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Kurs: MCAT > Rozdział 2

Lekcja 1: Biological sciences practice passage questions

Nervous system disorders II: MS


With a prevalence of approximately 30 individuals per 100,000 worldwide, multiple sclerosis (MS) is the leading cause of non-traumatic disability in young adults in the USA. The disease is characterized by an immune response against ‘self’ myelin cells, particularly within the CNS. Ultimately, the continued demyelination and inflammation of neural structures leads to the development of sclerotic plaques and the destruction of neural white matter. Neurological deficits arise as a product of these white matter lesions, and may be localized as a function of the symptoms observed. For example, loss of upper motor neurons may produce loss of feedback to spinal reflexes; brainstem damage may result in the loss of voluntary or coordinated motor function, weakness, bladder dysfunction, and/or sensory impairment. Damage to higher cortical structures notably contributes to cognitive dysfunction, depression, and vision loss.
In the lab, you are curious about the effects of demyelination in MS prior to actual neuronal injury or loss. You prepare 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 MS. Furthermore, you prepare two control groups derived from lab synthesized material that are known to act as perfect positive and negative controls. One of these accurately mimics a highly myelinated nerve; the other, a completely demyelinated nerve. Unfortunately, while you are suspending the samples between electrodes in Ringer’s solution, your supervisor removes all of your labels. Although you can identify all of the samples of a particular group, you cannot determine which group is which. Your supervisor encourages you to run the experiment anyway as an exercise to see if you can apply your current knowledge to determine the identities of each group. You are sure to record the lengths, diameters, volumes, and masses of all samples, and observe proper laboratory technique in preparing and incubating the samples. Electrodes are placed at either end of each spinal cord, 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.
Figure 1: Mean conduction velocities of unknown test groups.
Lines A through D represent the averaged conduction velocities with respect to axon diameter for each test group.
Which line in Figure 1 most likely corresponds to the test group of spinal cords isolated from rats with multiples sclerosis?
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