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.)
-To further study anti-nAChR antibody function, the researchers wanted to replicate the experiments described in the passage in other mammalian animal models, but their research proposal was rejected by other scientists. Why was the proposal rejected?

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.)
-To further study anti-nAChR antibody function, the researchers wanted to replicate the experiments described in the passage in other mammalian animal models, but their research proposal was rejected by other scientists. Why was the proposal rejected?

Quizplus · Accepted Answer

11ecba2d_100f_3741_a3bf_3f9de56c38ea_MD0008_00 The passage states that acetylcholine (ACh) binding to its nicotinic acetylcholine receptor (nAChR) induces muscle contraction, which also causes muscle tension. Because previous studies had shown that nAChR subunits could be antibody binding sites, researchers hypothesized anti-nAChR antibodies could bind nAChR and block the ACh attachment site. Researchers tested this hypothesis by comparing the effect of three different infusions on muscle tension: Infusion A (anti-nAChR antibodies only), Infusion B (ACh only), and Infusion C (ACh + anti-nAChR antibodies). The research objective was to investigate how varying concentrations of anti-nAChR antibodies affected muscle tension, so the ACh concentration of infusions B and C should have remained unchanged. This would have allowed researchers to assess how muscle tension was affected by varying concentrations of anti-nAChR antibodies only. However, Table 1 shows that the ACh concentration of the infusions was inconsistent. Infusion B contained 7.5 mL of a 2.5 mg/mL ACh solution (ie, 18.75 mg of ACh). In contrast, Infusion C contained 3.75 mL of the same 2.5 mg/mL ACh solution (ie, 9.36 mg of ACh). Muscle samples in the electrolyte bath exposed to Infusion B generated more muscle tension than those exposed to Infusion C. As a result, muscle tension measurements were invalid as they were affected by varying concentrations of both ACh and anti-nAChR antibodies, instead of being affected by anti-nAChR antibodies alone. (Choice A) Experiment 2 investigated the effect of various infusions on muscle tension. To quantify the positive or negative effect that each infusion has on muscle tension, a baseline condition (pre-infusion muscle tension, in this case) must be used as a control. (Choice C) Like all antibodies, autoimmune antibodies (eg, anti-nAChR antibodies) are synthesized by B cells and transported through the blood stream to all body tissues (including skeletal muscle). Exposure of skeletal muscle to anti-nAChR antibodies would be a standard and expected characteristic of the autoimmune disease mentioned in the passage and would not affect proposal approval or rejection. (Choice D) Mice and other rodents are mammals that are frequently used as animal models to investigate mammalian physiology and pathophysiology (ie, diseases). Therefore, it is feasible that experimental results from mouse studies can be reproduced in other mammals (eg, humans). Educational objective: When the research goal is to assess how changes in a specific independent variable affect a dependent variable, researchers should ensure that any other variables that are not under study remain controlled or unchanged.

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