MCAT - Organ Systems:: Musculoskeletal System Flashcards (50 Cards) — Study Online

Curriculum Overview

Comprehensive, high-yield MCAT study deck focusing on Musculoskeletal System. Features 50 rigorous, curriculum-aligned flashcards designed for advanced-level mastery. Core concepts covered include When Ca, key problem-solving heuristics, foundational formulas, and exam-tested application scenarios. Ideal for active recall review, spaced repetition study, and scoring in the top percentile.

Topics & Key Concepts

MCAT Only Type SERCA Direct Muscle Myosin Calcium Z-discs T-tubule

Sample Flashcard Questions & Answers

Showing 8 of 50 cards

Question #1 Active Recall

The functional contractile unit of a skeletal muscle fiber, defined as the region between two adjacent Z-discs, is called the:

- **A)** Myofibril
- **B)** Sarcolemma
- **C)** Sarcomere
- **D)** Sarcoplasmic reticulum

Answer & Explanation:

**Answer: C)**

The sarcomere, bounded by two Z-discs, is the basic repeating contractile unit of skeletal (and cardiac) muscle, containing the organized arrays of thick (myosin) and thin (actin) filaments whose sliding produces contraction.

Question #2 Active Recall

According to the sliding filament theory, muscle contraction occurs because:

- **A)** Both thick and thin filaments shorten individually
- **B)** Thin (actin) filaments slide past thick (myosin) filaments via cross-bridge cycling, without either filament changing length, shortening the sarcomere overall
- **C)** Myosin filaments contract by coiling into a smaller shape
- **D)** Actin filaments are enzymatically degraded to produce force

Answer & Explanation:

**Answer: B)**

Neither thick nor thin filaments change length during contraction; instead, myosin cross-bridges cyclically bind actin, pull it inward (power stroke), release, and rebind further along - progressively sliding actin past myosin and shortening the sarcomere (I band and H zone narrow) while filament lengths themselves stay constant.

Question #3 Active Recall

Troponin and tropomyosin, associated with the thin filament, regulate muscle contraction by:

- **A)** Directly generating ATP for the myosin head
- **B)** Actively pulling actin filaments during contraction
- **C)** Forming the thick filament backbone
- **D)** Blocking (tropomyosin) or exposing (via troponin's Ca2+-induced conformational change) the myosin-binding sites on actin, controlling whether cross-bridge cycling can occur

Answer & Explanation:

**Answer: D)**

At rest, tropomyosin physically blocks myosin-binding sites on actin. When Ca2+ binds troponin C, it induces a conformational shift that moves tropomyosin away from the binding sites, allowing myosin cross-bridges to attach and cycle - the molecular basis of Ca2+-dependent contraction regulation.

Question #4 Active Recall

The trigger for Ca2+ release from the sarcoplasmic reticulum during skeletal muscle excitation-contraction coupling is:

- **A)** Depolarization of the T-tubule membrane, sensed by voltage-gated dihydropyridine receptors (DHPRs) that mechanically couple to and open ryanodine receptors (RyRs) on the adjacent sarcoplasmic reticulum
- **B)** Direct diffusion of extracellular Ca2+ through the plasma membrane as the primary source
- **C)** ATP binding directly to troponin
- **D)** Acetylcholine binding directly to the sarcoplasmic reticulum

Answer & Explanation:

**Answer: A)**

In skeletal muscle, T-tubule depolarization is sensed by DHPRs, which are mechanically (not electrically) coupled to ryanodine receptors on the sarcoplasmic reticulum - depolarization directly triggers RyR opening and Ca2+ release, without requiring significant extracellular Ca2+ influx (unlike cardiac muscle's calcium-induced calcium release mechanism).

Question #5 Active Recall

The cross-bridge cycle step in which myosin releases from actin (allowing the cycle to repeat) requires binding of:

- **A)** Calcium directly to the myosin head
- **B)** A new ATP molecule to the myosin head, which causes detachment from actin (rigor mortis occurs when ATP is unavailable and myosin remains permanently bound to actin)
- **C)** ADP directly, without any nucleotide exchange
- **D)** Troponin binding directly to myosin

Answer & Explanation:

**Answer: B)**

ATP binding to the myosin head (not its hydrolysis) is what causes myosin to release from actin, resetting the cross-bridge for another cycle. Without ATP (as after death), myosin heads remain locked onto actin - the basis of rigor mortis - illustrating that ATP is required for muscle RELAXATION, not just contraction.

Question #6 Active Recall

Skeletal muscle relaxation occurs when cytosolic calcium is actively pumped back into the sarcoplasmic reticulum by:

- **A)** Simple passive diffusion
- **B)** The Na+/K+-ATPase
- **C)** SERCA (sarco/endoplasmic reticulum Ca2+-ATPase), an active pump that lowers cytosolic Ca2+, allowing tropomyosin to re-block myosin-binding sites
- **D)** Voltage-gated Ca2+ channels on the T-tubule

Answer & Explanation:

**Answer: C)**

SERCA actively pumps Ca2+ back into the sarcoplasmic reticulum against its concentration gradient (ATP-dependent), lowering cytosolic Ca2+ so that troponin releases Ca2+, tropomyosin re-blocks the myosin-binding sites, and the muscle relaxes.

Question #7 Active Recall

Type I (slow-twitch, oxidative) muscle fibers are best suited for which type of activity?

- **A)** Prolonged, low-intensity, endurance activity (e.g., maintaining posture, marathon running), due to high mitochondrial density, myoglobin content, and fatigue resistance
- **B)** Very brief, maximal-power activities like sprinting or powerlifting
- **C)** Activities requiring the fastest possible contraction speed with rapid fatigue
- **D)** No physiologic role in normal human movement

Answer & Explanation:

**Answer: A)**

Type I fibers have abundant mitochondria, myoglobin, and capillary density, favoring sustained oxidative ATP production and high fatigue resistance - ideal for endurance and postural activities, though they generate force more slowly and with lower peak power than type II fibers.

Question #8 Active Recall

Type IIb (fast-twitch, glycolytic) muscle fibers are best suited for which type of activity?

- **A)** Sustained low-intensity postural activity
- **B)** Long-duration aerobic exercise
- **C)** Activities requiring maximal fatigue resistance above all else
- **D)** Brief, high-intensity, powerful contractions (e.g., sprinting, jumping), relying primarily on anaerobic glycolysis for rapid ATP production but fatiguing quickly

Answer & Explanation:

**Answer: D)**

Type IIb fibers have fewer mitochondria and rely heavily on anaerobic glycolysis, allowing very rapid, forceful contraction but with quick fatigue - well suited to brief, powerful efforts rather than sustained activity, the opposite profile from type I fibers.

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