Musculoskeletal, Skin, and Connective Tissue

↑ pressure within a fascial compartment of a limb (defined by compartment pressure to diastolic blood pressure gradient of <30 mmHg) → venous outflow obstruction and arteriolar collapse → anoxia and necrosis. - Causes include significant long bone fractures, reperfusion injury, animal venoms. - Presents with severe pain and tense, swollen compartments with limb flexion.- Motor deficits are late sign of irreversible muscle and nerve damage.

Abnormal acetabulum development in newborns. - Results in hip instability/dislocation. - Commonly tested with Ortolani and Barlow maneuvers (manipulation of newborn hip reveals a "clunk"). - Confirmed via ultrasound (x-ray not used until ~4-6 months because cartilage is not ossified). Treatment: splint/harness

Posterior cervical: Scalp & neck, skin of arms & pectorals, thorax, cervical and axillary nodes Postauricular: External auditory meatus, pinna, scalp Preauricular: Eyelids & conjunctivae, temporal region, pinna Right supraclavicular node: Mediastinum, lungs, esophagus Left supraclavicular node: Thorax, abdomen via thoracic duct

Compression of ulnar nerve at wrist. Classically seen in cyclists due to pressure from handlebars.

Thenar (median) – Opponens pollicis, abductor pollicis brevis, flexor pollicis brevis (superficial head, deep head by ulnar nerve) Hypothenar (ulnar) – Opponens digiti minimi, abductor digiti minimi, flexor digiti minimi brevis Dorsal interossei (ulnar) – abduct the fingersPalmar interossei (ulnar) – adduct the fingers Lumbrials (1st/2nd, median; 3rd/4th, ulnar) – flex at the MCP join, extend PIP and DIP joints.

Innervation:- Motor – gluteus medius, gluteus minimus, tensor fascia latae Cause of injury:- Iatrogenic injury during intramuscular injection to superomedial gluteal region (prevent by choosing superolateral quadrant, preferably anterolateral region)  -Trendelenburg sign/gait – pelvis tilts because weight-bearing leg cannot maintain alignment of pelvis through hip abduction.- Lesion is contralateral to side of the hip that drops, ipsilateral to extremity on which the patient stands.

Abductors: Gluteus medius, gluteus minimus Adductors: Adductor magnus, adductor longus, adductor brevis Extensors: Gluteus maximus, semitendinosus, semimembranosus Flexors: Iliopsoas, rectus femoris, tensor fascia lata, pectineus, sartorius Internal rotation: Gluteus medius, gluteus minimus, tensor fascia latae External rotation: Iliopsoas, gluteus maximus, piriformis, obturator

Overuse injury of lateral knee that occurs primarily in runners.  Pain develops 2° to friction of iliotibial band against lateral femoral epicondyle.

Inflammation of plantar aponeurosis characterized by heel pain (worse with first steps in the morning or after period of inactivity) and tenderness.

Fluid-filled swelling overlying joint or tendon sheath, most commonly at dorsal side of wrist. Arises from herniation of dense connective tissue.

Paresthesia and weakness related to specific lumbosacral spinal nerves. Usually, the intervertebral disc herniates into central canal, affecting the inferior nerves (eg, herniation of L3/4 disc affects L4 spinal nerve, but not L3).Intervertebral discs generally herniate posterolaterally, due to the thin posterior longitudinal ligament and thicker anterior longitudinal ligament along the midline of the vertebral arteries. L3-L4 → Weakness of knee extension, ↓ patellar reflex L4-L5 → Weakness of dorsiflexion, difficulty in heel-walking L5-S1 → Weakness of plantar flexion, difficulty in toe-walking, ↓ Achilles reflex

Axilla/lateral thorax: Long thoracic nerve + lateral thoracic artery Surgical neck of humerus: Axillary nerve + posterior circumflex artery Midshaft of humerus: Radial nerve + deep brachial artery Distal humerus/cubital fossa: Median nerve + brachial artery Popliteal fossa: Tibial nerve + popliteal artery Posterior to medial malleolus: Tibial nerve + posterior tibial artery

T-tubules are extensions of plasma membrane in contact with the sarcoplasmic reticulum, allowing for coordinated contraction of striated muscles 1. Action potential opens presynaptic voltage-gated Ca2+ channels, inducing acetylcholine (ACh) release.2. Postsynaptic ACh binding leads to muscle cell depolarization at the motor end plate.3. Depolarization travels over the entire muscle clel and deep into the muscle via T-tubules.4. Membrane depolarization induces conformational changes in the voltage-sensitive dihydropyridine receptor (DHPR) and its mechanically coupled ryanodine receptor (RR) → Ca2+ release from the sarcoplasmic reticulum into the cytoplasm.5. Tropomyosin is blocking myosin-binding sites on the actin filament. Released Ca2+ binds to troponin C (TnC), shifting tropomyosin to expose the myosin-binding sites.6. The myosin head binds strongly to actin, forming a crossbridge. Pi is then released, inducing the power stroke.7. During the power stroke, force is produced as myosin pulls on the actin filament. Muscle shortening occurs, with shortening of H and I bands and between Z lines. The A band remains the same length. ADP is released at the end of the power stroke.8. Binding of new ATP molecule causes detachment of myosin head from actin filament. Ca2+ is resequestered.9. ATP hydrolysis into ADP and Pi results in myosin head returning to high-energy position (cocked). The myosin head can bind to a new site on actin to form a crossbridge if Ca2+ remains available.

Implicated drugs: Procainamide, isoniazid, hydralazine, minocycline, TNF-α inhibitors (eg, etanercept) Clinical features:- Abrupt-onset symptoms: Fever/fatigue, arthralgias/arthritis, rash, serositis- Predilection for slow acetylators Laboratory findings:- Anti-histone antibodies present in >95% of patients- Anti-dsDNA antibodies rarely seen

Thick filaments: MyosinThin filaments: Actin Sacromere = Z to Z- Actin is bound to Z lineM = Middle line, where myosin filaments anchor to structural elements.A band = corresponds to the thick filaments (myosin) including overlap with actin.  H band is a portion of the A band. Where myosin does not overlap with actin.I band = Where actin does not overlap with myosin. During muscle contraction, H and I bands decrease in length.

Failure of involution of the mammary ridge. - Usually asymptomatic- May swell or become tender similar to normal breast tissue before or during menses, pregnancy, and lactation. - Occr along the embryonic milk line between the axilla and perineum.- Bilateral in 50% of patients.- Hyperpigmentation, epidermal thickening