Musculoskeletal system patho

Skeletal Muscle Fibers

Single, very long cylindrical, multinucleate cells with obvious striations; voluntary control.

Cardiac Muscle Fibers

Branching chains of cells with 1-2 central nuclei, intercalated discs, and striations; involuntary control.

Smooth Muscle Fibers

Single, fusiform (spindle-shaped), uninucleate cells lacking striations; involuntary control.

Epimysium

Dense connective tissue sheath surrounding the entire skeletal muscle organ.

Perimysium

Connective tissue wrapper grouping individual muscle fibers into bundles called fascicles.

Endomysium

Delicate connective tissue layer enclosing each individual muscle fiber/cell.

Sarcomere

The fundamental contractile unit of a myofibril bounded by two Z-discs.

A-Band

Dark central region of sarcomere containing the full length of thick myosin filaments.

I-Band

Light outer region of sarcomere containing thin actin filaments only; bisected by Z-disc.

H-Zone

Central area within the A-band containing thick myosin filaments only; narrows during contraction.

Z-Disc (Z-Line)

Anchoring protein border for thin actin filaments defining sarcomere boundaries.

M-Line

Central protein line in the middle of H-zone that anchors thick myosin filaments.

Actin

Thin myofilament protein containing active binding sites for myosin heads.

Tropomyosin

Long fibrous protein wrapped around actin that blocks myosin-binding sites in resting muscle.

Troponin Complex

Three-subunit protein (TnT, TnI, TnC) attached to tropomyosin that binds Ca2+ to trigger contraction.

Troponin C (TnC)

Specific troponin subunit that binds calcium ions, causing a conformational shift in tropomyosin.

Myosin

Thick myofilament protein with tail region and globular heads containing ATP and actin binding sites.

Titin

Giant elastic protein anchoring myosin to the Z-disc; maintains passive tension and structural alignment.

Dystrophin

Cytoskeletal protein anchoring internal actin framework to the sarcolemma and extracellular matrix (DAPC).

Type I Muscle Fibers

Slow-twitch oxidative, red fibers with high myoglobin/mitochondria, low fatigability; used for endurance.

Type IIa Muscle Fibers

Fast-twitch oxidative-glycolytic, red/pink fibers with intermediate fatigue resistance and power.

Type IIx / IIb Muscle Fibers

Fast-twitch glycolytic, white fibers with large diameter, high glycogen/glycolytic enzymes, rapid fatigue.

Fiber Type Conversion

Exercise training converts Type IIx fibers into Type IIa fibers; Type I/II ratio determined by genetics and motor neuron.

Satellite Cells

Quiescent myogenic stem cells between basal lamina and sarcolemma; activate upon injury to repair and donate nuclei.

Muscle Hypertrophy

Increase in muscle fiber cross-sectional size via addition of myofibrils and contractile proteins (not cell number).

Size Principle of Recruitment

Orderly motor unit recruitment from smallest motor neurons (Type I) to largest motor neurons (Type II).

Neuromuscular Junction (NMJ)

Synapse between alpha-motor neuron terminal button and muscle fiber motor end plate.

Excitation-Contraction Coupling Steps

Action potential -> Presynaptic Ca2+ entry -> ACh release -> Nicotinic receptor binding -> Na+ influx -> T-tubule depolarization -> SR Ca2+ release.

Sarcoplasmic Reticulum (SR)

Specialized endoplasmic reticulum storing high Ca2+ concentration; releases Ca2+ via terminal cisternae.

Cross-Bridge Cycle Power Stroke

Myosin head releases inorganic phosphate, tilts to drag actin toward M-line, shortening the sarcomere.

Role of ATP in Muscle Contraction

ATP binding is required for myosin head detachment from actin; ATP hydrolysis re-cocks myosin head.

DMARDs (Disease-Modifying Antirheumatic Drugs)

Medications (Methotrexate, Leflunomide, Adalimumab) that slow RA progression; carry high risk of systemic toxicity.

Corticosteroid MSK Side Effects

Adrenocortical suppression, osteoporosis, muscle wasting/atrophy, and ligament/tendon laxity around injected joints.

NSAIDs Mechanism & Risks

Inhibit cyclooxygenase (COX) to decrease prostaglandin synthesis; risks include GI distress, ulcers, and masking pain.

Opioid PT Considerations

Schedule physical therapy ~2 hours post-administration for peak analgesia; monitor for respiratory depression and orthostatic hypotension.

Botox (Botulinum Toxin) Mechanism

Inhibits presynaptic acetylcholine (ACh) release at NMJ, reducing muscle contraction and spasticity.

Muscle Strain Phases & Referral

4 phases: overload disruption -> tissue injury/inflammation -> pain/inhibition -> repair. Grade III ruptures require orthopedic referral.

Return-to-Sport Criteria after Strain

Pain-free full ROM, strength restoration >=90% of uninjured side, functional testing pass, no compensatory movement.

Adhesive Capsulitis (Frozen Shoulder)

Fibrosis/contracture of shoulder capsule, RTC, and bursa; presents with capsular pattern loss (Flexion, Abduction, External Rotation) and night pain.

Myasthenia Gravis (MG) Pathophysiology

Autoimmune destruction/blocking of postsynaptic ACh receptors (or MuSK) at the NMJ by autoantibodies.

Myasthenia Gravis Cardinal Sign

Extreme skeletal muscle weakness and fatigue that worsens with activity and improves with rest or anticholinesterase drugs.

Myasthenia Gravis Clinical Diagnostic Tests

EMG decremental response with repeated stimulation; Tensilon (Edrophonium) test showing rapid temporary strength recovery.

Myasthenia Gravis Pharmacotherapy

Anticholinesterase agents (Pyridostigmine, Neostigmine) to prolong ACh in synaptic cleft; immunosuppressants (Prednisone).

Botulism Pathology & Symptoms

Clostridium botulinum neurotoxin blocks presynaptic ACh release; causes symmetric descending paralysis with intact sensation/cognition.

Duchenne Muscular Dystrophy (DMD)

X-linked recessive absence of dystrophin causing sarcolemma instability, progressive muscle necrosis, and fatty pseudohypertrophy.

Gower's Sign

Clinical sign in DMD where child uses hands to 'walk up' legs from floor due to severe proximal hip/quad weakness.

DMD Prognosis & Exercise Precautions

Wheelchair-dependent by age 13; death by age 20 from respiratory/cardiac failure. AVOID eccentric/overuse exercise; use submax low-impact (swimming).

Myofascial Pain Syndrome (MFPS)

Motor endplate dysfunction creating local muscle energy crisis, ischemia, sensitized nociceptors, and taut trigger points with referred pain.

MFPS vs. Fibromyalgia Differential

MFPS features localized taut bands, trigger points, local twitch responses, and NO systemic signs (unlike Fibromyalgia).

Down Syndrome MSK Manifestations

Trisomy 21 causing muscle hypotonia, joint hypermobility/ligament laxity, C1-C2 atlantoaxial subluxation risk, and scoliosis.

Spinal Muscular Atrophy (SMA)

Autosomal recessive (chromosome 5) degeneration/apoptosis of spinal cord anterior horn cells; causes progressive weakness and muscle wasting.

SMA Types Classification

Type I (Werdnig-Hoffmann; severe infantile), Type II (intermediate childhood), Type III (Kugelberg-Welander; mild juvenile walking).

Congenital Torticollis

Unilateral SCM contracture/fibrosis causing head tilt to ipsilateral side and rotation to contralateral side; plagiocephaly present.

Arthrogryposis Multiplex Congenita (AMC)

Non-progressive congenital rigid joint contractures in multiple limbs secondary to decreased in-utero movement; Amyoplasia is most common (Type II).