Memory tricks for muscles — anatomy and how they contract

Movement: muscles pull bones (levers) at joints — walking, breathing, swallowing, blinking. Maintain posture: constant low-level contraction of postural muscles keeps you upright against gravity — back, core, and neck muscles working continuously. Produce heat: ~85% of body heat comes from muscle metabolism (ATP hydrolysis) — critical for thermoregulation. Shivering is rapid involuntary muscle contraction to generate heat. Stabilize joints: muscles and tendons reinforce joint capsules — rotator cuff stabilizes shoulder, quadriceps stabilize knee. Muscle weakness → joint instability → injury.

Whole muscle: surrounded by epimysium (connective tissue sheath). Fascicle: bundle of muscle fibers — surrounded by perimysium. Visible as "grain" in meat. Muscle fiber (cell): individual muscle cell — multinucleated, surrounded by endomysium. Contains myofibrils. Myofibril: cylindrical organelle running length of fiber — contains the contractile proteins. Creates striated appearance. Sarcomere: functional unit of contraction — region between two Z lines. Contains overlapping thick (myosin) and thin (actin) filaments. Contraction = sarcomeres shorten → myofibrils shorten → fiber shortens → muscle shortens.

During contraction, actin (thin filaments) slide over myosin (thick filaments) — neither filament actually shortens. A band: stays the SAME length (contains myosin). I band: SHORTENS (actin-only zone between A bands). H zone: DISAPPEARS at full contraction (myosin-only zone fills with actin). Z lines: move CLOSER together. Sarcomere shortens. Memory trick: A stays, I shrinks. The myosin heads (cross-bridges) attach to actin → power stroke pulls actin toward M line → release → recock → repeat. Each cycle requires one ATP.

Motor neuron fires → ACh released at NMJ → end-plate potential → muscle action potential travels along sarcolemma → down T-tubules (transverse tubules) deep into fiber → activates dihydropyridine receptors → opens ryanodine receptors (RyR) on sarcoplasmic reticulum → Ca2+ floods into cytoplasm → Ca2+ binds troponin C → troponin-tropomyosin complex shifts → myosin-binding sites on actin EXPOSED → cross-bridge cycle begins → contraction. Relaxation: Ca2+ pumped back into SR by SERCA (requires ATP) → tropomyosin covers actin sites again → relaxation.

Size: maximus (largest), minimus (smallest), longus (long), brevis (short). Gluteus maximus, peroneus longus. Location: named for nearby bone or body region — tibialis (tibia), brachii (arm), femoris (femur). Action: flexor, extensor, adductor, abductor, rotator, levator (raises). Direction of fibers: rectus (straight/parallel to midline), transverse (perpendicular), oblique (diagonal). Rectus abdominis, transverse abdominis, external oblique. Number of origins: biceps (2 heads), triceps (3 heads), quadriceps (4 heads). Shape: deltoid (triangle), trapezius (trapezoid), serratus (serrated).

Prime mover (agonist): the muscle doing the main work — produces the desired movement. Biceps brachii is prime mover for forearm flexion. Antagonist: opposes the prime mover — relaxes to allow movement, contracts to slow/stop it. Triceps brachii is antagonist to biceps. Synergists: assist the prime mover — add force, reduce unwanted movements, stabilize. Brachialis assists biceps in forearm flexion. Fixators: stabilize the origin bone so the prime mover can act effectively — rotator cuff muscles fix scapula so deltoid can abduct arm. All muscle groups work together — no muscle acts truly in isolation.

Isotonic contractions: muscle tension stays relatively constant but muscle length changes. Two types: Concentric: muscle SHORTENS while developing tension — lifting a dumbbell up. Most common active movement. Eccentric: muscle LENGTHENS while developing tension — lowering a dumbbell (resisting gravity). Produces more force than concentric. More muscle damage (DOMS — delayed onset muscle soreness). Isometric: muscle develops tension but length does NOT change — holding a dumbbell still, pushing against a wall. Used for joint stabilization. Clinical relevance: eccentric strengthening is key in tendon rehabilitation (Achilles tendinopathy — heel drop exercises).

Trapezius: upper back/neck — elevates, retracts, and rotates scapula. "Trap shrug." Sternocleidomastoid (SCM): flexes and rotates neck. Two heads — sternal + clavicular. Torticollis = spasm of SCM. Pectoralis major: chest — adducts and medially rotates arm. Bench press muscle. Serratus anterior: "boxer's muscle" — protracts scapula (pushes away from body). Weakness → winging scapula. Rectus abdominis: "six-pack" — flexes vertebral column. External/internal obliques: rotate and laterally flex trunk. Erector spinae: extends vertebral column — back straightener. Latissimus dorsi: extends and adducts arm — "lat pull-down" muscle.

Rotator cuff (SITS — Supraspinatus, Infraspinatus, Teres minor, Subscapularis): stabilize shoulder, initiate abduction (supraspinatus). Most common tear. Biceps brachii: flexes forearm and supinates. Two heads — long (intertubercular groove) and short. Triceps brachii: extends forearm — only muscle on posterior arm. Quadriceps (rectus femoris + 3 vasti): extend knee — knee extension. Hamstrings (biceps femoris, semimembranosus, semitendinosus): flex knee + extend hip. Most commonly strained muscle. Gastrocnemius + soleus (triceps surae): plantar flex foot. Largest tendon in body = Achilles.