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MCAT

Unit 2: Lesson 3

Foundation 3: Organ Systems

Nervous system disorders II: MS

Problem

Multiple sclerosis (MS) is an autoimmune condition that leads to selective demyelination of the central nervous system. In patients with MS, multifocal plaques, or zones of demyelination, can be seen throughout the white matter of the brain and spinal cord. The pathogenesis of the disease involves an immune attack against CNS antigens mediated through activated CD4+ myelin-reactive T cells with a possible contribution by B cells. Much current understanding of immunopathogenesis of MS comes from the study of experimental autoimmune encephalomyelitis (EAE), an animal model of CNS inflammatory demyelination that can be induced by peripheral immunization with myelin protein components. EAE shares many of the histologic features of MS including active demyelination and axonal loss, all of which are presumably mediated by myelin specific T cells.
A researcher is examining the effects of demyelination in EAE prior to neuronal injury or loss and examines in vitro samples of spinal cords isolated from rats in two test groups: a group of normal, healthy rats and a group of rats with EAE. The researchers also prepares two control groups derived from lab synthesized material that are known to act as positive and negative controls. One of these accurately mimics a highly myelinated nerve; the other, a completely demyelinated nerve.
The samples are suspended between electrodes in Ringer’s solution. Electrodes are placed at either end of each spinal cord samples, and a small current is generated at the (+) end. A probe at the (-) end measures the voltage at the (-) electrode as a function of time to calculate the conduction velocity of each nerve sample. The results, with respect to nerve diameter, are shown in Figure 1. Lines A through D represent the averaged conduction velocities with respect to axon diameter for each test group.
Figure 1 Mean conduction velocities of unknown test groups
Which line in Figure 1 most likely corresponds to the test group of spinal cords isolated from rats with EAE?
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