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MCAT

Unit 2: Lesson 3

Foundation 3: Organ Systems

The vagus nerve and mice

Problem

A vagotomy is a surgical procedure in which the vagus nerve is resected. The vagus nerve is a part of the parasympathetic division of the autonomic nervous system. Scientists interested in exploring the relationship between the nervous system and other biological processes performed a vagotomy on rats and measured their physiological responses when exposed to an antigen.
40 rats were collected and separated into two groups. The first group had a true vagotomy performed on them, in which the vagal trunk was removed. The latter group was given a sham surgery, in which the rats’ vagus nerve was exposed but not cut. Both groups were given one month to recover from their surgery, and the weight of both groups was measured.
After weighing, both groups were given injections of either saline or lipopolysaccharide (LPS).
LPS elicits a strong immune response in animals, because the O-antigen is a target for recognition by host antibodies. It is found in the outer membrane of gram-negative bacteria. In rats, Interleukin-1 beta (IL-1β) specifically targets LPS, and is involved in inflammatory mediation, apoptosis, and cell differentiation.
Two hours after injection, all of the rats were decapitated and were measured for cranial concentrations of IL-1β mRNA in the hippocampus, hypothalamus, and pituitary. This process involved a polymerase chain reaction after reverse transcription. There were no significant levels of IL-1β mRNA in the pituitary for any group. Levels of IL-1β mRNA in the hippocampus and hypothalamus are shown in Figure 1.
Figure 1. Open bars represent the sham-operated rats, while the hatched bars represent the vagotomized rats. Panel A refers to the hippocampus, and Panel B refers to the hypothalamus.
Finally, the stomachs of the rats were removed and weighed. This information, along with the original body weights after surgery, is tabulated in Table 1.
Table 1. Body weight and stomach weight of vagotomized and sham-operated rats.
Vagotomy groupSham-surgery group
Body weight at surgery37.6 ± 0.41g36.7 ± 0.52g
Stomach weight after decapitation*0.87 ± 0.8g0.26 ± 0.01g
*This finding was significantly different between the two groups.
Source: Bluthe, R. Combe, C. Dantzer, R. Keith, K. Kent, S. Laye, S. Medina, C. Parnet, P. (1995).
What is a plausible purpose of having one group undergo a sham surgery?
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