Which fascicle arrangement, under voluntary or involuntary regulation of contraction, can control the opening or closing an orifice (opening)?

Correct Answer: C

Rationale: Circular fascicle arrangements, like those in the orbicularis oris or sphincters, are designed to control the opening or closing of orifices (e.g., mouth, anus) under voluntary or involuntary contraction. Their ring-like structure constricts or relaxes to regulate passage. Pennate fascicles (feather-like) maximize force, not orifice control. Triangular (e.g., pectoralis major) spread force broadly, not circularly. Parallel fascicles (e.g., sartorius) produce linear movement, not sphincter action. Circular arrangement is the correct answer, as it uniquely enables the constriction needed for orifice regulation, a critical function in muscles governing entry or exit points in the body.

Correct Answer: B

Rationale: The calcaneal (Achilles) tendon connects the gastrocnemius and soleus to the calcaneus. A slight tear from training is a muscle strain, an overstretching or tearing of muscle or tendon, treated with P.R.I.C.E. (protection, rest, ice, compression, elevation) and anti-inflammatories. A sprain involves ligaments, not tendons. Repetitive strain injuries result from overuse, not a single tear. Tendon strain isn't a standard term strains encompass tendon injuries. Muscle strain is the correct answer, as it fits the Achilles injury and treatment, common in runners.

Correct Answer: C

Rationale: Ligaments are tough, elastic bands connecting bone to bone, stabilizing joints like the knee's ACL linking femur to tibia. This limits excessive movement, maintaining skeletal integrity. Cartilage-to-bone connections occur via other tissues, not ligaments directly, as cartilage cushions, not links. Muscle-to-bone attachment is tendons' job, pulling bones for motion. Tendons-to-muscles isn't accurate tendons extend from muscle to bone, not connecting to other tendons. Ligaments' bone-to-bone role distinguishes them, passively restricting joint range while allowing flexibility, unlike tendons' active force transmission or cartilage's shock absorption. This structural specificity ensures joint stability across the body, vital for posture and movement, setting ligaments apart in the musculoskeletal framework.

Correct Answer: D

Rationale: The muscular system accounts for about 50% of body weight, primarily from skeletal muscles, with smooth and cardiac adding less mass. This reflects their extensive presence over 600 muscles supporting movement and structure. Twenty percent is too low, ignoring skeletal bulk. Forty percent is close but underestimates, per anatomical data. Seventy percent overstates, leaving little for bones, organs, and fat. Fifty percent aligns with standard physiology, emphasizing muscles' role in force generation and mass, balancing the body's composition, distinct from lower or higher guesses misaligned with muscle distribution.

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.