Skeletal System Functions

• The skeletal system serves various vital functions including:
  • Support: Provides a rigid framework for the body, giving it shape and support.
  • Protection: Encloses and protects vital organs (e.g., the skull protects the brain, ribs protect the heart and lungs).
  • Movement: Facilitates movement through attachment points for muscles that contract and pull on the bones.
  • Store Calcium and Phosphorus: Bones act as a reservoir for minerals, crucial for various bodily functions.
  • Blood Cell Production (Marrow): Bone marrow located within certain bones produces blood cells, including red blood cells, white blood cells, and platelets.

Bone Anatomy

• Microscopic Structure:

  • Haversian System: The structural unit of compact bone, consisting of concentric circles of calcified matrix (lamellae) around a central canal (Haversian canal) which contains blood vessels and nerves.

• Macroscopic Structure:

  • External components visible on handouts.

Skeletal System Components

• Periosteum:

  • Description: Dense, fibrous membrane covering the external surface of bones (outer layer).
  • Function: Contains blood vessels and serves as an attachment point for tendons and ligaments.

• Endosteum:

  • Description: Thin layer of connective tissue lining the internal surface of the bone (inner layer).
  • Cells: Contains osteoclasts (for bone resorption) and osteoblasts (for bone formation).

• Bone Cell Types:

  • Osteoblasts:
  • Function: Cells responsible for bone formation; they synthesize and secrete the bone matrix.
  • Osteocytes:
  • Function: Mature bone cells that maintain the bone matrix and its function.
  • Osteoclasts:
  • Function: Cells involved in bone resorption, helping to break down bone tissue when needed.

• Other Structures:

  • Medullary Cavity: Contains bone marrow (either red or yellow marrow).
  • Articular Cartilage (hyaline): Reduces friction at joints.
  • Diaphysis: Shaft of the bone, primarily comprised of compact bone.
  • Epiphysis: End part of the long bone, involved in joint formation covered by articular cartilage to reduce friction.
  • Metaphysis: Area where the diaphysis meets the epiphysis, important during bone growth.

Bone Cells and Functions

• Bone Composition:
  • Bone structure resembles a "wire mesh” reinforced with calcium and phosphorus, providing strength and rigidity.

Bone Development Overview

• Ossification: The process of bone formation.
  1. Intramembranous Ossification:
  • Description: Bone develops directly from fibrous connective tissue.
  • Examples: Few bones (e.g., facial bones, skull, and others 'odd-shaped').
  • Process: Begins at the center of a fibrous membrane and radiates outwards to form bone.
  1. Endochondral Ossification:
  • Description: Most common type of ossification; bone replaces existing hyaline cartilage.
  • Examples: Long bones, particularly in arms and legs.
  • Process: Starts with a hyaline cartilage model; bone growth occurs lengthwise at the epiphysis.

Bone Growth Mechanisms

• Growth in Length:

  • Occurs at the epiphyseal plate where new cartilage forms, is replaced by bone, and thereby increases bone length.
  • Zones of Growth:
  • Zone of Resting Cartilage: Quiescent (inactive) chondrocytes.
  • Zone of Proliferating Cartilage: Chondrocytes undergo mitosis, leading to an increase in the number of cells.
  • Zone of Hypertrophic Cartilage: Mature chondrocytes enlarge.
  • Zone of Calcified Cartilage: Chondrocytes die, leaving behind woven bone.

• Growth in Diameter:

  • Inner Layer (Endosteum): Osteoclasts break down bone.
  • Outer Layer (Periosteum): Osteoblasts add new muscle.

Regulation of Bone Growth

• Hormonal Regulation:
  • Growth Hormone: Secreted from the pituitary gland, stimulates growth.
  • Calcitonin: Hormone that promotes the deposition of calcium into the bones, stimulating osteoblast activity.
  • Parathyroid Hormone (PTH): Stimulates the release of calcium from bone, activating osteoclasts.

Joint Characteristics and Function

• Types of Joints:

  1. Fibrous Joints: Joined by connective tissue, generally immovable.
  2. Cartilaginous Joints: Joined by cartilage, allows slight movement.
  3. Synovial Joints: Contain a joint cavity, allowing for free movement.

• Joint Function Types:

  • Synarthrosis: Immovable joints (e.g., sutures of the skull).
  • Amphiarthrosis: Slightly movable joints (e.g., pubic symphysis).
  • Diarthrosis: Freely movable joints (e.g., knee and elbow joints).

• Structural Components of Synovial Joints:

  • Articular cartilage (hyaline) that decreases friction.
  • Joint capsule with synovial fluid helping to lubricate and nourish cartilage.

Movements of Synovial Joints

• Flexion/Extension: Decrease/increase angle between two bones.
• Abduction/Adduction: Movement away/towards the midline of the body.
• Circumduction: Circular movement at joints (shoulder and hip).
• Rotation: Bone rotates around its own axis.
• Special Movements: Elevation, depression, inversion, eversion, dorsiflexion, plantarflexion, and rotation.

Muscular System Overview

• General Functions:

  • Movement, maintaining posture, moving fluids through various systems (e.g., GI, blood vessels), generating heat, and stabilizing joints.

• Three Types of Muscle:

  1. Skeletal Muscle: Voluntary, striated, primarily involved in body movement.
  2. Cardiac Muscle: Involuntary, striated muscle found only in the heart, regulated by pacemaker cells.
  3. Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs.

Properties of Muscles

• Excitability: The ability to respond to stimuli (action potentials).
• Contractibility: Capability to shorten when stimulated.
• Elasticity: Ability to return to original shape after stretching.

Muscle Anatomy Components

• Connective Tissue Structures:
  • Fascia: Sheet of connective tissue around muscles.
  • Tendon: Band of connective tissue that attaches muscle to bones (continuation of fascia).
  • Aponeurosis: Flat layer continuation of fascia.

Muscle Fiber Anatomy

• Microscopic Structures:
  • Sarcolemma: Membrane of muscle fibers; contains multiple nuclei.
  • T-tubules: Extensions branching from the sarcolemma, aiding in electrical transmission.
  • Sarcoplasmic Reticulum: Stores calcium ions, facilitating contraction.

Sliding Filament Theory of Muscle Contraction

• Mechanism: Myosin heads pull actin filaments toward the M-line, resulting in a shortening of the sarcomere while the filaments themselves remain the same length.
• Effects during Contraction:
  • A band: Remains the same length.
  • I band: Shortens (may disappear).
  • H zone: Shortens (may disappear).
  • Zone of overlap: Lengthens.
  • Sarcomere: Shortens (Z disc to Z disc).

Neuromuscular Junction (NMJ) Functionality

• Interaction between Nerve and Muscle Fiber:
  • Action potentials at the axon terminal release neurotransmitters (e.g., acetylcholine) into the synaptic cleft.
  • Acetylcholine binds to receptors on the muscle, leading to a depolarization and subsequent muscle action potential.

Neuromuscular Junction Components

• Axonal Terminal: End of the nerve cell where neurotransmitters are released.
• Synaptic Cleft: Gap across which neurotransmitters travel to affect muscle fibers.
• Motor End Plate: Specialized region of the sarcolemma with receptors for acetylcholine, increasing surface area for binding.

Effects of Acetylcholine on Muscle Contraction

• When acetylcholine binds to receptors at the NMJ, sodium ions enter muscle fibers, leading to depolarization and contraction.
• Enzyme acetylcholinesterase breaks down acetylcholine after its action, preventing continuous stimulation of muscle fibers.

Clinical Implications

• Botulism: Toxin prevents acetylcholine release resulting in muscle weakness and possible death.
• Myasthenia Gravis: Autoimmune reaction attacking acetylcholine receptors leads to muscle weakness.
• Organophosphate Poisoning: Inhibits acetylcholinesterase, causing overstimulation of muscles due to persistent acetylcholine action.