You are studying a gene which, when mutant, causes oligodendrocytes to enwrap shorter sections of axon, so that nodes of Ranvier are more closely spaced. What is the likely effect on nerve impulses?

Question

Nodes of Ranvier are the spaces between sections of axon enwrapped with myelin. Because myelin creates an insulating layer that prevents ion flow, the progress of the action potential "jumps" from node to node, thus speeding up transmission down the axon.

A.  They will travel faster.
B.  They will travel slower.
C.  They will increase in amplitude.
D.  They will decrease in amplitude.
E.  They will travel laterally across connected axons.

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?
· What key words does the question contain?

Gather Content/Choose Answer
· What do you already know about nodes of Ranvier? How do they relate to nerve impulses?

Consider Alternatives
· What other information is related to the question?

Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

B.  They will travel slower.

Clarify Question
· What is the key concept addressed by the question?
        o The question is asking about the function of nodes of Ranvier in action potential conduction.
· What type of thinking is required?
        o This question is asking you to take your knowledge concerning nodes of Ranvier and to analyze the effects of changes to these nodes.
· What key words does the question contain?
        o Nodes of Ranvier, spaced, nerve impulses, oligodendrocytes. How are all these terms related?

Gather Content/Choose Answer
· What do you already know about nodes of Ranvier? How do they relate to nerve impulses?
        o To solve this problem, you’ll need to apply your knowledge of the form and function of nodes of Ranvier, which are unmyelinated regions of axons. Remember that myelin insulates axons by preventing ions from leaking out. Since ions only flow into the axon at nodes of Ranvier, this is where action potentials are generated. Following an action potential at one node, sodium rapidly diffuses to the next node, producing an action potential there in a saltatory manner.

Consider Alternatives
· What other information is related to the question?
        o The answers are all related to how node of Ranvier spacing could influence nerve impulses (action potentials). You should recall that action potentials are all-or-none responses that do not increase in amplitude with increased stimulation and that action potentials move along axons to dendrites of another neuron, not laterally across adjacent axons. 
Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o At this point, you should have everything you need to know to answer the question. Since you know that action potentials do not increase or decrease in amplitude and you also know that action potentials do not travel from axon to axon, but from axon to dendrite, you can eliminate most of the answer options. So the question is whether tighter spacing would increase or decrease action potential conduction speed. Since myelination speeds up action potentials by preventing ion leaking, and the more nodes of Ranvier that there are, the less overall myelination of the axon, you can deduce that more nodes would decrease the speed of nerve impulses. Therefore B must be the correct answer.

Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o If you figured out the correct answer, wonderful! If not, where in your thought process did things go wrong? If you arrived at an incorrect answer, did you remember that nerve impulses are all-or-none events that do not change amplitude? Did you remember that myelination speeds up action potential conduction and that action potentials only occur at nodes of Ranvier?
        o Was it clear to you that the type of thinking needed to answer this question is to analyze your knowledge about nerve impulses to predict the effects of a shift away from normal conditions? Consider where in the problem solving process your thinking went astray.