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NBME 23 Answers

nbme23/Block 2/Question#16

Which of the following best explains impaired ...

Increase in axonal capacitance

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submitted by mcl(228),

In case you wanna go super nerd and read about myelin, capacitance, and resistance, this guy does a good job.

nwinkelmann  This really helped me, at least the pictures did. Here's my interpretation of the pictures in not super scientific terms: capacitance is like the "capaciy" to keep ions close to the membrane. Myelin puts a barrier between the ions in the conductive environment (ECF or ICF) and the nerve membrane. The higher the capacitance, the closer the ions are to the membrane, so it's like the charge effect is "more potent" so harder to change the membrane potentia, whereas if the ions are farther from the membrane, the charge effect is "less potent" so easier to change the membrane potential and thus easier to depolarize. Thus, with myelin, there is decreased capacity of the ions to be close to the membrane, so in demyelinating conditions, the ions can be really close to the membrane, i.e. higher capacitance. +4  
sweetmed  this helped a lot! +  
roaaaj  Well explained! +  

-time constant= Resistance x Capacitance -lower time constant = faster conduction -myelin lowers capacitance - thereby lowering time constant and increasing conduction speed (lower capacity for neuron to hold charge may allow the charge to "jump" from node to node - I may be making that last part up but that is how I understand it)

Source: https://en.wikipedia.org/wiki/Myelin

"myelin speeds the transmission of electrical impulses called action potentials along myelinated axons by insulating the axon and reducing axonal membrane capacitance"

littletreetrunk  I think this makes total sense, but how does it not ALSO stop fast axonal transport? +2  
laminin  axonal transport is transport of organelles bidirectionally along the axon in the cytoplasm since myelin is on the outside of the axon demyelination doesn't affect this process. source: https://en.wikipedia.org/wiki/Axonal_transport "Axonal transport, also called axoplasmic transport or axoplasmic flow, is a cellular process responsible for movement of mitochondria, lipids, synaptic vesicles, proteins, and other cell parts to and from a neuron's cell body, through the cytoplasm of its axon." +1  
yotsubato  axonal transport is mediated by kinesin and dynein. Microtubule toxins like vincristine block these +2  

so why is "cessation of fast axonal transport" wrong? Don't myelinated axons, by definition, have fast conductance? So demyelinated axons would have "cessation of fast axonal transport", which is the answer A, right?

diabetes  i think it slows down ,no cessation . +