Musculoskeletal System

Skeletal Muscle Structure

Skeletal muscle is organized in a precisely nested hierarchy optimized for force generation. Each whole muscle (e.g., biceps brachii) is encased in epimysium and contains fascicles (bundles of muscle fibers) wrapped in perimysium. Individual muscle fibers (cells) are multinucleated syncytia formed by myoblast fusion, surrounded by endomysium. The sarcolemma (plasma membrane) conducts action potentials and invaginates into transverse tubules (T-tubules) at regular intervals. Internally, each fiber is packed with myofibrils—cylindrical organelles composed of repeating sarcomeres, the fundamental contractile units. A sarcomere is bounded by Z-lines and contains: thin filaments (actin, tropomyosin, troponin complex) anchored at the Z-line, thick filaments (myosin II) centered in the A-band, and titin, a giant elastic protein connecting myosin to the Z-line. The banding pattern visible under electron microscopy reflects this organization: I-band (actin only, light), A-band (myosin length, dark), H-zone (myosin only, within A-band), and M-line (myosin cross-links at center). The sarcoplasmic reticulum (SR), a specialized smooth ER, wraps each myofibril and stores Ca^(2+) in terminal cisternae flanking T-tubules.

Sliding-Filament Theory

The sliding-filament theory, proposed by Huxley and Hanson in 1954, states that muscle contraction results from thin (actin) filaments sliding past thick (myosin) filaments, shortening the sarcomere without changing filament lengths themselves. The molecular engine is the cross-bridge cycle: (1) ATP binds to the myosin head, causing release from actin. (2) ATP hydrolysis to ADP + Pi cocks the myosin head into a high-energy, extended conformation (recovery stroke). (3) The myosin head binds weakly to actin, then Pi release triggers the power stroke—the head pivots, pulling actin ~10 nm toward the sarcomere center and generating force. (4) ADP dissociates, and the myosin head remains tightly bound in rigor until a new ATP binds. Each cycle consumes one ATP per cross-bridge. The sarcomere shortens because cross-bridges pull thin filaments from opposite ends toward the center; the I-band and H-zone narrow, while the A-band remains constant. Key concept: the length-tension relationship—maximum force is produced at resting sarcomere length (2.0-2.2 microns) where thin-thick filament overlap is optimal; at shorter or longer lengths, force declines because fewer cross-bridges can form.

Excitation-Contraction Coupling

Excitation-contraction (E-C) coupling is the process by which an action potential on the sarcolemma triggers Ca^(2+) release from the SR, initiating cross-bridge cycling. The sequence: (1) A motor neuron releases acetylcholine (ACh) at the neuromuscular junction (NMJ), binding nicotinic ACh receptors on the motor end plate. (2) Ligand-gated Na+ channels open, producing an end-plate potential (EPP) that, if suprathreshold, initiates a muscle action potential. (3) The action potential propagates along the sarcolemma and deep into the fiber via T-tubules. (4) In skeletal muscle, T-tubule depolarization activates dihydropyridine receptors (DHPRs, L-type Ca^(2+) channels) which are mechanically coupled to ryanodine receptors (RyR) on the SR terminal cisternae—this conformational coupling does NOT require extracellular Ca^(2+) entry (unlike cardiac muscle, which uses Ca^(2+)-induced Ca^(2+) release). (5) RyR opens, flooding the cytoplasm with Ca^(2+) (~10^(-4) M SR to ~10^(-5) M cytosol). (6) Ca^(2+) binds troponin C, causing tropomyosin to shift and expose myosin-binding sites on actin. (7) Cross-bridge cycling proceeds. (8) Relaxation occurs when Ca^(2+)-ATPase (SERCA) pumps Ca^(2+) back into the SR, lowering cytosolic [Ca^(2+)]; tropomyosin re-covers binding sites. E-C coupling is the MCAT's classic example of signal transduction: electrical to chemical to mechanical.

Bone Structure and Remodeling

Bone is a dynamic connective tissue with a composite material structure: organic matrix (~35%, primarily type I collagen for tensile strength) and inorganic mineral (~65%, hydroxyapatite crystals Ca10(PO4)6(OH)2 for compressive strength). Two architectural forms exist: compact (cortical) bone forms the dense outer shell organized into osteons (Haversian systems)—concentric lamellae surrounding a central Haversian canal containing blood vessels and nerves; osteocytes reside in lacunae connected by canaliculi for nutrient exchange. Spongy (cancellous/trabecular) bone forms the inner meshwork, with trabeculae aligned along stress lines maximizing strength-to-weight ratio. Bone is constantly remodeled by two cell types: osteoblasts (bone-forming, derived from mesenchymal stem cells, secrete osteoid and alkaline phosphatase for mineralization) and osteoclasts (bone-resorbing, derived from hematopoietic monocytes, multinucleated, secrete acid and cathepsin K to dissolve mineral and collagen). The balance is hormonally regulated: parathyroid hormone (PTH) stimulates osteoclast activity (raising serum Ca^(2+)), while calcitonin inhibits osteoclasts; estrogen inhibits osteoclast apoptosis, so postmenopausal estrogen loss accelerates bone resorption (osteoporosis). Mechanical loading stimulates osteoblast activity via piezoelectric effects (Wolff's law: bone adapts to mechanical stress).

Joints and Connective Tissue

Joints are classified structurally and functionally. Structural classification: fibrous joints (immovable, e.g., skull sutures—dense regular connective tissue), cartilaginous joints (slightly movable, e.g., intervertebral discs—fibrocartilage, pubic symphysis), and synovial joints (freely movable, e.g., knee, shoulder). Synovial joints share a common architecture: articular cartilage (hyaline cartilage, avascular, reduces friction), a joint cavity filled with synovial fluid (secreted by synovial membrane, contains hyaluronic acid and lubricin for lubrication and nutrient transport), a fibrous articular capsule, and reinforcing ligaments. Tendons connect muscle to bone and transmit force; they are dense regular connective tissue with parallel collagen fibers optimized for unidirectional tensile loading. Ligaments connect bone to bone, stabilizing joints, and contain more elastin for slight stretch. Cartilage—hyaline (articular surfaces, epiphyseal plates), fibrocartilage (intervertebral discs, menisci), and elastic (ear, epiglottis)—is avascular and relies on diffusion from synovial fluid or perichondrium, explaining its poor healing capacity. The extracellular matrix of connective tissues consists of ground substance (proteoglycans, GAGs like chondroitin sulfate) and protein fibers (collagen, elastin, reticular fibers), and the specific composition determines tissue mechanical properties.

Motor-Unit Recruitment

A motor unit consists of a single alpha motor neuron and all the skeletal muscle fibers it innervates. This is the smallest functional unit of motor control. Motor units vary in size: small motor units (e.g., extraocular muscles, ~5 fibers per neuron) enable fine, precise movements; large motor units (e.g., quadriceps, ~1000+ fibers per neuron) generate high force but coarse control. Force gradation occurs through two mechanisms: (1) rate coding—increasing the firing frequency of the motor neuron increases force via temporal summation (unfused tetanus to fused tetanus at high frequencies), and (2) recruitment—activating more motor units. Recruitment follows the Size Principle (Henneman's principle): smaller motor neurons have lower thresholds and are recruited first; as force demand increases, progressively larger motor units are recruited. This is energetically efficient and ensures smooth force gradation. Motor units are also classified by fiber type: Type I (slow oxidative, red, high mitochondrial density, myoglobin-rich, fatigue-resistant, recruited first) and Type II (fast, further divided into IIa—fast oxidative-glycolytic, intermediate—and IIx/IIb—fast glycolytic, white, fatigable, recruited at highest forces). The motor unit is also the key to understanding neuromuscular disorders: myasthenia gravis (autoantibodies against ACh receptors leads to fewer functional motor units), ALS (motor neuron degeneration), and botulism (blocks ACh release produces flaccid paralysis of motor units).

How it works

Movement begins with a decision in the motor cortex, but the MCAT cares about the molecular chain. An action potential arrives at the NMJ, ACh releases, a muscle action potential fires, T-tubule depolarization triggers DHPR-RyR coupling, SR Ca^(2+) releases, Ca^(2+)-troponin C binds, tropomyosin shifts, myosin-actin cross-bridge cycling begins, sarcomeres shorten, force transmits through connective tissue to bone, and joints move. Meanwhile, bone is silently remodeling: osteoclasts resorb microcracks, osteoblasts fill the voids, and the skeleton adapts to the loads imposed by those same muscles. The entire system is a negative-feedback loop: muscles pull on bones across joints; proprioceptive afferents (muscle spindles, Golgi tendon organs) report length and tension back to the spinal cord, modulating motor-neuron firing and protecting against injury.

How it works

Movement begins with a decision in the motor cortex, but the MCAT cares about the molecular chain. An action potential arrives at the NMJ, ACh releases, a muscle action potential fires, T-tubule depolarization triggers DHPR-RyR coupling, SR Ca^(2+) releases, Ca^(2+)-troponin C binds, tropomyosin shifts, myosin-actin cross-bridge cycling begins, sarcomeres shorten, force transmits through connective tissue to bone, and joints move. Meanwhile, bone is silently remodeling: osteoclasts resorb microcracks, osteoblasts fill the voids, and the skeleton adapts to the loads imposed by those same muscles. The entire system is a negative-feedback loop: muscles pull on bones across joints; proprioceptive afferents (muscle spindles, Golgi tendon organs) report length and tension back to the spinal cord, modulating motor-neuron firing and protecting against injury.

Comparisons

• B/B (Sarcomere structure): Know the banding pattern cold—I-band (actin only), A-band (myosin length), H-zone (myosin only). During contraction, I-band and H-zone shrink; A-band unchanged. Questions often ask which band disappears or changes.
• C/P (Cross-bridge thermodynamics): ATP hydrolysis (~30 kJ/mol) provides free energy for the myosin power stroke; the cross-bridge cycle is an excellent example of coupling an exergonic reaction (ATP to ADP + Pi) to mechanical work.
• B/B (Calcium signaling): E-C coupling illustrates a second-messenger cascade—Ca^(2+) is the intracellular messenger linking membrane depolarization to myofilament activation, analogous to Ca^(2+) roles in neurotransmitter release and fertilization.
• B/B (Endocrine connections): PTH, calcitonin, vitamin D (calcitriol), and estrogen all converge on bone remodeling—disruption of any hormone (e.g., hyperparathyroidism, menopause) has predictable skeletal consequences tested on the MCAT.
• C/P (Collagen mechanics): Type I collagen's triple-helix structure and covalent cross-linking explain bone's tensile strength; collagen disorders like osteogenesis imperfecta (defective type I collagen) produce brittle bones.
• P/S (Motor learning): Motor-unit recruitment is refined through practice—skill acquisition involves more efficient recruitment patterns (fewer motor units for the same task), linking muscle physiology to behavioral science.

Common confusions

• Confusing the role of ATP in contraction vs. relaxation. ATP is required for myosin to RELEASE actin (step 1 of cross-bridge cycle). Without ATP, myosin stays locked in rigor (rigor mortis). ATP is NOT what drives the power stroke—that energy came from hydrolysis in the previous cycle.
• Mixing up skeletal vs. cardiac E-C coupling. Skeletal muscle uses mechanical DHPR-RyR coupling (no extracellular Ca^(2+) needed). Cardiac muscle uses Ca^(2+)-induced Ca^(2+) release (CICR)—a small extracellular Ca^(2+) influx through DHPR triggers SR Ca^(2+) release.
• Thinking Ca^(2+) directly causes contraction. Ca^(2+) binds troponin C, which moves tropomyosin off myosin-binding sites. Ca^(2+) enables cross-bridge cycling; the cross-bridges themselves generate force. Students who skip the troponin-tropomyosin step lose points.
• Misidentifying which bands change during contraction. A-band is constant (myosin filament length); I-band and H-zone narrow. If a question shows a sarcomere and asks what happens during contraction, the correct answer is 'I-band width decreases.'
• Forgetting that bone resorption is osteoclast-mediated, not osteoblast. PTH stimulates osteoclasts (via RANKL on osteoblasts)—osteoblasts don't resorb bone. This is an endocrine trap: know which hormone acts on which cell type.
• Confusing tendon vs. ligament function. Tendon = muscle-to-bone (force transmission); Ligament = bone-to-bone (joint stability). Injury scenarios test this: an ACL tear is a ligament injury, not a tendon injury.
• Assuming all motor units within a muscle are identical. The Size Principle means slow-oxidative (Type I), low-threshold units fire first; fast-glycolytic (Type IIx) units are recruited only at high force. A passage may show EMG data reflecting this recruitment order.
• Applying smooth or cardiac muscle properties to skeletal muscle. Skeletal muscle is striated, voluntary, and uses somatic motor neurons with nicotinic ACh receptors. Smooth muscle is non-striated, involuntary, and uses varicosities with both muscarinic and adrenergic receptors—don't conflate them in autonomic pharmacology questions.

Quick review

• Sarcomere: Z-line to Z-line. I-band = actin only, A-band = myosin length, H-zone = myosin only. Contraction shrinks I-band and H-zone.
• Cross-bridge cycle: ATP binds, release, hydrolysis cocks head, Pi release = power stroke, ADP release. Rigor without ATP.
• Skeletal E-C coupling: AP to T-tubule to DHPR mechanically opens RyR to SR Ca^(2+) release. No extracellular Ca^(2+) required.
• Ca^(2+) binds troponin C, tropomyosin moves, myosin-binding sites exposed.
• Relaxation: SERCA pumps Ca^(2+) back into SR. Active transport — requires ATP.
• Osteoblasts build bone (mesenchymal origin); osteoclasts resorb bone (hematopoietic origin, multinucleated).
• PTH increases osteoclast activity (increases serum Ca^(2+)); calcitonin decreases osteoclasts (decreases serum Ca^(2+)); estrogen inhibits osteoclast apoptosis.
• Motor unit = 1 alpha-motor neuron + all fibers it innervates. Size Principle: small (low-threshold) then large (high-threshold).
• Type I fibers: slow oxidative, red, fatigue-resistant. Type IIx: fast glycolytic, white, fatigable.
• Tendons = muscle-to-bone; Ligaments = bone-to-bone. Both dense regular connective tissue but different functions.
• Synovial joint components: articular (hyaline) cartilage, synovial fluid (hyaluronic acid), joint capsule, reinforcing ligaments.
• NMJ: ACh release, nicotinic receptors, EPP, muscle AP. Myasthenia gravis: anti-AChR antibodies.