Passage
The nicotinic acetylcholine receptor (nAChR) is a protein embedded in the muscle fiber membrane that facilitates cell-to-cell communication across the neuromuscular junction. When acetylcholine (ACh) binds nAChR, electrical depolarization within skeletal muscle fibers is induced and ultimately gives rise to contractile activity. The nAChR is composed of five protein subunits symmetrically organized around a central ion pore. Previous studies demonstrated that these subunits may serve as epitopes (antibody binding sites) in autoimmune disease. By reversibly binding nAChR subunits, anti-nAChr antibodies may temporarily block the attachment site for ACh, resulting in muscle weakness.Researchers harvested muscle samples from mice to investigate the effects of anti-nAChR antibodies on muscle tension, the pulling force transmitted by muscles during contraction.Experiment 1First, skeletal muscle samples (tibialis anterior) of adult male mice were surgically harvested (n = 30) , washed, and stripped of connective tissues. Next, the isolated muscle samples were placed in a 150-mL oxygen-infused, electrolyte-rich bath containing glucose and Ca²⁺ ions that were able to enter the muscle fibers and enable maximal contractile force. One end of the submerged muscle was anchored in place while the other was attached to an apparatus that measures muscle tension and modifies muscle length. Using electrodes, action potentials were elicited in the harvested muscle tissue to evaluate contractile function (Figure 1) .
Figure 1 Action potential elicited from a harvested skeletal muscle sampleExperiment 2The researchers divided the 30 muscle samples into three groups of 10. Resting muscle tension was measured, and the electrolyte-rich baths of each group were further infused with a 7.5-mL solution containing mouse-specific anti-nAChR antibodies, ACh, or a mixture of both (Table 1) . Post-infusion (peak) muscle tension was measured for 30 minutes, with results displayed in Figure 2.Table 1 Solutions Infused into the Electrolyte-Rich Bath
Figure 2 Pre-infusion resting muscle tension and post-infusion peak muscle tension in harvested muscle samples (Note: * indicates p < 0.05.)
-Why was Ca²⁺ added to the oxygen-infused, electrolyte-rich bath? To allow Ca²⁺ to:

Question

Passage
The nicotinic acetylcholine receptor (nAChR) is a protein embedded in the muscle fiber membrane that facilitates cell-to-cell communication across the neuromuscular junction. When acetylcholine (ACh) binds nAChR, electrical depolarization within skeletal muscle fibers is induced and ultimately gives rise to contractile activity. The nAChR is composed of five protein subunits symmetrically organized around a central ion pore. Previous studies demonstrated that these subunits may serve as epitopes (antibody binding sites) in autoimmune disease. By reversibly binding nAChR subunits, anti-nAChr antibodies may temporarily block the attachment site for ACh, resulting in muscle weakness.Researchers harvested muscle samples from mice to investigate the effects of anti-nAChR antibodies on muscle tension, the pulling force transmitted by muscles during contraction.Experiment 1First, skeletal muscle samples (tibialis anterior) of adult male mice were surgically harvested (n = 30) , washed, and stripped of connective tissues. Next, the isolated muscle samples were placed in a 150-mL oxygen-infused, electrolyte-rich bath containing glucose and Ca²⁺ ions that were able to enter the muscle fibers and enable maximal contractile force. One end of the submerged muscle was anchored in place while the other was attached to an apparatus that measures muscle tension and modifies muscle length. Using electrodes, action potentials were elicited in the harvested muscle tissue to evaluate contractile function (Figure 1) .

Figure 1 Action potential elicited from a harvested skeletal muscle sampleExperiment 2The researchers divided the 30 muscle samples into three groups of 10. Resting muscle tension was measured, and the electrolyte-rich baths of each group were further infused with a 7.5-mL solution containing mouse-specific anti-nAChR antibodies, ACh, or a mixture of both (Table 1) . Post-infusion (peak) muscle tension was measured for 30 minutes, with results displayed in Figure 2.Table 1 Solutions Infused into the Electrolyte-Rich Bath

Figure 2 Pre-infusion resting muscle tension and post-infusion peak muscle tension in harvested muscle samples (Note: * indicates p < 0.05.)
-Why was Ca²⁺ added to the oxygen-infused, electrolyte-rich bath? To allow Ca²⁺ to:

Quizplus · Accepted Answer

11ecba2d_100c_781d_a3bf_17d1167814d0_MD0008_00 Calcium (Ca²⁺) is a mineral that plays an important role in the contraction of skeletal muscle fibers. The sarcoplasmic reticulum (SR) is a specialized endoplasmic reticulum in muscle fiber that is responsible for tightly regulating intracellular calcium concentration by sequestering calcium ions during periods of relaxation and releasing them into the cytosol during periods of contraction (in response to action potentials). Cytosolic calcium drives muscle contraction by first binding troponin, which then allows the actin and myosin filaments of the sarcomere to slide across one another. The passage states that harvested muscle samples were placed in an oxygen-infused, electrolyte-rich bath that contained Ca²⁺ ions; it also states that these ions could enter the muscle fibers and enable maximal contractile force. Based on this information, the calcium ions in the bath were able to enter and aggregate inside the SR of the muscle fiber samples, enabling their contractile activity. (Choice B) Muscle fibers primarily generate ATP through glucose metabolism. Although calcium is involved in many biological processes (eg, cell signaling, bone remodeling), the unique role for calcium in muscle fibers specifically relates to contraction. (Choice C) Iron, not calcium, facilitates oxygen binding by the proteins myoglobin and hemoglobin. Myoglobin stores oxygen within muscle fibers, and hemoglobin is an oxygen carrier molecule found in red blood cells that allows oxygen to be transported throughout the body. (Choice D) At the neuromuscular junction, the entry of calcium ions into presynaptic neurons leads to the release of ACh from the presynaptic vesicles and into the synaptic cleft, where ACh then binds its receptor on the muscle fiber membrane. However, because the muscle fibers in the electrolyte bath were completely isolated, no presynaptic neurons were present, and calcium only served to supply the SR of the muscle fibers. Educational objective: The sarcoplasmic reticulum (SR) is a muscle fiber organelle that tightly regulates intracellular calcium (Ca²⁺) concentration. The release of calcium from the SR promotes muscle fiber contraction whereas the transport of calcium into the SR promotes and maintains muscle fiber relaxation.

Passage the Nicotinic Acetylcholine Receptor (NAChR) Is a Protein Embedded in in the Muscle