Skeletal System Functions and Bone Classification

Functions of the Skeletal System

• The skeletal system provides structure and support and is the body's framework.
• The skeletal system roles include movement, protection, and homeostasis.
• Bones act as levers when working with muscles and producing movement.
• Joints function as fulcrums.
• Protection: Skull protects the brain, the rib cage shields the heart and lungs, and the vertebrae encase the spinal cord.
• Storage of minerals and lipids: Calcium and phosphate are essential for physiological processes, and lipids are stored in yellow bone marrow as an energy reserve.
• Blood cell production (hematopoiesis) occurs in red bone marrow, producing red blood cells, white blood cells, and platelets.
• Support: Acts as the structural framework for the body, similar to a steel framework in a building.

Classification of bones

• Bones are categorized based on their shape and structure.
• Sutural bones are small, irregularly shaped bones between skull bones and vary among individuals.
• Irregular bones are complex shapes, such as vertebrae and pelvic bones.
• Short bones have a boxy appearance, such as carpal and tarsal bones.
• Flat bones are thin with parallel surfaces, such as the sternum, ribs, and scapulae, protect organs, and provide muscle attachment.
• Long bones are long and slender, such as the femur, humerus, fingers, and toes; the femur is the largest bone.
• Sesamoid bones are small, round bones embedded in tendons, such as the patella.
• Projections: Attachment sites for muscles, tendons, and ligaments.
• Openings & Depressions: Passageways for blood vessels and nerves.

Bone Tissue Structure

• Bone tissue is a dense matrix type of connective tissue that is reinforced by collagen fibers.
• Bone Matrix Components:
• Calcium phosphate is 2/3 of bone mass which provides hardness and strength.
• Collagen Fibers are 1/3 of bone mass offering flexibility and resistance to tension.
• Osteogenic cells are stem cells that produce osteoblasts.
• Osteoblasts are immature bone-forming cells that secrete osteoid.
• Osteocytes are mature bone cells that maintain the matrix.
• Osteoclasts are multinucleate cells that break down bone tissue (osteolysis).

Compact Bone (Dense Bone)

• Structure: Organized into osteons (Haversian systems).
• The central canal (Haversian canal) provides a passage for blood vessels and nerves.
• Lamellae: Concentric layers surrounding the central canal.
• Lacunae are small spaces housing osteocytes.
• Canaliculi are passageways allowing nutrient exchange
• Location: Outer layer of all bones, providing strength and rigidity.

Spongy Bone (Cancellous Bone)

• Structure: Contains an open network of trabeculae (struts & plates) and lacks osteons.
• Function: Houses red bone marrow for hematopoiesis and reduces the weight of bones, making movement easier.
• Location: Found in the epiphyses of long bones and inside flat bones.

Bone Formation (Ossification)

• There are two main types of bone formation.
• Endochondral Ossification: Most bones form this way.
• Cartilage model formation: Chondrocytes enlarge and die, leaving cavities.
• Bone collar formation: Blood vessels grow around cartilage edges.
• Primary ossification center: Osteoblasts form spongy bone.
• Medullary cavity formation: Bone remodeling creates a marrow cavity.
• Secondary ossification centers: Develop in epiphyses.
• Epiphyseal cartilage: Allows bone growth until it ossifies.
• Intramembranous Ossification: Flat bones like the skull and mandible.
• Mesenchymal cells differentiate into osteoblasts.
• Osteoid secretion begins mineralization.
• Spongy bone forms, followed by compact bone layers.

Bone Remodeling and Homeostasis

• Bone remodeling: Continuous breakdown and rebuilding.
• Osteoclasts break down bone, releasing calcium.
• Osteoblasts rebuild bone.
• Regulated by hormones & mechanical stress.

Calcium Homeostasis

• Parathyroid hormone (PTH) is released when calcium is low, stimulates osteoclast activity.
• Calcitonin is released when calcium is high and inhibits osteoclasts.
• Calcitriol (vitamin D) enhances calcium absorption in the intestines.

Fractures and Healing Process

• Types of Fractures:
• Closed (Simple): Bone does not break skin.
• Open (Compound): Bone pierces through skin.
• Comminuted: Bone shatters.
• Greenstick: Incomplete break which is common in children.
• Transverse: Break at a right angle to the bone.
• Spiral: Caused by twisting forces.

Fracture Healing

• Fracture hematoma formation: Blood clot at injury site.
• Callus Formation:
• Internal callus is spongy bone that forms inside.
• External callus is cartilage and bone that forms outside.
• Spongy Bone Formation: Callus is replaced by spongy bone.
• Bone Remodeling: Spongy bone is replaced by compact bone.

Aging and Bone Health

• Bone mass declines after age 30–40.
• Women lose 8% per decade.
• Men lose 3% per decade.
• Osteopenia is mild bone loss common with aging.
• Osteoporosis is severe bone loss that leads to brittle bones.
• Risk factors include hormonal imbalances, sedentary lifestyle, and poor nutrition.
• Prevention: Weight-bearing exercises, adequate calcium & vitamin D intake, and medications (e.g., bisphosphonates).

Introduction to Joints

• Joints (articulations) are the sites where two or more bones meet, and their structure determines their function and range of motion, while allowing movement and providing stability and support.
• Importance of joints:
• Enable movement while walking, running, and grasping objects.
• Provide structural support to the skeleton.
• Absorb shock and distribute force during physical activities.
• Crucial for orthopedics, sports medicine, and rehabilitation.

Classification of Joints

• A. Structural Classification (Based on Anatomy): Joints are classified based on their connective tissue composition and presence of joint cavities.
• Fibrous joints: Bones connected by dense connective tissue allow little to no movement.
• Suture: Immovable joints between skull bones.
• Gomphosis: A peg-in-socket joint (e.g., teeth in sockets).
• Syndesmosis: Bones connected by ligaments (e.g., tibia & fibula).
• Cartilaginous Joints: Bones connected by cartilage, allowing limited movement.
• Synchondrosis: Cartilage bridge between bones (e.g., epiphyseal plate in growing bones).
• Symphysis: Bones connected by fibrocartilage (e.g., pubic symphysis).
• Bony Joints (Synostosis): Two bones fused into a single structure (e.g., epiphyseal lines in mature bones).
• Synovial Joints: Joints with a fluid-filled cavity allowing free movement (e.g., knee, shoulder, hip).

B. Functional Classification (Based on Movement)

• Classified based on their range of motion.
• Synarthrosis (immovable): Found in skull sutures and gomphoses.
• Amphiarthrosis (slightly movable): Found in syndesmoses and symphyses.
• Diarthrosis (freely movable): All synovial joints.

Synovial Joints

• These are the most mobile joints in the body.
• Consists of a joint capsule, synovial fluid, and accessory structures.
• Key Components include:
• Articular Capsule: Encloses the joint cavity and contains synovial fluid.
• Synovial Fluid: Lubricates joints, absorbs shock, and nourishes cartilage.
• Articular Cartilage: Covers bone ends to reduce friction.
• Accessory Structures: -- Ligaments: Reinforce and stabilize joints. -- Tendons: Connect muscles to bones, aiding movement. -- Menisci: Fibrocartilage pads that cushion and stabilize joints (e.g., knee menisci). -- Bursae: Fluid-filled sacs that reduce friction between tendons and bones.
• Functions of Synovial Fluid:
• Reduces friction.
• Provides nutrients to cartilage.
• Acts as a shock absorber.

Movements at Synovial Joints

• Allow a variety of movements that are categorized by their axes of motion and direction.
• Types of Motion include:
• Gliding: Two flat surfaces sliding past each other (e.g., between carpals).
• Angular Movements:
• Flexion: Decreasing joint angle (e.g., bending the elbow).
• Extension: Increasing joint angle (e.g., straightening the knee).
• Hyperextension: Extension beyond normal range.
• Abduction: Moving away from the midline (e.g., lifting arm sideways).
• Adduction: Moving toward the midline.
• Circumduction: Circular movement combining flexion, extension, abduction, and adduction.
• Rotational Movements:
• Medial Rotation: Turning inward.
• Lateral Rotation: Turning outward.
• Pronation & Supination: Forearm rotations (palm down vs. palm up).
• Special Movements:
• Inversion & Eversion: Twisting of the foot.
• Dorsiflexion & Plantar Flexion: Upward and downward foot movements.
• Opposition: Thumb movement for gripping.
• Protraction & Retraction: Forward and backward movement.
• Elevation & Depression: Upward and downward movement.

Major Joints and Their Functions

• Elbow and Knee Joints (Hinge Joints)
• Elbow joint: Formed by the humerus, radius, and ulna. -- Stabilized by: Radial collateral, annular, and ulnar collateral ligaments. -- Common injuries: Tennis elbow, UCL tears (Tommy John surgery).
• Knee Joint: Composed of the femur, tibia, and patella. -- Menisci: Shock-absorbing fibrocartilage pads. -- Ligaments: ACL, PCL, MCL, LCL stabilize the knee. -- Common injuries: ACL tears, meniscus damage, osteoarthritis.
• Shoulder and Hip Joints (Ball-and-Socket Joints)
• Shoulder joint: Greatest range of motion but prone to dislocations. -- Stabilized by: Rotator cuff muscles, glenoid labrum, and ligaments. -- Common injuries: Dislocations, rotator cuff tears.
• Hip Joint: Strong, weight-bearing joint with a deep socket. -- Stabilized by: Ligaments (iliofemoral, pubofemoral, ischiofemoral) and acetabular labrum. -- Common conditions: Hip fractures, osteoarthritis, hip labral tears.

Aging and Joint Health

• Degenerative changes occur with age in joints, leading to stiffness and mobility reduction.
• Effects of aging:
• Reduced synovial fluid: Causes increased friction.
• Cartilage degeneration: leads to osteoarthritis.
• Weakened ligaments & tendons: Reduces flexibility and stability.
• Common age-related joint disorders:
• Osteoarthritis: Cartilage breakdown due to wear and tear.
• Rheumatoid arthritis: Autoimmune disease causing joint inflammation.
• Gout: Uric acid crystal buildup in synovial fluid.

Joint Disorders and Injuries

• Herniated disc: Nucleus pulposus bulges, which compresses spinal nerves.
• Sprains: Ligament damage due to overstretching.
• Bursitis: Inflammation of bursae, causing joint pain.
• Dislocations: Bones forced out of alignment.
• Torn ligaments (e.g., ACL tear) are common sports injuries requiring surgery.

Joint Health and Injury Prevention

• Exercise: Weight-bearing activities maintain joint flexibility and strength.
• Diet: Calcium, vitamin D, and protein support joint health.
• Injury prevention: Proper warm-ups before activity, strength training for ligament support, and wearing protective gear in sports.

Introduction to Muscle Tissue

• Muscle tissue is responsible for movement, force generation, and numerous physiological functions.
• There are three primary types:
• Skeletal muscle is under voluntary control and moves the body by pulling on bones.
• Cardiac muscle is involuntary and pumps blood through the cardiovascular system.
• Smooth muscle is involuntary and moves fluids and solids through internal organs.

Common Properties of Muscle Tissue

• Excitability: Ability to respond to stimuli such as nerve impulses.
• Contractility: Ability to shorten forcefully when stimulated.
• Extensibility: Ability to stretch beyond resting length.
• Elasticity: Ability to return to original length after stretching.

Functions of Skeletal Muscle

• Producing movement by pulling on tendons.
• Maintaining posture and body position.
• Supporting soft tissues, e.g., abdominal organs.
• Guarding body entrances and exits, e.g., digestive and urinary tracts.
• Maintaining body temperature through heat production.
• Storing nutrients like amino acids from muscle proteins.

Organization of Skeletal Muscle

• Skeletal muscles are composed of muscle fibers, connective tissues, blood vessels, and nerves.
• All work together to produce movement.
• Connective Tissue Layers:
• Epimysium: Surrounds the entire muscle.
• Perimysium: Surrounds fascicles (bundles of muscle fibers).
• Endomysium: Surrounds individual muscle fibers.
• Tendons are cord-like structures that connect muscles to bones.
• Aponeuroses are broad sheets that connect muscles to other structures.
• Extensive blood supply to meet high metabolic demands.
• Motor neurons send signals for muscle contraction.

Skeletal Muscle Fibers

• They are specialized for contraction containing:
• Sarcolemma: Plasma membrane of a muscle fiber, generating action potentials.
• Sarcoplasm: Cytoplasm with myoglobin, mitochondria, and glycogen.
• T-tubules (Transverse tubules): Conduct impulses deep into the fiber.
• Sarcoplasmic reticulum (SR): Stores and releases calcium ions.
• Myofibrils & Myofilaments include:
• Thin filaments are composed of actin.
• Thick filaments are composed of myosin.
• Sarcomeres are functional contractile units containing:
• Z-lines: Define boundaries.
• A-band: Contains thick and thin filaments.
• I-band: Contains only thin filaments.
• M-line: Central stabilizing structure.
• H-zone: Area with only thick filaments.

Muscle Contraction

• Occurs when electrical signals trigger calcium release, leading to actin-myosin interactions.
• The Neuromuscular Junction (NMJ):
• Axon terminal: Releases acetylcholine (ACh).
• Motor end plate: Contains ACh receptors.
• Synaptic cleft: Space where ACh diffuses.
• Acetylcholinesterase (AChE): Breaks down ACh to stop contraction.

Excitation-Contraction Coupling

• Action potentials travel along the sarcolemma and T-tubules.
• Triggers calcium release from the SR.
• Calcium binds to troponin and shifts tropomyosin to expose actin's binding sites.
• Myosin heads attach to actin to form cross-bridges.

Sliding Filament Theory

• Myosin pulls actin toward the M-line, shortening the sarcomere.
• ATP is required to detach myosin heads and reset them.
• Muscle relaxation occurs when calcium is pumped back into the SR.

Tension in Muscle Fibers

• The amount of tension a muscle fiber produces depends on:
• Number of cross-bridges formed: more cross-bridges results in greater force.
• Length-tension relationship: Optimal sarcomere length maximizes force.
• Frequency of stimulation:
• Twitch: Single contraction-relaxation cycle.
• Wave summation: Successive stimuli increase tension. -- Incomplete tetanus: Near-maximum tension with brief relaxations. -- Complete tetanus: Sustained contraction with no relaxation.

Types of Muscle Contractions

• Motor Unit Activation & Contraction Types:
• A motor unit consists of a motor neuron and all the muscle fibers it controls.
• Recruitment: Increasing motor unit activation to generate force.
• Types of muscle contractions: -- Isotonic contractions: --- Concentric: Muscle shortens e.g., lifting a weight. --- Eccentric: Muscle lengthens despite tension e.g., lowering a weight. -- Isometric contractions: Muscle generates tension but does not change the length e.g., holding a plank.

Energy to Power Contractions

• Muscle contractions require ATP, which is generated through multiple pathways:
• Creatine phosphate (CP) system: quick ATP regeneration for short bursts.
• Glycolysis (anaerobic metabolism): fast ATP production that produces lactic acid.
• Aerobic metabolism: Most efficient ATP production using oxygen.
• Muscle Metabolism During Activity:
• At rest: Uses fatty acids, stores glycogen & CP.
• Moderate activity: Relies on aerobic metabolism.
• Peak activity: Uses glycolysis leading to lactic acid buildup.

Muscle Performance and Conditioning

• Types of muscle fibers:
• Fast fibers: High power, quick fatigue, and anaerobic metabolism.
• Slow fibers: Endurance, fatigue-resistant, and rich in mitochondria.
• Intermediate fibers: Balanced between power and endurance.
• Effects of Training:
• Anaerobic training: Increases muscle size (hypertrophy), power, and strength.
• Aerobic training: Improves endurance, oxygen delivery, and mitochondria efficiency.
• Aging effects: Muscle loss (atrophy), slower recovery, and reduced elasticity.

Cardiac Muscle Tissue

• Specialized for continuous contractions, ensuring blood circulation.
• Characteristics:
• Branched, striated cells with intercalated discs.
• Pacemaker cells generate automatic contractions.
• Long refractory periods prevent tetanus.
• Depends on aerobic metabolism which prevents fatigue.

Smooth Muscle Tissue

• Typically found in hollow organs, blood vessels, and digestive organs.
• Characteristics of Smooth Muscle:
• Spindle-shaped, single-nucleated, non-striated cells.
• No sarcomeres, with actin and myosin arranged in a web-like structure.
• Contracts slowly but sustainably.
• Types of Smooth Muscle:
• Multiunit Smooth Muscle: Independent contractions e.g., the iris of the eye.
• Visceral Smooth Muscle: Coordinated contraction via gap junctions e.g., intestines.

Fascicle Arrangement and Muscle Function

• Muscles are made up of fascicles, which are bundles of muscle fibers.
• These are arranged in different patterns that influence muscle function, strength, and range of motion.
• Types of fascicle arrangements:
• Parallel Muscles: -- Fascicles run parallel to the muscle's long axis. -- Can shorten significantly (~30% of resting length). -- Example is the Biceps brachii, Rectus abdominis.
• Convergent Muscles: -- Broad origin with fascicles converging at a single insertion. -- Can pull in multiple directions. -- Example is the Pectoralis major.
• Pennate Muscles: -- Fascicles form an angle with the tendon. -- More force production but less range of motion. -- Types: --- Unipennate: All fibers on one side of tendon (Extensor digitorum). --- Bipennate: Fibers on both sides (Rectus femoris). --- Multipennate: Tendon branches within the muscle (Deltoid).
• Circular Muscles (Sphincters): -- Fascicles arranged concentrically around openings. -- Control passage through body openings. -- Example is the Orbicularis oris.
• Muscle Function and Fascicle Arrangement: -- Parallel muscles have a greater range of motion. -- Pennate muscles generate more force. -- Circular muscles act as sphincters to regulate body openings.

Levers and Muscle Efficiency

• Muscles generate movement by acting on bones.
• Bones function as levers.
• Key Lever Components:
• Fulcrum (F): Fixed pivot point e.g., a joint.
• Applied Force (AF): The effort exerted by a muscle.
• Load (L): The weight or resistance moved.
• Types of Levers in the Human Body:
• First-Class Lever: -- Fulcrum is between the applied force and load such as a seesaw. -- Example is the Neck extension (atlanto-occipital joint).
• Second-Class Lever: -- Load is between the applied force and fulcrum such as a wheelbarrow. -- Example: Plantar flexion (standing on tiptoes). -- Advantage: Small effort moves a large load.
• Third-Class Lever (Most Common in the Body): -- Applied force is between the fulcrum and load such as tweezers. -- Example: Biceps brachii flexing the forearm. -- Advantage: Maximizes speed and range of motion effectively.

Effects on Movement and Injury Risk

• First-class levers balance force and speed.
• Second-class levers maximize strength but reduce speed.
• Third-class levers maximize speed but require more force, increasing the risk of strain injuries e.g., biceps tendon rupture.

Origins, Insertions, and Muscle Interactions

• Muscles function by attaching to bones at specific points.
• Origin: fixed, less movable attachment.
• Insertion: more movable attachment.
• Muscle Roles in Movement:
• Agonist (Prime Mover): Main muscle responsible for movement. -- Example: Biceps brachii (elbow flexion).
• Antagonist: Opposes the agonist's movement. -- Example: Triceps brachii opposes elbow flexion.
• Synergist: Assists the agonist. -- Example: Brachialis assists the biceps brachii.
• Fixator: Stabilizes a joint, preventing unwanted movement. -- Example: Rotator cuff muscles stabilize the shoulder.
• Effects of Muscle Imbalances:
• Weak antagonists result in poor posture e.g., rounded shoulders.
• Agonist dominance leads to increased injury risk e.g., hamstring strains in athletes with dominant quadriceps.

Naming Skeletal Muscles

• Skeletal muscles are named based on characteristics such as location, shape, and function.
• Naming Criteria: -- Region of the Body: Temporalis (temporal region), Brachialis (arm), Femoris (thigh). -- Direction of Fibers: Rectus (straight), Oblique (angled), Transversus (horizontal). -- Structural Characteristics: --- Shape: Deltoid (triangle), Trapezius (trapezoid), Rhomboid (diamond). --- Size: Maximus (largest), Minimus (smallest), Longus (long). --- Number of Origins: Biceps (two heads), Triceps (three heads), Quadriceps (four heads). -- Origin and Insertion: Sternocleidomastoid (origin is the sternum & clavicle & insertion is the mastoid process). -- Function or Action: Flexor (bends a joint), Extensor (straightens a joint), Abductor (moves away from the midline).
• Importance of Muscle Names: -- Understanding naming conventions simplifies muscle identification. -- Recognizing patterns in muscle names improves memory retention and application in healthcare and fitness.

Practical Applications and Injury Prevention

• Muscle Mechanics and Injury Risk: -- Lever mechanics impact force production and injury susceptibility. -- Muscle imbalances lead to inefficient movement and strain. --- Overuse injuries e.g., Achilles tendonitis in second-class levers can be prevented through balanced training and stretching. --- Rehabilitation and Training Considerations: ---- Strengthening antagonists improves stability and prevents injuries. ---- Optimizing lever mechanics enhances movement efficiency. ---- Understanding muscle roles aids in injury recovery and physical therapy.