Bones and the Skeletal System

Important Functions of Bones

• Support: Bones provide the framework for the body and support soft organs.
• Protection: Bones shield and protect vital organs.
• Movement: Bones act as levers for muscles to pull on, enabling movement.
• Mineral Storage: Bones store calcium and phosphorus, releasing them into the bloodstream as needed.
• Blood Cell Production: Hematopoiesis occurs in bone marrow.
• Triglyceride Storage: Fat is stored in yellow marrow and used as an energy source.
• Hormone Production: Osteocalcin regulates blood glucose homeostasis and insulin secretion.

The Skeleton

• The human skeleton has 206 named bones.
• The skeleton divides into the axial and appendicular skeletons based on location.

Axial Skeleton

• Consists of the skull, vertebral column, and rib cage.
• Protects vital organs.

Appendicular Skeleton

• Includes the bones of the upper and lower limbs.
• Includes girdles that attach limbs to the axial skeleton.
• Enables movement.

Skeletal Variations

• Gender differences include bone size and density, with male skeletons typically larger and denser.
• Female pelvis is broader than male for childbirth.
• Maturation occurs around age 25.
• After age 50, there is a slow loss of bone density.
• Spinal curvatures include scoliosis (lateral curvature), kyphosis (hunchback), and lordosis (swayback).
• Environmental factors like inadequate calcium or vitamin D can lead to early degeneration.

Classifications of Bones

• Long bones are longer than they are wide.
• Short bones are cube-shaped (as wide as they are long).
• Flat bones are thin, flat, and slightly curved.
• Irregular bones have complicated shapes.
• Sesamoid bones are small, round, and embedded in tendons (e.g., patella).

Bone Structure

• Bones are organs composed of osseous tissue, nervous tissue, cartilage, connective tissue, muscle, and epithelial tissue.
• Compact bone is a dense outer layer made of osteons for strength.
• Spongy bone is a porous inner layer that reduces weight and absorbs shock.

Structure of Long Bones

• Diaphysis: Tubular shaft forming the axis, made of compact bone.
• Medullary cavity: Stores marrow within the diaphysis.
• Epiphysis: End of the bone, consisting of spongy bone with an outer compact bone layer.
• Epiphyseal plate (in youth) becomes the epiphyseal line (in adulthood).
• Articular cartilage covers the ends of long bones, providing a smooth, flexible surface for low-friction movement, shock absorption, and load distribution.

Membranes

• Periosteum: Outer surface of compact bone (except where there is articular cartilage).
• Endosteum: Thin membrane lining the compact bone and medullary cavity.

Hematopoietic Tissue

• Red bone marrow forms new blood cells.

Cells of Bone Tissue

• Osteogenic Cells: Stem cells that differentiate into osteoblasts or bone-lining cells, involved in growth and repair.
• Osteoblasts: Bone-forming cells that secrete bone matrix (calcium and collagen) for growth and healing.
• Osteocytes: Mature bone cells that monitor and maintain bone by regulating remodeling and mineral homeostasis.
• Osteoclasts: Bone-resorbing cells that break down bone, originating from hematopoietic stem cells, aiding in remodeling and repair.
• Bone-lining cells: Flat cells that cover the bone surface, regulating mineral movement; periosteal cells on external surfaces, endosteal cells on internal surfaces.

Balancing Act: Osteoblasts and Osteoclasts

• Osteoblasts and osteoclasts maintain bone density.
• Osteoporosis occurs when osteoblasts don't produce enough bone, or bone is broken down by osteoclasts faster than osteoblasts can produce it.

Clinical Application: Osteoporosis

• Osteoporosis is a metabolic bone disease characterized by loss of mineralized bone mass and deterioration of bone structure, increasing fracture risk.
• Risk factors include age, gender (women more likely), family history, poor diet, smoking, excessive alcohol consumption, and a sedentary lifestyle.

Bone Development

• Ossification/osteogenesis is the process of bone tissue formation.
• Bone formation occurs during embryonic/fetal development, postnatally (growth until early adulthood), and throughout life for continuous changes and injury repair.

Embryonic Development

• Begins in the second month of fetal development.
• Endochondral Ossification: Replacement of hyaline cartilage with bone (except for flat bones).
• Osteoblasts invade the cartilage model, replacing it with bone in "Primary Ossification".
• Osteoclasts clear away the medullary cavity from the center of the diaphysis as bone formation progresses.
• Secondary ossification centers develop at the epiphyses of long bones.
• Intramembranous Ossification: Development of flat bones (skull & clavicles) through the transformation of mesenchymal tissue into bone.
• Mesenchymal cells differentiate into osteoblasts and secrete bone matrix (collagen, minerals), forming a network of spongy bone.
• Increased matrix leads to denser tissue and solid layers of compact bone.

Postnatal Bone Growth

• Length: Occurs at the epiphyseal plate through cartilage cell division by chondrocytes.
• New cartilage pushes older cartilage towards the diaphysis, adding length to the bone.
• Osteoblasts convert cartilage into bone.
• The epiphyseal plate ossifies in late adolescence, ending lengthening (females at 18, males at 21).
• Width: Osteoblasts in the periosteum add new bone tissue to the outer surface.
• Osteoclasts in the endosteum break down bone tissue in the medullary cavity.

Hormonal Control

• Growth Hormone: Stimulates epiphyseal plate activity during infancy and childhood.
• Thyroid Hormone: Regulates bone growth rate by modulating growth hormone activity and ensuring proper growth plate development.
• Sex hormones accelerate adolescent bone growth (growth spurts) and induce epiphyseal plate closure, ending growth.

Bone Remodeling or Fracture Repair

• Continuous process to maintain strength, repair damage, and adapt to stresses.
• Key components are deposition (new bone matrix by osteoblasts) and resorption (bone matrix broken down by osteoclasts).

Regulating Remodeling: Hormonal Control

• Negative feedback loop involves PTH (parathyroid hormone) and calcium.
• Increased calcium leads to secretion of calcitonin, which moves calcium into bone, stimulating osteoblast activity.
• Decreased calcium leads to secretion of PTH, which signals osteoclasts to resorb more bone.
• High blood calcium causes the thyroid to secrete calcitonin, moving calcium into bone and stimulating osteoblasts.
• Low blood calcium causes PTH glands to secrete PTH, osteoclasts reabsorb bone and release calcium into the blood.
• Osteoblast activity increases with physical activity, increasing bone density and strength.
• Osteoclast activity increases with inactivity, increasing resorption and weakening bone.

Fractures

• Fractures are breaks in the bone caused by injury (direct force/trauma), stress/fatigue (repeated wear and tear), or weakening from pathological conditions.
• Classification of Fractures:
  • Nondisplaced: Bone ends remain aligned.
  • Displaced: Bone ends are misaligned.
  • Complete: Broken all the way through into 2 separate fragments.
  • Incomplete: Hasn't divided into separate pieces.
  • Open/Compound: Skin is penetrated.
  • Closed/Simple: Skin remains intact.

Bone Repair

• Purpose: Restore bone to its original structure and strength.
• Takes ~6 months
• Four major stages that begin with the formation of a blood clot:
  • Hematoma Formation: Bleeding at the injury site leads to clot formation with symptoms of swelling, bruising, pain, and tenderness.
  • Soft Callus Formation: Cartilage and fibrous tissue replace clots, bridging the fracture and providing stability, with reduced swelling, decreased mobility, and increased stiffness.
  • Hard Callus Formation: Osteoblasts produce new bone tissue, replacing the soft callus with a hard callus, with a visible lump and firm/tense skin.
  • Bone Remodeling: Hard callus is remodeled into mature (compact) bone, returning the bone to its original shape/strength, with a normalized appearance and slight variations in bone contour.

Joints

• Joints, or articulations, are where two bones meet, allowing for movement and flexibility.

Classification of Joints

• Structural Classification: Physical composition and type of connective tissue (fibrous, cartilaginous, synovial).
• Functional Classification: Degree/range of motion allowed by the joint (synarthroses, amphiarthroses, diarthroses).
• Synarthroses: Immovable joints.
• Amphiarthroses: Slightly movable joints.
• Diarthroses: Freely movable joints, allowing for a wide range of motion.

Structural Classifications

• Fibrous or synesthetic joints are connected by dense fibrous connective tissue.
• Sutures: Rigid, interlocking joints found in the flat bones of the skull, with a thin layer of DCT that ossifies before adulthood.
• Syndesmoses: Connected by ligaments with varying degrees of movement based on fiber length; short fibers allow little to no movement, longer fibers allow a larger degree of movement.
• Gomphoses: Peg-in-socket joints, only found in the mouth (teeth).
• Cartilaginous Joints: Held together by cartilage.
• Synchondroses: Connected by hyaline cartilage, these synarthrotic (immovable) joints can be temporary (epiphyseal plate/growth plate) or permanent (first rib connected to manubrium).
• Symphyses: Held together by a pad of fibrocartilage, providing resistance and compressive abilities; amphiarthrotic joints found in the pelvis and intervertebral discs of the spine.

Synovial Joints

• Diarthrotic Joints: Bones are separated by a fluid-filled cavity.
• Articular Cartilage: Smooth, slippery tissue covering the ends of bones, preventing/reducing friction.
• Synovial Cavity: Small space between articulating bones filled with fluid.
• Synovial Fluid: Viscous fluid within the synovial cavity that lubricates joints, nourishes cartilage, and absorbs shock.
• Articular Capsule: Double-layered structure surrounding the joint; the external fibrous layer is strong and made of dense irregular CT, and the inner synovial membrane is loose CT where fluid is synthesized.

Synovial Joint Structure

• Reinforcing Ligaments: Strong bands of CT; capsular ligaments are thickened parts of the joint for support and stabilization, extracapsular ligaments are outside the joint for stabilization, and intracapsular ligaments are inside the joint capsule for stabilization.
• Nerves and Blood Vessels: Provide a sense of joint position (proprioception), pain sensation, and circulation.
• Fatty pads: Layers of adipose tissue between the fibrous layer of the capsule and the synovial membrane, providing cushioning.
• Articular discs: Fibrocartilage that separates articular surfaces, enhancing how the ends of two bones connect, stabilizing the joint, and reducing wear and tear.

Additional Synovial Structures

• Bursae: Small, fluid-filled sacs between bones and soft tissue that reduce friction and cushion pressure points.
• Tendon Sheaths: Tubular structures that provide lubrication, reducing friction as tendons slide over bone.

Factors Affecting Synovial Joint Stability

• Shape of Articular Surfaces: The fit and contour of bones affect stability; deeper socket joints (hips) offer more stability than shallow socket joints (shoulders).
• Ligaments: A greater number of ligaments at a joint enhances stability.
• Muscle Tone: Muscle tension around the joint helps stabilize it; well-conditioned muscles provide better support.

Functional Classification of Joints

• Nonaxial Joints: Gliding/sliding movement between bones without rotation (plane or "gliding" joint), e.g., intercarpal joints of the wrist and intertarsal joints of the ankle.
• Uniaxial Joints: Movement around a single axis, in one direction/degree.
• Hinge joints: Cylindrical end of one bone fits into another, allowing flexion and extension.
• Pivot joints: Rounded end of one bone fits into a ring of another bone, allowing rotation.
• Biaxial Joints: Allows movement around two axes.
• Condyloid Joints: Oval-shaped bone fits into the cavity of another bone, allowing flexion, extension, abduction, and adduction.
• Saddle Joints: Two bones each with concave and convex surfaces, allowing flexion, extension, abduction, and adduction.
• Multiaxial Joints: Allows the highest degree of motion around multiple axes.
  • Ball-and-Socket Joints: One bone has a ball-shaped end that fits into a cup-like socket, allowing flexion, extension, abduction, adduction, rotation, and circumduction.

Initiating Joint Movement

• Joint movement process:
  • Muscle contraction: Muscles shorten and pull on attached bones.
  • Tendon connection: Tendons link muscles to bone, transferring contraction force.
  • Joint action: Muscle pulling moves connected bones at the joint, enabling movements.

Types of Joint Movement

• Gliding: Sliding or slippage of one bone over another (e.g., movement in vertebral joints).
• Angular: Change in angle (decrease/increase) between two bones.
  • Flexion: Bending of a limb at a joint, decreases angle.
  • Extension: Straightening limb at a joint, increasing angle.
  • Hyperextension: Movement beyond anatomical position (e.g., head, shoulder, spine, knee).
  • Abduction: Moving a limb away from the midline.
  • Adduction: Moving a limb toward the midline (e.g., shoulders).
• Circumduction: Movement of a limb in a circular or cone pattern (e.g., shoulder, hip, wrist, and ankle).
• Rotational: Turning around an axis, toward/away from the midline.
  • Lateral rotation: External rotation, away from the midline.
  • Medial rotation: Internal rotation, toward the midline (e.g., head, vertebral column).
• Special: Doesn't fit into any other category.
  • Pronation: Palms face posteriorly.
  • Supination: Palms face anteriorly (e.g., forearm).
  • Dorsiflexion: Upper side of the foot moves towards the shin.
  • Plantarflexion: Sole of the foot moves towards the floor, leaving toes pointing downward (e.g., ankle).
  • Inversion: Twisting of the sole of the foot inward toward the midline.
  • Eversion: Twisting or turning of the sole of the foot outward, away from the midline (e.g., ankle).
  • Elevation: Movement of a body part in a superior or upward motion.
  • Depression: Movement of a body part inferiorly or downward (e.g., shoulder, mandible, rib cage).

Review of Select Synovial Joints

• Shoulder or Glenohumeral Joint

  • Multiaxial, ball-and-socket joint formed by the articulation between the head of the humerus and the glenoid cavity of the scapula.
  • The most freely moving joint with the largest ROM but the least stable.
  • Articular cartilage covers the humeral head and glenoid cavity to reduce friction.
  • The labrum is a fibrocartilage lining that improves stability.
  • Coracohumeral ligament: Coracoid process of the scapula to the greater tubercle of the humerus, providing stability against downward dislocation.
  • Glenohumeral ligaments: 3 ligaments (superior, middle, inferior) from the glenoid cavity of the scapula to the humerus.

• Elbow or Humeroulnar Joint

  • Uniaxial, hinge joint formed by the trochlea of the humerus and trochlear notch of the ulna.
  • Articular cartilage covers ends of the humerus and ulna.
  • Annular ligament wraps around the head of the radius allowing rotation.
  • Ulnar Collateral ligament provides stability to the inner elbow by preventing excessive outward movement.
  • Radial Collateral ligament stabilizes the outer elbow, preventing excessive inward angling of the forearm.

• Hip or Coxafemoral Joint

  • Multiaxial, ball-and-socket joint between the head of the femur and the acetabulum of the hip bone.
  • The deepest joint is very stable and allows a good ROM.
  • Articular cartilage covers the femoral head and acetabulum, reducing friction.
  • Labrum is a fibrocartilage ring surrounding the acetabulum, helping to deepen the socket.
  • Iliofemoral Ligament: Ileum to femur, prevents hyperextension (strongest).
  • Pubofemoral ligament: Pubis to femur, limits abduction and extension.
  • Ishiofemoral ligament: Ischium to femur, stability during internal rotation.

• Knee

• Uniaxial, modified hinge joint made of 3 articulations; the largest and complex

• Femoropatellar – Distal femur and patella

• Lateral and medial tibiofemoral – lateral and medial condyles between the femur and the tibia

• Articular Catilage: covers the ends of the femur and tibia, which reduce friction

• Menisci: Help distribute weight evenly to reduce stress, cushion and distribute the load

• Anterior Cruciate Ligament: Runs diagonally across the front knee and prevents tibia from sliding forward or hyper extending

• Posterior Cruciate Ligament: Runs diagonally posterior

• Medial and lateral Collateral Ligaments: runs along inner and outer knee edges preventing rotation and side-to-side movements

Common Joint Disorders

• Cartilage Tears: Partial or complete tear of cartilaginous structures due to compression or shearing forces.
• Clinical Presentation: Pain, swelling, decreased ROM, and potential for locking/binding due to cartilage fragments in the joint space.
• Repair: Intervention is necessary due to the inability of cartilage to heal itself.
  • Arthroscopic Surgery:
    • Chondrectomy or removal of cartilage.
      • Partial leaves the joint less stable but stays mobile
      • Complete leads to the development of osteoarthritis due to bone-to-bone friction.
      • Transplant reserved for younger patients.
• Sprains Ligaments are Streched or town d/t twisting or over stretching Clinical Presentation: Pain, swelling and limited movement Repair
  • Poor ligament vascularization leads to slow healing
  • Partial tears take time to heal, while complete tears need immobilization or surgical reconstruction
  • Surgical Repair – Replacing (using grafts), sewing ends back together
• Dislocations
  • Bones are out of anatomical position, commonly through trauma or blunt force
  • Clinical Presentation: Causes severe pain, swelling, visual deformity, and immobility
  • Reduction: moving back into position via external manipulation
  • Immobiling
• Inflammatory Conditions
• Bursitis: d/t over use, injury or repetitive motions
• Tendonitis: inflammation due to overuse and repetitive moments
• Clinical presentation is pain, swelling stiffness decreased mobility
• More than 100 inflammatory degenerative diseases that damage the joint which all have unique treatments and causes

Types of Inflammatory Condidtions

• Osteoarthritis is a degenerative disease causing bone to bone friction. Commonly from wear and tear or past injuries causing swelling, pain and stiffness that worsens overtime
• Rheumatoid Arthritis an autoimmune disease that affects the joints causing chronic inflammation and pain which can lead to joint damage/deformity.
• Gout is caused by build up of uric acid (hyperuricemia) which leads to inflammation, causes severe pain, redness and swelling during the attacks

Muscle Tissue

• Muscle tissue is soft tissue composed of muscle fiber cells found throughout the body.
• It contracts and relaxes to facilitate movement.
• Muscles make up approximately 40-50% of body mass.

Types of muscle tissue

• 3 Types; Skeletal, Cardiac and Smooth

Skeletal

• Most abundant, attach and cover skeleton
• Rapid Contraction and consumes high energy
• Facilitates movement

Cardiac

• only one in the heart with involuntary control

Smooth

• Walls of hallow organs with slow and rhythmic functions
• Cell shape and appearance are dependent on its type

Characteristics of muscle tissue

• Excitability or Responsiveness: Ability to receive/respond to stimuli
• Contractibility: Ability to shorten forcibly when stimulated Exctensibility: Ability to be stretches without tearing
• Elasticity: Ability to recoil, return to resting length

Muscle Functions

• To produce various types of movement
• helps Maintain posture
• Stabilize Joints
• Generate heat through Thermoregulation (80% of body heat is maintained through contraction

Skeletal Muscle Anatomy

• Innervated with nerves that control all functions through impulses
• Rich with Blood for oxygen, nutrients and energy
• The connective tissue protects, contracts muscles

Connective tissue sheaths

• Holds Muscle cells together, 3 types Epimysium, Perimysium and Endomysium
• Epimysium: Surrounds entire muscle and it functions for protecting and contraction
• Perimysium: Surrounds Bundles/Fascicles and allows for blood and nerve vessels
• Endomysium: Thin layer that surrounds individual muscle fibers and contains capillaries and nerves

Attachments

• To a fixed or less moveable point through origin
• moveable through to more movable point called insertions
• can be direct or indirect (muscle to tendon and then connected to a bone)
• Indirect allows for great elasticity and flexibility

Microanatomy of Muscle fibers

• Sarcolemma: A plasma membrane that surrounds muscle fiber
• SarcoPlasm: Cytoplasm of muscle cells that are rich with Organelles, Enzymes and Nutrients
• Myofibrils Contractile Units of muscle ibers, Densely packed, long-cylindrical bundles
• Sarcoplasmic Reticulum
• T-Tubules: Both work together during muscle activity Myofilaments
• smallest structures of muscle; Action and Myosin which also have the help of Regulating Proteins (Troponin and Tropomyosin

Skeletal Muscle Contraction

1)A Motor Neuron is sent 2)A Ch is released, which sends signals through a NeuroMuscular Junction 3)Receptors recive Ach which generate Action Potential This then leads to Calcium being released by the Sarcoplasmic Reticulum, Which leads to Muscle contracting through Myosin pulling on Actin filaments

Neuromuscular Junction Events

Nerve signal from motor neuron goes down the axon and release AcH Ach carried signal from brain to muscles, diffuses across the cleft and binds to Receptor Allows Na+ into the cell

Excitation-Contraction

Na Ions depolarize membrane to allow potential to transmit along the T- Tubes Occurs rapidly which ensures to transmit throughout the entire cells for optimal contraction After reaching T-Tubes, The Sarcoplasmic Reticulum releases Ca++ into the cytoplasm of the muscle fibers

Cross Bridge Cycling

(Sliding Filament) - where final and force is generation

Muscle Twitch

brief contraction or movement that has three phases: 1: Latent (Milliseconds between stimulus and start of contraction) 2: Contraction (Where the muscle activity contracts) 3: Realxation (Muscle relaxes and returns to original state) Influencing factors of contractions are: Frequent stimulation, many Muscle fibers and Degree of Muscle Stretch