Where are the calcium ions stored in a muscle fiber?

Correct Answer: C

Rationale: Calcium ions, essential for muscle contraction, are stored in the sarcoplasmic reticulum, a specialized endoplasmic reticulum surrounding myofibrils. Upon a nerve signal, it releases calcium into the sarcoplasm to bind with troponin, initiating actin-myosin interaction. T-tubules conduct the signal inward but don't store calcium they facilitate its release. The sarcolemma is the muscle fiber's membrane, encasing the cell, not a storage site. Mitochondria produce ATP, powering contraction, but lack calcium storage capacity. The sarcoplasmic reticulum's role as a calcium reservoir is unique, ensuring precise regulation of contraction and relaxation, distinguishing it from conduction, structural, or energy-producing components, critical for the sliding filament mechanism in muscle physiology.

Correct Answer: D

Rationale: The gluteus maximus is named for size (maximus, largest) and location (gluteus, buttock region), the biggest muscle there, extending and rotating the thigh. Shape (e.g., trapezius) or action (e.g., flexor) could apply elsewhere, but here, size and gluteal position define it. Combining both aspects highlights its prominence and placement, distinct from single-trait names, reflecting its anatomical and functional significance in the hip.

Correct Answer: A

Rationale: Rectus abdominis, along the abdomen, flexes the vertebral column, bending the spine forward, as in sit-ups. Serratus anterior protracts scapula. Sternocleidomastoid flexes the neck, not full column. Trapezius extends the head or adducts scapula. Rectus abdominis' anterior pull distinguishes it, key for trunk flexion, unlike scapular or neck-specific actions.

Correct Answer: B

Rationale: Quadriceps occupy the anterior thigh, comprising four muscles (rectus femoris, vastus lateralis, medialis, intermedius) that extend the knee. Anterior abdomen hosts rectus abdominis. Posterior thigh has hamstrings. Posterior arm holds triceps. Quadriceps' thigh-front location sets them apart, key for leg movement, distinct from other regions.

Correct Answer: B

Rationale: The A band in a sarcomere represents the region containing thick myosin filaments, overlapping with thin actin filaments in striated muscle. During contraction, the sliding filament theory explains that actin slides over myosin, shortening the sarcomere as Z lines draw closer. However, the A band's length defined by the fixed length of myosin filaments remains constant. The I band (actin-only) and H zone (myosin-only) shrink as overlap increases, but the myosin span doesn't change, keeping the A band stable. Z lines anchor actin, not define the A band, and actin-only regions are the I band, not A. This unchanged A band length is a key feature in muscle mechanics, reflecting the structural integrity of myosin and the dynamic overlap driving contraction, a fundamental concept in understanding sarcomere function.