Human Skeletal, Muscular, and Joint Systems

Overview

This lecture explains the structure and function of the human skeletal, muscular, and joint systems, focusing on key concepts for exams. It covers the divisions of the skeleton, types and structure of bones and cartilage, types of joints and their diseases, and the structure and function of muscles, including mechanisms of contraction.

Human Skeleton: Structure and Divisions

• The human skeleton is an endoskeleton made up of bones and cartilage, providing support, protection, movement, mineral storage, and blood cell production.
• It is divided into:
  • Axial skeleton (80 bones): includes the skull, rib cage, and vertebral column.
    • Skull: 29 bones (8 cranial, 14 facial, 6 ear ossicles, 1 hyoid bone). Cranial bones include 4 paired (parietal, temporal) and 4 unpaired (frontal, occipital, sphenoid, ethmoid). Facial bones include 6 paired (palatine, maxilla, lacrimal, nasal, inferior concha, zygomatic) and 2 unpaired (mandible, vomer).
    • Rib cage: 12 pairs of ribs (24 total): 7 true ribs (directly attached to sternum), 3 false ribs (attached via costal cartilage), and 2 floating ribs (not attached to sternum).
    • Vertebral column: 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 sacral fused, 4 coccygeal fused). The first two cervical vertebrae are atlas and axis. The sacrum (5 fused) and coccyx (4 fused) form the base.
  • Appendicular skeleton (126 bones): includes limbs and girdles.
    • Pectoral girdle: 2 clavicles (collar bones), 2 scapulae (shoulder blades).
    • Upper limbs: humerus, radius (thumb side), ulna (little finger side), 8 carpals (wrist), 5 metacarpals (palm), 14 phalanges (fingers).
    • Pelvic girdle: 2 hip bones (each formed by fusion of ilium, ischium, and pubis), joined at the pubic symphysis (cartilaginous joint).
    • Lower limbs: femur (thigh), patella (knee cap), tibia (shin), fibula (outer bone), 7 tarsals (ankle), 5 metatarsals (foot), 14 phalanges (toes).

Bone and Cartilage

• Bone is the hardest connective tissue, providing protection, support, movement (with muscles), mineral storage (mainly calcium), and blood cell production (hematopoiesis).
  • Structure:
    • Periosteum: outer protective layer.
    • Endosteum: inner protective layer.
    • Epiphysis: ends of long bones, mainly spongy bone with red marrow (site of blood cell production).
    • Diaphysis: shaft, contains yellow marrow (fat storage).
  • Bone cells:
    • Osteoblasts: form new bone, mononucleated, responsible for mineralization.
    • Osteocytes: mature bone cells, maintain bone tissue, found in lacunae, regulate response to stress.
    • Osteoclasts: multinucleated, resorb bone, phagocytic, found at sites of old or injured bone.
  • Bone matrix: 65–70% inorganic (mainly hydroxyapatite, a form of calcium phosphate), 30–35% organic (mostly collagen).
• Cartilage is softer than bone but still firm, found at bone ends and in specific structures.
  • Types:
    • Hyaline cartilage: most abundant, found at ends of long bones and throughout the respiratory tract (except epiglottis).
    • Elastic cartilage: found in epiglottis and external ear (pinna).
    • Fibrocartilage: found in intervertebral discs and knee joints.
  • Cartilage cells are called chondrocytes.
  • Cartilage lacks blood vessels, nerves, and lymphatics; receives nutrients by diffusion.
  • Contains type II collagen (bone contains type I).

Structure of Bones: Compact and Spongy

• Compact bone:
  • Dense and strong, forms the outer layer of bones.
  • Composed of structural units called osteons (Haversian systems).
    • Each osteon has concentric rings (lamellae) around a central (Haversian) canal containing blood vessels.
    • Osteocytes are housed in small spaces called lacunae between lamellae.
    • Canaliculi are tiny channels connecting lacunae, allowing nutrient and waste exchange between osteocytes.
    • Volkmann’s canals (perforating canals) run perpendicular to Haversian canals, connecting them and carrying blood vessels from periosteum to endosteum.
• Spongy bone:
  • Porous, highly vascular, found mainly in epiphyses of long bones.
  • Contains red bone marrow (site of hematopoiesis).
  • Osteocytes are irregularly placed within the trabeculae.

Joints and Arthritis

• Joints (articulations) are points where two bones or a bone and cartilage meet.
  • Types of joints:
    • Fibrous joints: no joint cavity, immovable (e.g., skull sutures, tooth sockets).
    • Cartilaginous joints: bones connected by cartilage, allow slight movement (e.g., intervertebral discs, pubic symphysis, costal cartilage).
    • Synovial joints: freely movable, have a joint cavity filled with synovial fluid, surrounded by a fibrous capsule and synovial membrane.
      • Types: hinge (elbow, knee), ball and socket (shoulder, hip), pivot (atlas/axis), saddle (thumb), gliding (between vertebrae), ellipsoidal/condyloid (wrist, metacarpophalangeal joints).
• Arthritis: inflammation of joints, causing pain, swelling, and stiffness.
  • Osteoarthritis: most common, degenerative, often in elderly, can be inherited.
  • Rheumatoid arthritis: autoimmune, most dangerous, affects synovial membrane, often in hands and wrists.
  • Gouty arthritis: due to uric acid crystal deposition, commonly affects the big toe, a metabolic disorder.
  • Acute arthritis can be caused by bacterial infection and treated with antibiotics.

Muscles: Types and Structure

• Types of muscles:
  • Smooth muscle: involuntary, unstriated, found in walls of hollow organs (e.g., digestive tract, blood vessels), slow contraction, fatigue-resistant, uninucleated.
  • Cardiac muscle: involuntary, striated (irregular), found only in the heart, intermediate contraction speed, fatigue-resistant, uninucleated, contains intercalated discs for synchronized contraction.
  • Skeletal muscle: voluntary, striated (regular), attached to bones (mainly compact bone), rapid contraction, can fatigue, multinucleated (nuclei at periphery).
• Skeletal muscle structure:
  • Surrounded by epimysium (outermost layer).
  • Bundles called fascicles are wrapped in perimysium.
  • Individual muscle fibers are covered by endomysium.
  • Muscle fiber membrane is called sarcolemma (or myolemma).
  • T-tubules (transverse tubules) are invaginations of the sarcolemma, transmit nerve impulses into the fiber.
  • Sarcoplasm contains glycogen and myoglobin (muscle hemoglobin).
  • Sarcoplasmic reticulum (smooth ER) stores and releases calcium for contraction.
  • Multiple nuclei are located near the plasma membrane.
  • Myofibrils (1–2 micrometers in diameter) are made of myofilaments (thick and thin).
    • Thick filaments: made of myosin (diameter ~15–16 nm, 300 myosin molecules per filament).
    • Thin filaments: made of actin (main protein), tropomyosin (covers myosin binding sites), and troponin (binds calcium, regulates contraction).
  • Sarcomere: functional unit of contraction, extends from one Z-line to the next.
    • I band: light, contains only thin filaments, bisected by Z-line.
    • A band: dark, contains thick filaments (with overlapping thin filaments), includes H zone (only thick filaments) and M line (center).
    • During contraction, I band and H zone shorten, A band remains unchanged, Z-lines move closer, sarcomere shortens.
• Muscle contraction mechanism:
  • Nerve impulse triggers release of acetylcholine at neuromuscular junction.
  • Impulse travels along sarcolemma and T-tubules, causing sarcoplasmic reticulum to release calcium.
  • Calcium binds to troponin C, causing tropomyosin to move and expose myosin binding sites on actin.
  • Myosin heads bind to actin (cross-bridge formation), ATP is required for detachment and re-cocking of myosin heads.
  • ATP is also needed for calcium reuptake into sarcoplasmic reticulum (muscle relaxation).
  • Without ATP, myosin remains attached to actin, causing muscle rigidity.

Key Terms & Definitions

• Endoskeleton: Internal support structure made of bone and cartilage.
• Epiphysis: End part of a long bone, contains spongy bone and red marrow.
• Diaphysis: Shaft or central part of a long bone, contains yellow marrow.
• Periosteum: Outer protective layer of bone.
• Endosteum: Inner protective layer of bone.
• Osteon (Haversian System): Structural unit of compact bone.
• Osteoblasts: Bone-forming cells.
• Osteocytes: Mature bone cells that maintain bone tissue.
• Osteoclasts: Cells that break down bone tissue.
• Lacunae: Spaces housing osteocytes in bone matrix.
• Canaliculi: Small channels connecting lacunae for nutrient transfer.
• Volkmann’s Canal: Channels running perpendicular to Haversian canals, connecting them.
• Hematopoiesis: Production of blood cells in bone marrow.
• Chondrocytes: Cartilage cells.
• Arthritis: Inflammation of one or more joints.
• Sarcomere: Functional contractile unit of muscle fiber.
• Striations: Bands seen in skeletal and cardiac muscle due to filament arrangement.
• Intercalated discs: Specialized connections in cardiac muscle for synchronized contraction.
• Motor unit: A motor neuron and all the muscle fibers it innervates.
• Triad: A T-tubule plus two terminal cisternae of the sarcoplasmic reticulum.

Action Items / Next Steps

• Review lecture notes, focusing on bone structure, joint types, and muscle contraction mechanisms.
• Complete assigned MCQs and practice questions provided with the video.
• Prepare for the next lecture on evolution.
• Take screenshots of key diagrams and tables for visual memory.
• Pay special attention to frequently asked exam topics, such as the division of the human skeleton, bone and cartilage structure, types of joints, and muscle contraction steps.