Joints and Muscle System Comprehensive lec Notes

Joint Classification

• Structural categories

  • Synostosis

  • Bony fusion resulting from ossification of fibrous/cartilaginous joints

  • No joint cavity; immovable (functional class: synarthrosis)

  • Fibrous joints

  • Sutures, syndesmoses, gomphoses

  • Connected by dense regular CT; no cavity

  • Functional range: synarthrosis (suture) → amphiarthrosis (syndesmosis)

  • Cartilaginous joints

  • Synchondroses (hyaline cartilage) & symphyses (fibrocartilage)

  • Allow slight movement (amphiarthrosis) or none (1° epiphyseal synchondrosis)

  • Synovial joints

  • Bone ends separated by fluid-filled cavity, enclosed in capsule

  • Always diarthrotic (freely movable)

• Functional categories (degree of movement)

  • Synarthrosis – immovable (e.g., sutures, epiphyseal plates, alveolar gomphoses)

  • Amphiarthrosis – slightly movable (e.g., tibiofibular syndesmosis, pubic symphysis)

  • Diarthrosis – freely movable (all synovial joints; sub-types: plane, hinge, pivot, condylar, saddle, ball-and-socket)

Synovial Joint Basic Anatomy (5 Mandatory Structures)

• Fibrous (articular) capsule

  • Dense irregular CT continuous with periosteum

  • Resists tensile forces & stabilizes articulation

• Articular cartilage

  • Thin hyaline layer (~2!–!4\;\text{mm}) on epiphyses

  • Reduces friction (coefficient <0.005) & absorbs shock via fluid weeping

• Synovial cavity

  • Microscopic potential space (expands with effusion)

  • Houses \approx0.5\;\text{mL} fluid in small joints

• Synovial fluid

  • Ultra-filtrate of plasma + hyaluronan + lubricin

  • Functions: lubrication, nutrient delivery (cartilage is avascular), waste removal, shock distribution (non-Newtonian; viscosity drops with shear = thixotropy)

• Synovial membrane

  • Areolar CT with inner A & B synoviocytes

  • Produces hyaluronan; lines capsule except over cartilage

Accessory Structures of Synovial Joints

• Ligaments

  • Dense regular CT; intrinsic (capsular) vs extrinsic (extra/intra-capsular)

  • Mechanical stabilization, motion guidance, proprioceptive feedback

• Tendons

  • Connect muscle → bone; add dynamic stability; strain energy storage (Achilles)

• Bursae

  • Flattened sacs lined by synovial membrane, filled with fluid

  • Reduce friction between skin/tendon/ligament & bone (subacromial, prepatellar)

• Tendon sheaths

  • Tubular bursae enveloping long tendons (digits, biceps brachii)

  • Permit sliding with minimal wear

• Fat pads

  • Adipose cushions (e.g., infrapatellar) that fill space, accommodate shape change, vascular reserve

• Articular discs / Menisci

  • Fibrocartilaginous wedges (knee, TMJ, sternoclavicular, radiocarpal)

  • Improve fit, redistribute load, enhance shock absorption, limit translation

Knee Joint Specializations

• Menisci

  • Medial (C-shaped) & lateral (O-shaped) fibrocartilage

  • Key functions: deepen tibial plateau, distribute compressive forces (↓ peak stress ≈ 50\%), proprioception

• Ligaments

  • Extracapsular: patellar ligament, medial (tibial) collateral (MCL), lateral (fibular) collateral (LCL), oblique & arcuate popliteal

  • Intracapsular: anterior cruciate (ACL) – resists anterior tibial glide & rotation; posterior cruciate (PCL) – resists posterior glide; transverse ligament; meniscofemoral

  • Terrible triad: ACL + MCL + medial meniscus tear via valgus + rotation trauma

Intervertebral Joints & Discs

• Synovial facet (zygapophyseal) joints: plane diarthroses controlling spine flexibility pattern

• Intervertebral symphyses (between bodies)

  • Annulus fibrosus – concentric fibrocartilage lamellae angling \pm30^{\circ}; contains nucleus

  • Nucleus pulposus – hydrated gel (≈80\% water at birth) rich in type-II collagen & proteoglycans; axial shock absorber

  • Height loss over day due to fluid shift (≈1\;\text{cm})

• Herniated disc

  • Nucleus protrudes through annulus (posterolateral common) → nerve root compression (radiating pain, paresthesia)

Arthritides & Selected Injuries

• Osteoarthritis (degenerative)

  • Progressive articular cartilage erosion, subchondral sclerosis, osteophytes; pain worsens with use

• Rheumatoid arthritis

  • Autoimmune synovitis → pannus formation, joint erosion; symmetrical; assoc. with HLA!\text{-}DR4

• Gouty arthritis

  • \text{Uric acid} crystal deposition (monosodium urate) in synovium, cartilage (first MTP classic)

• Other injuries

  • Sprain (ligament stretch/tear)

  • Strain (muscle/tendon)

  • Bursitis, tendinitis, dislocation (luxation vs subluxation)

Stability vs. Mobility Continuum

• Factors enhancing stability (↓ ROM)

  • Deep socket (acetabulum), strong capsular & extracapsular ligaments, high muscle tone across joint, labrum/menisci, negative intra-articular pressure

• Factors enhancing mobility (↑ ROM)

  • Shallow socket (glenoid), loose capsule, fewer reinforcing ligaments, smooth/large cartilage surfaces, high ratio joint cavity to bone volume

• Trade-off principle: hip (stable) vs shoulder (mobile)

Synovial Structures: Stability/Shock vs Mobility

• Stability/Shock absorbers: articular discs, menisci, strong intracapsular ligaments, labra, dense capsule, muscular co-contraction

• Mobility promoters: thin capsule zones, bursae + tendon sheaths reduce friction, smooth hyaline cartilage, synovial fluid viscosity reduction with movement

Muscle Tissue Types

• Skeletal (striated, voluntary, multinucleated peripheral nuclei); moves skeleton, heat generation

• Cardiac (striated, involuntary, single central nucleus, intercalated discs; autorhythmic via pacemaker cells)

• Smooth (nonstriated, involuntary, fusiform cells, dense bodies; found in vessel walls, viscera, arrector pili)

Skeletal Muscle Structural Hierarchy & CT Wrappings

• Whole muscle → Fascicle → Muscle fiber (cell) → Myofibril → Myofilament (thick \text{myosin}, thin \text{actin}, elastic \text{titin})

• Connective tissue sheaths

  • Epimysium (dense irregular CT around muscle)

  • Perimysium (fibrous sheath around fascicle; carries vessels & nerves)

  • Endomysium (areolar reticular CT around each fiber; contains capillaries & satellite cells)

Sliding Filament Theory (Histology Basis)

• Sarcomere (Z→Z) as contractile unit

• During contraction:

  • Ca^{2+} released from sarcoplasmic reticulum binds troponin C

  • Tropomyosin moves, exposing actin-myosin binding sites

  • Myosin heads (energized by ATP hydrolysis) form cross-bridges → power stroke pulls actin toward M line

  • Result: I bands & H zone shorten; A band length constant; overall fiber shortens

• Cycle ends when [Ca^{2+}] falls & ATP binds myosin causing detachment

Skeletal Muscle Fiber Types

• Type I (slow oxidative)

  • High myoglobin, many mitochondria, fatigue-resistant; postural muscles

• Type IIa (fast oxidative-glycolytic)

  • Intermediate; rapid but moderate fatigue

• Type IIb/x (fast glycolytic)

  • Low myoglobin, anaerobic, powerful bursts; eye, hand muscles

Muscle Naming Criteria & Examples

• Size (gluteus maximus > minimus)

• Location (brachialis – arm)

• Action (flexor digitorum)

• Shape (deltoid – triangular)

• Fiber direction (rectus abdominis – straight)

• Number of heads (triceps brachii – 3)

• Attachments (sternocleidomastoid – sternum + clavicle + mastoid)

• Relative position (external vs internal intercostals)

Tendon of Origin vs Insertion

• Origin: proximal/stationary attachment; generally more stable bone

• Insertion: distal/movable attachment; muscle action moves insertion toward origin

Neuromuscular Junction (NMJ)

• Components

  • Axon terminal with synaptic vesicles (ACh)

  • Synaptic cleft (≈30\;\text{nm})

  • Motor end plate (sarcolemma junctional folds with ACh receptors)

• Events

  • AP → Ca^{2+} influx → ACh release → binds receptors → Na^+ influx → end-plate potential → AP along sarcolemma → contraction

• Acetylcholinesterase terminates signal; clinical relevance: myasthenia gravis, botulinum toxin, curare

Motor Unit & Recruitment

• Motor unit = one α-motor neuron + all muscle fibers it innervates (ranges: eye \approx10\;\text{fibers/unit} vs gastrocnemius \approx2000)

• Differential recruitment

  • Size principle: smaller (slow) units activated first; larger (fast) units for forceful tasks

  • Allows fine control + energy efficiency + graded tension

Key Muscle Groups

• Rotator cuff: supraspinatus, infraspinatus, teres minor, subscapularis ("SITS")

• Quadriceps femoris: rectus femoris, vastus lateralis, vastus medialis, vastus intermedius

• Hamstrings: biceps femoris (long & short heads), semitendinosus, semimembranosus

Agonist / Antagonist / Synergist Relationships

• Agonist (prime mover) – produces primary action (e.g., biceps brachii in elbow flexion)

• Antagonist – opposes agonist, provides control (triceps brachii)

• Synergist – assists agonist via extra force or stabilization (brachialis, brachioradialis)

• Fixator – subtype of synergist stabilizing origin (scapular muscles during arm motion)

Integrated Movement: Muscles, Bones, CT, Nerves & Joints

• Skeletal muscles generate force transmitted through tendons → bones; joints act as fulcrums; ligaments/capsule constrain path; nerves coordinate timing; proprioceptors provide feedback for precision

• Lever classes: 1st (atlanto-occipital), 2nd (plantarflexion), 3rd (elbow flexion – most common; favors speed/ROM over force)

Fascicle Arrangement & Functional Implications

• Parallel (sartorius) – large ROM, less force

• Convergent (pectoralis major) – versatile line of action

• Pennate (uni: extensor digitorum; bi: rectus femoris; multi: deltoid) – higher fiber packing → greater force, reduced ROM

• Circular (orbicularis oris) – sphincters

Mysteries of Muscle (Representative Q&A)

• Why do muscles feel stiff post-mortem? → Rigor mortis from ATP depletion locking actin-myosin bridges

• How can stretching increase ROM? → Viscoelastic CT creep, sarcomere addition, muscle spindle accommodation

• Why does eccentric contraction cause more soreness? → Greater microtears & titin strain → inflammatory response (DOMS)