Demyelinating disease
- Dashboard
- Demyelinating disease
Most Recent Score Report
| Questions Correct | Questions Answered | Time Spent | Status | Attempt Date | |
|---|---|---|---|---|---|
| -- | -- | -- | -- | -- |
Previous Score Reports
| Questions Correct | Questions Answered | Time Spent | Status | Attempt Date | |
|---|---|---|---|---|---|
| -- | -- | -- | -- | -- |
Demyelinating disease
Array
(
[passage] => WP_Post Object
(
[ID] => 554600
[post_author] => 12815
[post_date] => 2024-12-23 18:09:03
[post_date_gmt] => 2024-12-23 23:09:03
[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
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. Our understanding of the immunopathogenesis of MS has been informed by 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.
Researchers are examining the effects of demyelination in EAE prior to neuronal injury or loss. To do this, they compare in vitro conduction velocities of four groups of nerve samples: spinal cord samples isolated from a group of normal, healthy rats, spinal cord samples isolated from a group of rats with EAE, and two control groups derived from lab-prepared materials that serve positive and negative controls. One of these controls accurately mimics a highly myelinated nerve; the other, a completely demyelinated nerve. The samples are suspended between two electrodes in solution. A small current is applied to one electrode and the voltage is measured as a function of time at the other electrode; from this measurement, the conduction velocity of each nerve sample can be determined. Figure 1 shows mean conduction velocities versus nerve diameter for each test group.
Figure 1 Mean conduction velocities with respect to axon diameter for test groups A–D
[post_title] => Demyelinating disease
[post_excerpt] =>
[post_status] => publish
[comment_status] => closed
[ping_status] => closed
[post_password] =>
[post_name] => demyelinating-disease
[to_ping] =>
[pinged] =>
[post_modified] => 2024-12-23 18:09:03
[post_modified_gmt] => 2024-12-23 23:09:03
[post_content_filtered] =>
[post_parent] => 0
[guid] => https://medlifemastery.com/?post_type=passage&p=554600
[menu_order] => 0
[post_type] => passage
[post_mime_type] =>
[comment_count] => 0
[filter] => raw
)
[questions] => Array
(
[0] => Array
(
[quiz_unique_key] => 602779517
[question] => In contrast to the white matter mentioned in the passage, which description accurately describes the contents of the grey matter of the brain?
[value] => Array
(
[answer] => 1
[description] => Reason for Correct Answer:
The brain is composed of both grey matter and white matter. Grey matter largely functions to receive incoming information and regulate outgoing information, whereas white matter serves to transmit signals to other regions of the brain, spinal cord, and body.
The white color of white matter is largely due to the presence of myelinated structures – specifically, nerve axons that serve to relay signals.
Grey matter is notable for abundant synapses, which connect regions of the brain.
An abundance of neuron cell bodies accurately describes the grey matter of the brain and spinal cord.
Here is a picture of the gray matter in the brain. Here, the gray matter surrounds white matter, whose axons connect different parts of the brain’s gray matter.
Here is a picture of the gray matter in the spinal cord. Here, the gray matter sits centrally, and white matter runs peripherally (closer to the body to which it transmits signals).
https://en.wikipedia.org/wiki/File:Grey_and_white_matter_in_the_spinal_cord.gif
)
[answers] => Array
(
[0] => Array
(
[each_answer] => A. Abundant neuron cell bodies
)
[1] => Array
(
[each_answer] => B. Absence of synapses
)
[2] => Array
(
[each_answer] => C. Abundant myelinated nerve axons
)
[3] => Array
(
[each_answer] => D. Absence of glial cells
)
)
)
[1] => Array
(
[quiz_unique_key] => 1403770772
[question] => What neural mediated process may be preserved, or even heightened, after damage to the white matter of the brainstem?
[value] => Array
(
[answer] => 1
[description] => Reason for Correct Answer:
The brainstem transmits information between the brain and the spinal cord.
Loss of brainstem white matter would be associated with loss of motor function/coordination of the arms/legs and sensory loss due to damage to upper motor neurons.
If only the brainstem is affected, then lower motor neurons innervating skeletal muscles would be unaffected.
Spinal reflex arcs are responsible for deep tendon (stretch) reflexes such as the knee-kick (patellar) reflex. Though these spinal reflex arcs normally receive (inhibitory) feedback from the brain, reflexes happen without the brain’s input. Damage to the central nervous system, including the brainstem, therefore does not affect deep tendon reflexes and often results in heightened reflexes due to loss of inhibitory feedback.
)
[answers] => Array
(
[0] => Array
(
[each_answer] => A. Deep tendon (stretch) reflex
)
[1] => Array
(
[each_answer] => B. Lower limb coordination
)
[2] => Array
(
[each_answer] => C. Upper limb coordination
)
[3] => Array
(
[each_answer] => D. Swallowing
)
)
)
[2] => Array
(
[quiz_unique_key] => 1403770772
[question] => Which line in Figure 1 most likely corresponds to the test group of spinal cords isolated from rats with EAE?
[value] => Array
(
[answer] => 3
[description] => Reason for Correct Answer:
The passage states that two of the lines represent control groups.
Both a positive and a negative control group are included, which we can assume are represented by the two extremes on our plot: lines A and D. Because myelination increases conduction velocities, line A should represent the “heavily myelinated” positive control (fastest velocity per diameter), and line D should represent the “completely demyelinated” negative control.
The remaining two lines, B and C, must then each either represent the healthy rats or the EAE rats.
Because EAE is characterized by the destruction and loss of myelin, nerves from the EAE rats should also exhibit lower velocities than healthy controls, and the line for this group should be closer to the negative control/demyelinated group. Therefore, line C most likely corresponds to the test group of spinal cords isolated from rats with EAE. This graph shows a summary of the groups:
)
[answers] => Array
(
[0] => Array
(
[each_answer] => A. Line A
)
[1] => Array
(
[each_answer] => B. Line B
)
[2] => Array
(
[each_answer] => C. Line C
)
[3] => Array
(
[each_answer] => D. Line D
)
)
)
[3] => Array
(
[quiz_unique_key] => 1403770772
[question] => In addition to axonal damage, MS is likely associated with damage to which cell type?
[value] => Array
(
[answer] => 1
[description] => Reason for Correct Answer:
The passage states that MS causes selective demyelination in the central nervous system.
It’s reasonable to conclude that MS would cause damage to myelin-producing cells. The two myelin-producing cells in the nervous system are oligodendrocytes and Schwann cells.
However, whereas Schwann cells are found in the peripheral nervous system, oligodendrocytes are found in the central nervous system, where MS exerts its effects. Therefore, MS likely causes damage to oligodendrocytes in the central nervous system.
)
[answers] => Array
(
[0] => Array
(
[each_answer] => A. Oligodendrocyte
)
[1] => Array
(
[each_answer] => B. Schwann cell
)
[2] => Array
(
[each_answer] => C. Ependymal cell
)
[3] => Array
(
[each_answer] => D. Microglia
)
)
)
[4] => Array
(
[quiz_unique_key] => 1403770772
[question] => In addition to the mechanism described in the passage, which mechanism would most likely generate an animal model of MS?
[value] => Array
(
[answer] => 3
[description] => Reason for Correct Answer:
The passage describes one mechanism for generating EAE rats, which serves as an animal model for MS: peripheral immunization with myelin protein components. The purpose of this immunization is to trigger the rat’s own immune cells to start reacting to myelin, eventually attacking myelin on their own cells and causing “experimental autoimmune encephalomyelitis (EAE),” which is very similar to MS (where the patient’s immune system also attacks and degrades its own myelin).
An alternative form of creating an MS animal model must involve creating an animal in which immune cells are attacking myelin/myelin-containing cells. We can therefore eliminate Choices A and B, because peripheral immunization with CD4/CD8 receptors or with demyelinated axons would NOT trigger an immune response against myelin.
Transferring actual reactive immune cells or antibodies into an animal is also a potential way to model an autoimmune disease process. However, the passage states that MS is mediated through activated CD4+ myelin-reactive T cells (cell-mediated immunity).
Therefore, the transfer of T cells that are myelin activated and will attack myelin in the rats (Choice C) would create a better model of MS than transferring antibodies, which are part of the B-cell mediated/humoral immune response.
)
[answers] => Array
(
[0] => Array
(
[each_answer] => A. Peripheral immunization with CD4 and CD8 receptor molecules taken from MS patients
)
[1] => Array
(
[each_answer] => B. Peripheral immunization with demyelinated axons taken from MS patients
)
[2] => Array
(
[each_answer] => C. Passive transfer of activated myelin-specific CD4+ lymphocytes from MS patients
)
[3] => Array
(
[each_answer] => D. Passive transfer of anti-myelin antibodies from MS patients
)
)
)
[5] => Array
(
[quiz_unique_key] => 1325138223
[question] => Compared to healthy controls, nerve cell membranes taken from MS patients would exhibit which of the following?
[value] => Array
(
[answer] => 1
[description] => Reason for Correct Answer:
Nerve cell membranes taken from MS patients will have less myelin sheath.
The myelin sheath acts as an electrical insulator; it is a barrier that prevents the leakage of ions where the sheath is located. (This helps propagate nerve impulses faster, as the nerve signal jumps from node to node between areas with myelin sheath, where there is lower resistance of ion flow.) Thus, myelin increases resistance across the neuronal membrane.
Capacitance refers to the ability of a membrane to store electrical charge. The myelin sheath decreases the capacitance of the membrane because it increases the distance between charges inside and outside of the nerve cell.
Therefore, a nerve with less myelin, like one taken from an MS patient, would have decreased resistance and increased capacitance across its membrane. The loss of myelin would not directly cause defects in ion channels nor would it result in “superconductivity.”
Here are some images explaining how this related to the speed of the action potential:
For more info, see: https://biology.stackexchange.com/questions/8282/why-is-saltatory-conduction-in-myelinated-axons-faster-than-continuous-conductio
)
[answers] => Array
(
[0] => Array
(
[each_answer] => A. Increased capacitance
)
[1] => Array
(
[each_answer] => B. Increased resistance
)
[2] => Array
(
[each_answer] => C. Defects in ion channels
)
[3] => Array
(
[each_answer] => D. Superconductivity
)
)
)
)
[total_question] => 6
[correct_answers] => Array
(
[554600|1] => A
[554600|2] => A
[554600|3] => C
[554600|4] => A
[554600|5] => C
[554600|6] => A
)
[hide_display_feedback_settings] =>
[hide_solutions] =>
)
Figure 1
