peripheral components.

• Central Nervous System (CNS): Brain and spinal cord; processes information.

• Peripheral Nervous System (PNS): Nerves outside CNS; transmits signals.

Neurons and Neuroglia

Neurons are the functional units of the nervous system; neuroglia support them.

• Neuron Structure: Cell body, dendrites, axon.

• Types of Neurons: Sensory, motor, interneurons.

• Neuroglia: Support, protect, and nourish neurons.

Action Potentials

Neurons communicate via electrical impulses called action potentials.

• Resting Potential: Difference in charge across membrane.

• Depolarization: Sodium influx reverses membrane potential.

• Repolarization: Potassium efflux restores resting state.

• Propagation: Action potential travels along axon.

Equation:

Where V is voltage, I is current, and R is resistance (Ohm's Law, relevant for nerve conduction).

Neurotransmitters and Synaptic Transmission

Neurotransmitters are chemicals that transmit signals across synapses.

• Types: Acetylcholine, dopamine, serotonin, norepinephrine, GABA, glutamate.

• Function: Excitatory or inhibitory effects on postsynaptic cells.

Types of Potentials and Transmission

Neurons exhibit different types of electrical potentials and transmission mechanisms.

• Graded Potentials: Local changes in membrane potential.

• Action Potentials: All-or-none electrical impulses.

• Saltatory Conduction: Rapid transmission along myelinated axons.

• Continuous Conduction: Slower transmission along unmyelinated axons.

HTML Table: Comparison of Bone Cells

The following table summarizes the main types of bone cells and their functions.

HTML Table: Types of Joints

The following table compares the main types of joints by structure and movement.

HTML Table: Types of Muscle Tissue

The following table summarizes the three types of muscle tissue.

Example: Application of Bone Remodeling

Bone remodeling allows bones to adapt to stress, repair microdamage, and regulate calcium levels. For example, weight-bearing exercise stimulates osteoblast activity, increasing bone density.

Additional info: Academic context and tables have been expanded for clarity and completeness.