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What are the four types of bone cells and their main functions?
Answer: Osteoprogenitor cells (stem cells that divide into osteoblasts), osteoblasts (build new matrix), osteocytes (mature bone cells that maintain matrix, housed in lacunae), osteoclasts (large multinucleate cells that resorb bone).
Osteoblasts and osteoclasts work together continuously in remodeling: blasts build bone, clasts break it down.
Compare compact bone and spongy (cancellous) bone.
Answer: Compact bone is dense, forms the outer shell, organized into osteons (Haversian systems); spongy bone is porous, made of trabeculae, found at bone ends and contains red marrow.
Compact bone provides strength and protection; spongy bone is lighter and houses hematopoietic marrow.
Describe the structure of an osteon (Haversian system).
Answer: Concentric rings of bone matrix (lamellae) surrounding a central (Haversian) canal containing blood vessels and nerves; osteocytes sit in lacunae connected by canaliculi.
Canaliculi allow nutrient and waste exchange between osteocytes and the central canal.
What makes up the organic vs inorganic components of bone matrix?
Answer: Organic: collagen fibers (give flexibility/tensile strength). Inorganic: hydroxyapatite (calcium phosphate crystals) (give hardness/compressive strength).
Removing minerals makes bone rubbery; removing collagen makes bone brittle.
Differentiate periosteum and endosteum.
Answer: Periosteum: fibrous membrane covering the outer surface of bone (except joint surfaces), anchors tendons/ligaments. Endosteum: thin membrane lining the internal medullary cavity and trabeculae.
Label the main regions of a long bone: diaphysis, epiphysis, metaphysis.
Answer: Diaphysis = shaft (compact bone, medullary cavity); epiphysis = bone ends (spongy bone, articular cartilage); metaphysis = region between diaphysis and epiphysis containing the epiphyseal plate/line.
What is the epiphyseal plate and what happens to it after growth stops?
Answer: A layer of hyaline cartilage where bone lengthens; after growth ceases it ossifies into the epiphyseal line.
What fills the medullary cavity of a long bone?
Answer: Yellow (fatty) marrow in adults; red marrow (hematopoietic) in infants and certain bones like the sternum, ilium, and vertebrae.
Compare intramembranous and endochondral ossification.
Answer: Intramembranous: bone forms directly from mesenchymal tissue (e.g., flat bones of skull, clavicle). Endochondral: bone forms by replacing a hyaline cartilage model (most bones, including long bones).
List the key steps of endochondral ossification.
Answer: 1) Cartilage model forms. 2) Primary ossification center forms in the diaphysis; perichondrium becomes periosteum. 3) Cartilage calcifies and chondrocytes die, forming a cavity. 4) Blood vessels invade, osteoblasts form bone. 5) Secondary ossification centers form in epiphyses. 6) Cartilage remains only at the articular surfaces and epiphyseal plate.
What is bone remodeling and which cells carry it out?
Answer: The continuous replacement of old bone with new bone tissue; osteoclasts resorb bone and osteoblasts deposit new matrix.
Remodeling allows bone to repair microdamage, adapt to stress, and release calcium as needed.
State Wolff's Law.
Answer: Bone grows and remodels in response to the mechanical stresses placed on it — bone is laid down where stress is high and resorbed where stress is low.
Explains why weight-bearing exercise increases bone density and immobilization causes bone loss.
What is the effect of parathyroid hormone (PTH) on blood calcium?
Answer: PTH raises blood calcium by stimulating osteoclast activity (bone resorption), increasing renal calcium reabsorption, and promoting activation of vitamin D (which increases intestinal calcium absorption).
Released from parathyroid glands in response to low blood Ca2+.
What is the effect of calcitonin on blood calcium?
Answer: Calcitonin lowers blood calcium by inhibiting osteoclast activity and reducing bone resorption.
Released from thyroid C cells in response to high blood Ca2+; opposes PTH.
What role does vitamin D play in calcium homeostasis?
Answer: Activated vitamin D (calcitriol) increases intestinal absorption of calcium and phosphate, and works with PTH to mobilize bone calcium.
What structures make up the axial skeleton?
Answer: The skull, vertebral column (26 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 1 sacrum, 1 coccyx), rib cage (12 pairs of ribs + sternum), and the hyoid bone.
The axial skeleton forms the body's central axis and protects the brain, spinal cord, and thoracic organs.
What structures make up the appendicular skeleton?
Answer: The pectoral (shoulder) girdle, upper limbs, pelvic girdle, and lower limbs — all bones of the limbs and their girdles.
The appendicular skeleton enables movement and manipulation of the environment.
Name the major bones of the upper limb from proximal to distal.
Answer: Clavicle and scapula (pectoral girdle) → humerus → radius and ulna → carpals → metacarpals → phalanges.
Name the major bones of the lower limb from proximal to distal.
Answer: Hip (pelvic) bone → femur → patella → tibia and fibula → tarsals → metatarsals → phalanges.
Compare fibrous, cartilaginous, and synovial joints with an example of each.
Answer: Fibrous: connected by dense connective tissue, little/no movement (e.g., skull sutures). Cartilaginous: connected by cartilage, slight movement (e.g., intervertebral discs, pubic symphysis). Synovial: fluid-filled joint cavity, freely movable (e.g., knee, shoulder).
What structures make up a typical synovial joint?
Answer: Articular cartilage (covers bone ends), joint (articular) capsule, synovial membrane producing synovial fluid (lubricates and nourishes), and ligaments for stability.
List the six types of synovial joints with an example of each.
Answer: Hinge (elbow), ball-and-socket (shoulder/hip), pivot (atlantoaxial joint), saddle (thumb carpometacarpal), condyloid/ellipsoid (wrist), plane/gliding (intercarpal joints).
Define flexion/extension and abduction/adduction.
Answer: Flexion decreases the joint angle; extension increases it. Abduction moves a limb away from the midline; adduction moves it toward the midline.
Define pronation/supination and inversion/eversion.
Answer: Pronation rotates the forearm so the palm faces posteriorly/down; supination rotates it so the palm faces anteriorly/up. Inversion turns the sole of the foot medially; eversion turns it laterally.
Describe the structural hierarchy of skeletal muscle from whole muscle to sarcomere.
Answer: Muscle (wrapped in epimysium) → fascicle (wrapped in perimysium) → muscle fiber/cell (wrapped in endomysium) → myofibril → sarcomere (the functional contractile unit).
Identify the sarcomere bands/zones: Z-disc, A band, I band, H zone, M line.
Answer: Z-disc = sarcomere boundary where thin filaments anchor; A band = full length of thick filaments (dark); I band = thin filaments only (light); H zone = thick filaments only, center of A band; M line = center of sarcomere anchoring thick filaments.
What are thick and thin filaments made of?
Answer: Thick filaments = myosin. Thin filaments = actin plus the regulatory proteins tropomyosin and troponin.
Tropomyosin blocks myosin-binding sites on actin at rest; troponin binds Ca2+ to move tropomyosin aside.
What is the role of T-tubules and the sarcoplasmic reticulum (SR)?
Answer: T-tubules conduct the action potential deep into the muscle fiber; the SR stores and releases Ca2+ that triggers contraction.
Describe the cross-bridge cycle of the sliding filament mechanism.
Answer: 1) Ca2+ binds troponin, moving tropomyosin off the binding site. 2) Myosin head (with ADP+Pi) binds actin forming a cross-bridge. 3) Power stroke: myosin releases Pi/ADP, pulling the thin filament inward. 4) ATP binds myosin, detaching it from actin. 5) ATP hydrolysis re-cocks the myosin head for another cycle.
Without new ATP, myosin stays bound to actin (as in rigor mortis).
During contraction, which sarcomere bands shorten and which stay the same length?
Answer: I band and H zone shorten (and may disappear); A band length stays the same; Z-discs move closer together, shortening the sarcomere.
Filaments themselves do not shorten — they slide past each other.
Describe the sequence of events at the neuromuscular junction (NMJ).
Answer: 1) Action potential reaches the axon terminal. 2) Voltage-gated Ca2+ channels open, Ca2+ influx triggers ACh release from synaptic vesicles. 3) ACh diffuses across the synaptic cleft and binds nicotinic receptors on the motor end plate. 4) Ligand-gated Na+ channels open, depolarizing the sarcolemma (end-plate potential). 5) Acetylcholinesterase (AChE) breaks down ACh, ending the signal.
Outline excitation-contraction coupling.
Answer: The sarcolemma action potential travels down T-tubules, where voltage-sensing DHP receptors mechanically open ryanodine receptor Ca2+ channels on the SR, releasing Ca2+ into the sarcoplasm to initiate the cross-bridge cycle.
Relaxation occurs when Ca2+ is pumped back into the SR via the SERCA pump, allowing tropomyosin to re-cover binding sites.
What are the main sources of ATP for muscle contraction, from fastest to slowest?
Answer: 1) Stored ATP (seconds). 2) Phosphocreatine system (regenerates ATP rapidly, ~10-15 sec). 3) Anaerobic glycolysis (~30-60 sec, produces lactate). 4) Aerobic (oxidative) respiration (sustained, slower but far more ATP).
Creatine phosphate + ADP → creatine + ATP (via creatine kinase), the fastest way to replenish ATP.
Compare Type I, Type IIa, and Type IIx skeletal muscle fibers.
Answer: Type I (slow oxidative): slow contraction, highly fatigue-resistant, relies on aerobic metabolism, used for posture/endurance. Type IIa (fast oxidative-glycolytic): fast contraction, moderately fatigue-resistant, mixed metabolism. Type IIx/IIb (fast glycolytic): fastest contraction, fatigues quickly, relies on anaerobic glycolysis, used for short powerful bursts.
Define a muscle twitch and its three phases.
Answer: A twitch is the brief contraction produced by a single action potential. Phases: latent period (Ca2+ release, no tension yet), contraction phase (cross-bridge cycling, tension rises), relaxation phase (Ca2+ reuptake, tension falls).
What is summation and tetanus in muscle physiology?
Answer: Summation: successive stimuli arriving before full relaxation cause twitches to add together, increasing force. Tetanus: at high enough stimulation frequency, individual twitches fuse into one sustained, smooth maximal contraction (unfused tetanus shows small oscillations; fused tetanus is smooth and maximal).
Name the actions of the deltoid, pectoralis major, and trapezius.
Answer: Deltoid: abducts the arm at the shoulder. Pectoralis major: flexes, adducts, and medially rotates the arm. Trapezius: elevates, retracts, and rotates the scapula; extends the neck.
Name the actions of the biceps brachii and triceps brachii.
Answer: Biceps brachii: flexes the elbow and supinates the forearm. Triceps brachii: extends the elbow.
Name the actions of the rectus abdominis, latissimus dorsi, and gluteus maximus.
Answer: Rectus abdominis: flexes the trunk (vertebral column). Latissimus dorsi: extends, adducts, and medially rotates the arm. Gluteus maximus: extends and laterally rotates the thigh at the hip.
Name the actions of the quadriceps femoris, hamstrings, and gastrocnemius.
Answer: Quadriceps femoris: extends the knee (and flexes the hip via rectus femoris). Hamstrings: flex the knee and extend the hip. Gastrocnemius: plantarflexes the foot and flexes the knee.
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