MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 3b Muscular System Contraction

What this deck covers

This deck focuses on 3b Muscular System Contraction, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

How to use these flashcards

Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What key feature distinguishes skeletal muscle cells from smooth muscle cells?

Answer: Skeletal muscle is striated; smooth muscle is nonstriated. Striations in skeletal muscle arise from organized sarcomeres, enabling voluntary contraction, while smooth muscle lacks this for slower, sustained activity.

Flashcard 2: What structural line runs through the center of the sarcomere and anchors thick filaments?

Answer: M line. The M line provides structural support by cross-linking thick filaments at the sarcomere midpoint, maintaining alignment during contraction.

Flashcard 3: During skeletal muscle contraction, which sarcomere regions shorten: I band, H zone, or A band?

Answer: I band and H zone shorten; A band stays constant. Contraction pulls Z lines closer, shortening I bands and H zones as thin filaments slide inward, while A band length reflects constant thick filament size.

Flashcard 4: What is the sliding filament model statement that best describes sarcomere shortening?

Answer: Thin filaments slide past thick filaments; filament lengths do not change. This model explains force generation without filament shortening, as myosin cross-bridges pull actin filaments toward the sarcomere center.

Flashcard 5: What ion directly binds troponin C to initiate skeletal muscle contraction?

Answer: Ca$^{2+}$. Calcium ions trigger conformational changes in troponin, exposing myosin-binding sites on actin to allow cross-bridge formation.

Flashcard 6: What is the immediate effect of Ca$^{2+}$ binding to troponin in skeletal muscle?

Answer: Tropomyosin shifts to expose myosin-binding sites on actin. Calcium binding induces troponin movement, shifting tropomyosin to uncover actin's binding sites for myosin heads.

Flashcard 7: What neurotransmitter is released at the neuromuscular junction to stimulate skeletal muscle?

Answer: Acetylcholine (ACh). Released by motor neurons, ACh binds postsynaptic receptors to depolarize the muscle fiber, initiating the excitation-contraction cascade.

Flashcard 8: What receptor type on the motor end plate binds ACh to depolarize skeletal muscle?

Answer: Nicotinic ACh receptor (ligand-gated cation channel). This receptor opens upon ACh binding, allowing Na$^+$ influx to generate an end-plate potential that propagates as an action potential.

Flashcard 9: What structure in skeletal muscle conducts the action potential into the fiber interior?

Answer: T tubules (transverse tubules). T tubules invaginate from the sarcolemma, ensuring rapid action potential spread to deep myofibrils for synchronous contraction.

Flashcard 10: What intracellular organelle stores and releases Ca$^{2+}$ for skeletal muscle contraction?

Answer: Sarcoplasmic reticulum. This specialized endoplasmic reticulum sequesters Ca$^{2+}$ during relaxation and releases it upon stimulation to trigger contraction.

Flashcard 11: What event directly causes Ca$^{2+}$ release from the sarcoplasmic reticulum in skeletal muscle?

Answer: T-tubule depolarization activates DHP receptors, opening ryanodine receptors. DHP receptors act as voltage sensors, mechanically linking to ryanodine receptors to release stored Ca$^{2+}$ from the SR lumen.

Flashcard 12: What is the role of ATP binding to myosin during the cross-bridge cycle?

Answer: ATP binding causes myosin to detach from actin. ATP binding reduces myosin's affinity for actin, allowing detachment and preparation for the next cycle after the power stroke.

Flashcard 13: What is the role of ATP hydrolysis by myosin ATPase during the cross-bridge cycle?

Answer: Hydrolysis cocks the myosin head into a high-energy state. ATP hydrolysis provides energy to position the myosin head in a cocked state, ready for attachment to actin upon site exposure.

Flashcard 14: What step of the cross-bridge cycle is the power stroke?

Answer: Release of Pi (then ADP) drives myosin head pivot and actin sliding. Phosphate and ADP release triggers the myosin head to pivot, pulling actin filaments and generating the force of contraction.

Flashcard 15: What is rigor mortis in terms of actin-myosin interactions?

Answer: ATP depletion prevents myosin detachment, locking cross-bridges. Without ATP, myosin remains bound to actin in a rigid state, causing postmortem muscle stiffness until proteolysis occurs.

Flashcard 16: What is the primary mechanism that terminates skeletal muscle contraction by lowering cytosolic Ca$^{2+}$?

Answer: SERCA pumps Ca$^{2+}$ back into the sarcoplasmic reticulum. SERCA actively transports Ca$^{2+}$ against its gradient using ATP, restoring low cytosolic levels to allow tropomyosin to block actin sites.

Flashcard 17: What are the three major muscle tissue types found in the human body?

Answer: Skeletal, cardiac, and smooth muscle. These tissues differ in structure, function, and location, with skeletal attached to bones, cardiac in the heart, and smooth in organs for involuntary movement.

Flashcard 18: What is the H zone in a sarcomere?

Answer: Region with only thick filaments (no thin filament overlap). The H zone represents the central area of the A band where thin filaments do not reach in relaxed muscle, vanishing during contraction.

Flashcard 19: What key feature distinguishes cardiac muscle from skeletal muscle?

Answer: Intercalated discs (gap junctions and desmosomes) in cardiac muscle. These structures allow rapid electrical signal propagation and mechanical linkage between cells, enabling synchronized heart contractions.

Flashcard 20: What is the functional unit of a striated muscle fiber that shortens during contraction?

Answer: Sarcomere. It consists of organized actin and myosin filaments that interact to produce force via the sliding filament mechanism.

Flashcard 21: What sarcomere boundary is defined by the Z line (Z disc)?

Answer: A sarcomere runs from Z line to Z line. Z lines anchor thin filaments, defining the sarcomere's repeating units that collectively shorten during muscle contraction.

Flashcard 22: What proteins form the thin filament in skeletal muscle?

Answer: Actin, tropomyosin, and troponin. Actin provides the backbone, tropomyosin regulates access, and troponin senses calcium to initiate contraction.

Flashcard 23: What protein primarily forms the thick filament in skeletal muscle?

Answer: Myosin II. Myosin II molecules form bipolar thick filaments that interact with actin to generate force through cross-bridge cycling.

Flashcard 24: What sarcomere band contains thick filaments and is dark on microscopy?

Answer: A band. The A band's density results from myosin thick filaments overlapping with actin, appearing anisotropic under polarized light microscopy.

Flashcard 25: What sarcomere band contains only thin filaments and is light on microscopy?

Answer: I band. The I band's lighter appearance stems from actin thin filaments without myosin overlap, appearing isotropic under microscopy.