Musculoskeletal system Study Guide

📖 Core Concepts Musculoskeletal system – organ system that produces movement by integrating muscles, bones, and connective tissues. Skeletal system – provides support, protection, mineral storage, and houses marrow for blood cell production. Muscular system – three tissue types (cardiac, smooth, skeletal); only skeletal muscle is voluntarily controlled and moves the skeleton. Connective tissues – tendons (muscle‑to‑bone), ligaments (bone‑to‑bone), cartilage (smooth joint surfaces), bursae (fluid‑filled cushions). Joints – points of bone‑to‑bone contact; classified by mobility: synarthrosis (little/no motion), amphiarthrosis (limited), diarthrosis (free motion, mostly synovial). Bone marrow – red (hematopoiesis) vs. yellow (fat storage). Mineral homeostasis – bone deposits calcium when blood levels are high and releases calcium when low. --- 📌 Must Remember 206 bones in the adult skeleton (≈300 at birth). Axial skeleton = skull + vertebral column; appendicular skeleton = limbs + girdles. Bone classification: long, short, flat, irregular, sesamoid. Skeletal muscle contraction → motor‑neuron depolarization → ACh release → muscle‑fiber action potential → calcium release → actin‑myosin cross‑bridge cycling. Red marrow = blood cell production; yellow marrow = energy reserve. Synovial fluid reduces friction; produced by the synovial membrane. Ligaments prevent dislocation and excessive motions (e.g., hyper‑extension). Tendons act like springs, storing and releasing elastic energy. --- 🔄 Key Processes Muscle contraction cascade Motor‑neuron action potential → acetylcholine release → sarcolemma depolarization → Ca²⁺ release from sarcoplasmic reticulum → tropomyosin shift → myosin heads bind actin → power stroke → ATP re‑energizes myosin. Bone calcium regulation High plasma Ca²⁺ → osteoblasts deposit Ca²⁺ into bone matrix. Low plasma Ca²⁺ → osteoclasts resorb bone, releasing Ca²⁺ into bloodstream. Joint lubrication Synovial membrane secretes synovial fluid → fluid fills joint capsule → reduces friction during movement. Tendon energy storage Tendon stretches during loading → elastic potential energy stored → released during unloading to aid propulsion. --- 🔍 Key Comparisons Long vs. Short vs. Flat vs. Irregular vs. Sesamoid bones Long: shaft (diaphysis) + ends (epiphyses) – e.g., femur (lever). Short: roughly cube‑shaped – e.g., carpals (shock absorber). Flat: broad plates – e.g., skull, ribs (protect). Irregular: complex shapes – e.g., vertebrae (support & protect). Sesamoid: embedded in tendons – e.g., patella (protects tendon, changes force angle). Joint mobility Synarthrosis: fibrous, immovable (e.g., sutures). Amphiarthrosis: cartilaginous, limited (e.g., intervertebral discs). Diarthrosis (Synovial): freely movable, fluid‑lubricated. Muscle tissue types Cardiac: involuntary, striated, single nucleus, intercalated discs. Smooth: involuntary, non‑striated, spindle‑shaped, single nucleus. Skeletal: voluntary, striated, multinucleated, attached to bone. Marrow types Red: hematopoiesis (blood cells). Yellow: adipose storage (energy). --- ⚠️ Common Misunderstandings “All muscles are voluntary.” Only skeletal muscle is under conscious control; cardiac and smooth muscles are involuntary. “Bones are only structural.” Bones also store minerals and produce blood cells. “Ligaments and tendons are the same.” Ligaments connect bone‑to‑bone; tendons connect muscle‑to‑bone and transmit force. “Synovial joints are the only joints.” They are the most mobile, but other joints (amphiarthroses, synarthroses) exist. --- 🧠 Mental Models / Intuition Lever model – Bones act as levers, joints as fulcrums, muscles provide the effort force. Visualize a seesaw: larger moment arm = greater torque for the same muscle force. Spring‑damper model – Tendons behave like springs; muscles act as active “dampers” that control the release of stored energy. Factory model for marrow – Red marrow = “factory floor” churning out blood cells; yellow marrow = “warehouse” storing fuel (fat). --- 🚩 Exceptions & Edge Cases Sesamoid bones are not part of the standard five‑group classification in every textbook, but they arise within tendons where friction is high. Red marrow persists in adults in the vertebrae, sternum, pelvis, and proximal ends of long bones; other long‑bone shafts become yellow marrow. Some diarthroses are not synovial (e.g., the temporomandibular joint has a fibrocartilaginous disc). --- 📍 When to Use Which Diagnosing joint pain → first determine joint type: No motion → consider synarthrosis pathology (e.g., suture infection). Limited motion → amphiarthrosis issues (e.g., disc herniation). Free motion → evaluate synovial structures (fluid, cartilage, ligaments). Assessing bone health → focus on mineral storage function (serum Ca²⁺, phosphate) and marrow activity (CBC for red‑marrow function). Choosing muscle‑related explanation → use skeletal muscle model for voluntary movement; cardiac for heart‑related questions; smooth for hollow‑organ motility. --- 👀 Patterns to Recognize “Bone‑muscle‑tendon” triad in movement questions – always trace the force from muscle → tendon → bone → joint. “Ligament + direction” – ligaments are named for the bones they connect and the direction they restrict (e.g., anterior cruciate ligament prevents anterior translation of tibia). “Red vs. yellow marrow” – whenever a question mentions blood cell production, think red marrow; when it mentions energy reserves during starvation, think yellow marrow. “Synovial fluid + friction” – any mention of smooth joint motion points to synovial joints. --- 🗂️ Exam Traps Mistaking ligament for tendon – a choice describing “connects muscle to bone” is a tendon, not a ligament. Confusing joint classification – a “false joint” refers to a synarthrosis (fibrous), not a synovial joint. Assuming all bones store calcium – only the mineral matrix does; the amount varies with age (more storage in children’s developing bones). Red‑marrow location – answer choices that place red marrow only in long‑bone diaphyses are wrong; it resides mainly in flat bones and axial skeleton in adults. Muscle type relevance – a question about involuntary heart contraction is not about skeletal muscle; choosing “skeletal” is a trap. ---