A Case Study on the Nervous System 3

Part III—Diagnostic Tests





Dr. Thrush looked over the results of Phyllis’s Magnetic Resonance
Imaging (MRI) and evoked potential tests. She was thinking how glad she
was that Steven had accompanied Phyllis on this visit as she showed
them the MR images of Phyllis’ brain.

“The machine took pictures of Phyllis’ brain in slices. The dark
areas are the brain tissue, Phyllis, and the white areas in the middle
and around the outside are the cerebrospinal fluid.”

“Is that normal? Am I OK?” Phyllis asked. “And what are those little
white dots in the tissue?”

“The white dots are what concerned me,” replied the doctor. “So we
did another type of MRI, called dark fluid, so that your cerebrospinal
fluid would not show up white.”

“But I still see the white dots in my brain tissue. What does that
mean?”

The doctor looked down, knowing that this was never an easy thing to
tell a patient.

“I am afraid that this indicates that there are plaques or scars in
your brain, and that you may have multiple sclerosis.”

Tears began to form in her eyes as Phyllis squeezed her husband’s
hand.

“I am afraid that the evoked potential test and the elevated levels
of myelin basic protein in you cerebrospinal fluid indicate the same
thing.”





Questions

1. Test your knowledge of the function of chemical synapses by filling
   in the flow diagram to the right.
   
2. What type of cell is myelin?
   
3. What is the function of myelin in nerve cells?
   
4. In myelinated axons, where are action potentials
   generated?
   
5. Where, then, are voltage gated sodium channels
   concentrated in myelinated axons?
   
6. What happens to myelin in people who suffer from
   multiple sclerosis?
   
7. Why is there an elevated level of myelin basic protein
   in the cerebrospinal fluid?
   
8. What would be the effect on action potential conduction
   at a region of axon where the disease had its effect?
   
9. What effect would this have on the coordination of
   movements if this took place in areas involved in motor control of
   finger movements?