Medicine Exam 5: MSK Physiology & Fractures

Lecture 1

1. Define osteoblasts, osteocytes and osteoclasts and identify their basic functions.

· Osteoblasts: Primary bone-producing cells; produce calcium phosphate → hydroxyapatite; synthesize osteoid. Osteoclasts: Major resorptive cells; dissolve hydroxyapatite; migrate over bone surfaces. Osteocytes: Most abundant; transformed osteoblasts; mechanoreceptors; signal remodeling; maintain homeostasis.

2. Recognize the function of chondrocytes, proteoglycans and collagen.

· Chondrocytes: Produce collagen; maintain ECM; anaerobic. Proteoglycans: Strengthen bone; form compression-resistant networks. Collagen: Bulk of bone matrix; structural framework; most abundant macromolecule.

3. Identify, recognize, and define cortical bone, cancellous bone, & periosteum.

· Cortical: 85% skeleton; solid; concentric lamellae. Cancellous: 15%; trabeculae meshwork. Periosteum: Double-layered connective tissue; outer = vessels/nerves; inner = Sharpey fibers anchor tendons/ligaments.

4. Identify, recognize and explain the purpose and process of bone remodeling.

· Stimulus → cytokines → osteoclasts digest bone → resorption cavity. Osteoblasts line cavity → express osteoid → mineralize → new bone. Sequence = 4–6 months.

5a. Hematoma formation

· Damaged vessels hemorrhage; fibrin & platelets form meshwork.

5b. Procallus formation

· Fibroblasts, capillary buds, osteoblasts → granulation tissue; cartilage precursor.

5c. Callus formation

· Osteoblasts form woven bone; phosphate + calcium harden callus.

5d. Replacement

· Callus replaced with lamellar/trabecular bone.

5e. Remodeling

· Periosteal/endosteal surfaces remodeled to pre-injury shape.

6. Define joint (articulation) and identify the primary function of joints.

· Joint = articulation; where 2+ bones join; provide stability & mobility.

7. Recognize and define joint classification by movement: synarthrosis, amphiarthrosis, diarthrosis.

· Synarthrosis: Immovable. Amphiarthrosis: Slightly moveable. Diarthrosis: Freely moveable.

8. Recognize, define and differentiate joint classification by connective structures: fibrous, cartilaginous, synovial.

· Fibrous: Bone united by fibrous tissue; types = suture, syndesmosis, gomphosis. Cartilaginous: Symphysis (fibrocartilage pad); synchondrosis (hyaline cartilage). Synovial: Most moveable; complex structure.

9a. Articular capsule

· Fibrous tissue covering bone ends; Sharpey fibers attach to periosteum; reinforced by ligaments/tendons.

9b. Synovial membrane

· Inner lining; vascular subintima & cellular intima; rapid repair; secretes hyaluronate.

9c. Synovial cavity

· Enclosed fluid-filled space between bones; surrounded by synovial membrane.

9d. Synovial fluid

· Superfiltrated plasma; lubricates; nourishes cartilage; contains hyaluronic acid, synovial cells, leukocytes.

9e. Articular cartilage

· Hyaline cartilage; reduces friction; distributes forces; composed of chondrocytes, collagen, proteoglycans, water; no vessels/nerves.

10. Layers of skeletal muscle: fascia, epimysium, perimysium, endomysium, tendon, ligaments.

· Fascia: Surrounds/separates muscles. Epimysium: Surrounds muscle. Perimysium: Bundles fibers into fascicles. Endomysium: Surrounds individual fibers. Tendons: Muscle to bone. Ligaments: Bone to bone.

11. Define voluntary, striated, intrafusal, & extrafusal skeletal muscle.

· Striated: Skeletal muscle; voluntary via nervous system. Intrafusal: Sensory spindles detect stretch. Extrafusal: Generate movement force.

12. Major components of muscle fiber and their functions.

· Sarcolemma: Encases fiber; propagates impulses. Sarcoplasm: Cytoplasm; stores O₂/Fe²⁺. T-tubules: Signal conduction. SR: Ca²⁺ regulation. Myofibrils: Contain sarcomeres; actin/myosin.

13. Define motor unit and its significance.

· Motor unit = anterior horn cell + axon + muscle fibers; controls contraction.

14a. Excitation

· Action potential → sarcolemma → T-tubules → Ca²⁺ enters cell.

14b. Coupling

· T-tubule depolarization → SR releases Ca²⁺ → binds proteins → actin-myosin coupling.

14c. Contraction

· Myosin pulls actin → sliding → myofibril shortens → cross-bridge forms.

14d. Relaxation

· Ca²⁺ reabsorbed into SR → cross-bridges detach → sarcomere lengthens.

15. Isometric vs dynamic muscle contractions

· Isometric: Constant length, ↑ tension, no movement. Dynamic (isotonic): Constant tension, movement; eccentric = lengthening, concentric = shortening.

16a. Fracture

· Break in bone continuity; force exceeds strength.

16b. Dislocation

· Joint surfaces lose contact; causes = trauma, congenital, laxity, RA.

16c. Subluxation

· Partial loss of joint contact; e.g. hip dysplasia; often diagnosed by history.

17a. Stable (nondisplaced)

· Fracture with normal alignment.

17b. Displaced

· Fragments misaligned.

17c. Pathological

· Fx at abnormal bone site; causes = tumors, osteoporosis, RA, Paget’s, rickets, radiation, meds.

17d. Stress

· Repetitive strain; cumulative force; e.g. running.

17e. Open

· Compound fx; skin compromised.

17f. Compression

· Bone wedged/compressed; often vertebrae.

18a. Transverse

· Horizontal break; perpendicular through cortices; direct trauma.

18b. Oblique

· Fx line angled through cortices.

18c. Spiral

· Curved fx; twisting force; may mimic oblique; hx important.

18d. Comminuted

· >2 fragments; high energy injury.

18e. Torus

· Incomplete fx; cortex bulges; pediatric.

18f. Greenstick

· Bowing + cortical disruption on one side; pediatric.

Bone Function: What’s so special about bones?

· Provide form & structure · Permit movement · Protect vital organs · Bone marrow is source of stem · Help maintain mineral homeostasis

Bone Function: Bone marrow is source of stem cells

In flat bones · e.g. skull, sternum, ribs, pelvis Generate both hematopoietic & nonhematopoietic cells · blood & immune cells · bone, cartilage, & fat cells

Bone Function: Help maintain mineral homeostasis

· Store & release calcium, phosphate, carbonate, magnesium

Bone Cells & Related

· Osteoblasts, osteoid · Osteoclasts · Osteocytes · Chondrocytes · Collagen fibers · Proteoglycans

Osteoblasts

· Primary bone-producing cells · Produce inorganic calcium phosphate · Converted to hydroxyapatite · Active on the outer surfaces of bone (single layer) · Synthesize osteoid

osteoid

· non-mineralized bone matrix

Osteoclasts

· Migrate over bone surfaces · Major resorptive cells of bone · Acidify hydroxyapatite to dissolve it

Osteoclasts Function: Migrate over bone surfaces

· Travel to resorption areas · Areas are prepared & stripped of osteoid by enzymes

Osteocytes

· Most abundant cells in bone · Transformed osteoblasts · Mechanoreceptor

Osteocytes Characteristic: Transformed osteoblasts

· Trapped/surrounded in osteoid as it hardens · Hardened bone matrix = lacuna · Mechanoreceptor · Responds to stressors such as weightbearing/load · Signal remodeling · to both osteoblasts & osteoclasts · Maintain homeostasis

Osteocytes Characteristic: Mechanoreceptor

· Responds to stressors such as weightbearing/load Signal remodeling · to both osteoblasts & osteoclasts · Maintain homeostasis

Chondrocytes

· Cartilage cells · Produce collagen · Maintain homeostasis of extra-cellular matrix · Low blood supply = anaerobic functioning

Chondrocytes Function: Maintain homeostasis of extra-cellular matrix

· Network of proteins/molecules that surround & support cell

Collagen fibers

· Make up bulk of bone matrix · Matrix formation is a multi-step process initiated by osteoblasts · Most abundant macromolecule in the body · Structural framework for nearly all tissues

Proteoglycans

· Large complexes of polysaccharides attached to a common protein core · Strengthen bone

Proteoglycans Function: Strengthen bone

· Form compression-resistant networks between collagen fibers

Bone Tissue

· Cortical (compact) bone · Cancellous (spongy) bone · Periosteum

Cortical (compact) bone

· 85% of the skeleton · Highly organized, solid, extremely strong · Concentric layers of bone matrix = lamellae

Cancellous (spongy) bone

· 15% of the skeleton · Less complex (irregular meshwork) · Lamellae are not in layers but in plates/bars = trabeculae · Pattern of meshwork determined by direction of stress (that that bone feels)

Periosteum

· All bones covered by double-layered connective tissue · Outer layer: blood vessels & nerves Inner layer · anchored to bone by collagenous (Sharpey) fibers; These fibers penetrate bone; These help hold/attach tendons & ligaments to the periosteum

Bone Remodeling

· Sequence takes 4-6 months

Bone Remodeling A

· Stimulus activates cytokine system = osteoclast formation · Osteoclasts attach to bone matrix · Osteoclasts digest bone & create resorption cavity

Bone Remodeling B

· Osteoblasts line walls of resorption cavity · Express osteoid & alkaline phosphatase = Form sites for calcium & phosphorous deposition

Bone Remodeling C

· Osteoid mineralizes & new bone is formed · Successive layers are laid down

Bone Repair

· Remodeling can repair microscopic injuries; gross/larger injuries require repair

Hematoma formation

· Damaged vessels hemorrhage; · Fibrin & platelets form framework/meshwork

Procallus formation

· Fibroblasts, capillary buds, osteoblasts produce granulation tissue · Cartilage is formed as precursor to bone

Callus formation

· Osteoblasts in the procallus form membranous/woven bone (callus) · Phosphate + calcium hardens callus

Replacement

· Basic multicellular units of callus replaced with lamellar or trabecular bone

Remodeling

· Periosteal & endosteal surfaces remodeled to appropriate size & shape of bone before injury

Bone Repair Sequence

· Hematoma → Procallus → Callus → Replacement → Remodeling

What is a joint?

· Articulation · Where 2 or more bones attach/join · Provide stability and mobility

Types of Joints

Based on Movement · Synarthrosis · Amphiarthrosis · Diarthrosis Based on Connectivity · Fibrous o Suture o Syndesmosis o Gomphosis · Cartilaginous o Symphysis o Synchondrosis · Synovial (diarthrosis)

Synarthrosis

· Immovable joint

Amphiarthrosis

· Slightly moveable joint

Diarthrosis

· Freely moveable joint

Fibrous

· Bone united directly to other bone via fibrous connective tissue · 3 types = Suture, Syndesmosis, Gomphosis

Suture

· Thin layer of fibrous tissue interlocking flat bones · e.g. skull bones of child · by adulthood, fibrous tissue replaced with bone

Syndesmosis

· 2 bones united by ligament or membrane · ligaments are flexible & stretch · allows for some movement · e.g. tib-fib articulation

Gomphosis

· Conical projection fits into complementary socket · held in place by ligament · e.g. tooth in mandible

Cartilaginous

· 2 types: Symphysis and Synchondrosis

Symphysis

· Bones united by pad or disc of fibrocartilage · thin layer of hyaline cartilage covers articulating surface · pad acts as shock absorber & stabilizer · e.g. pubic symphysis & vertebrae

Synchondrosis

· Hyaline cartilage (not fibrocartilage) connects 2 bones · slight movement is allowed · e.g. joint between ribs & sternum (costal cartilage)

Synovial (diarthrosis)

· Most moveable & most complex consists of multiple components · Articular capsule · Synovial membrane (synovium) o 2 layers: Vascular subintima, Cellular intima · Joint (synovial) cavity · Synovial fluid · Articular cartilage

Articular capsule

· Fibrous connective tissue that covers the ends of bones (where they articulate) · Sharpey fibers attach capsule to periosteum · ligaments/tendons reinforce capsule · dense supply of nerve, blood & lymphatic vessels · nerves sensitive to: motion, compression, tension, vibration, and pain

Synovial membrane (synovium)

· Inner lining of joint capsule · rich supply of blood & lymphatic vessels = rapid repair & regeneration · found in non-articular portion of joint · 2 layers: Vascular subintima, Cellular intima

Vascular subintima

· loose fibrous connective tissue; elastin; fat cells; fibroblasts; macrophages; mast cells

Cellular intima

· Macrophages (A cells): ingest/remove bacteria/debris · Fibroblasts (B cells): secrete hyaluronate

Joint (synovial) cavity

· Enclosed, fluid-filled space between articulating surface of 2 bones · surrounded by synovial membrane · filled with synovial fluid

Synovial fluid

· Superfiltrated plasma · lubricates joint surfaces · nourishes articular cartilage · hyaluronic acid’s biomechanical properties = lubrication · also contains free-floating synovial cells & leukocytes = clear joint debris & microorganisms

Articular cartilage

· Layer of hyaline cartilage that covers end of bone · reduce friction · distribute weight-bearing & shearing forces · composed of chondrocytes, collagen, proteoglycans, and water NO blood vessels, lymph vessels, nerves · Good: no pain; Bad: no regeneration

Articular cartilage: Water content

· water content ranges from 60–80% of net wt; rapid entry/exit of water contributes to resiliency of tissue

Muscular work

· Sequence of contraction & relaxation

Muscle Distribution

· adult: 40% of body weight · child: 50% of body weight

Muscle composition

· 75% water, 20% protein, 5% organic & inorganic compounds · 32% of protein stores for energy/metabolism

Muscle changes with age

· ages 30–60yo = ↓ muscle mass by 0.5lb/yr

Striated

· Skeletal muscle commonly referred to as striated based on appearance on microscopy · under direct voluntary control via nervous system · 2 main categories: Intrafusal and Extrafusal

Intrafusal muscle fibers

· Bundle of these fibers = spindle · spindles are sensory receptors = detect stretch

Extrafusal muscle fibers

· Major component of skeletal muscle · generate force in movement