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22 Cards in this Set
- Front
- Back
What three pathologies is repetitive nerve stim most useful for?
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Myasthenia gravis, Lamber-Eaton Myasthenic Syndrome, and botulinism.
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How many molecules of acetylcholine are in each quanta?
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approximately 10k
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What are the three stores of quanta? How many are in each store and where are they?
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Primary or immediately available have 1k quanta and are just beneath the presynaptic nerve terminal membrane. Secondary or mobilization store has 10k quanta and resupply the primary after a few seconds. Tertiare store of 100k are in the axon and soma.
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How is the amount of Ach released related to the amount of Na channels opened on the postsynaptic membrane?
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The more quanta bound to the Ach receptors, the more sodium is allowed in. Thus, the more Ach released, the larger the endplate potential.
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In relation to the EPP and action potential, what is the safety factor?
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the amount of extra depolaration elicited by the EPP above and beyond necessity for an all or none action potential to occur.
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Describe slow RNS in relation to EPP in a normal situation
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2-3 Hz stim causes progressive depletion of quanta released with decline in EPP each time. However, due to safety factor there is always enough quanta. After a few seconds, the secondary store further replenishes the lost quanta.
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Describe rapid RNS in relation to EPP in a normal situation
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it takes 100ms to evacuate influxed calcium out of the terminal bouton. Stim faster than this causes accumulation of Ca2+, causing increased EPP with each successive stim.
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Describe what happens with slow and rapid RNS in pathologic NMJ conditions.
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slow RNS causes progressive depletion of quanta wherein threshold may not be reached. Rapid RNS can cause a subsequent increase in EPP back above baseline.
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What are the three physiological assumptions underlying RNS modeling in NMJ disorders?
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1) m=pn where m is the quanta released with each stim, p is the probability of release based on the concentration of calcium, n is the number of quanta immediately available. 2) the mobilization store takes effect after 1-2 seconds. 3) 100 ms are required to pump Ca out of the presynaptic terminal.
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At 3 Hz RNS, what is the largest absolute drop in EPP? When is the EPP the lowest?
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Between the first and second stim. The EPP is lowest in the stim just before mobilization of secondary stores. This takes around 1-2 seconds, so between the third and sixth stim.
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In regards to the formula m = pn, describe what's abnormal with MG and LEMS.
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All is normal with MG. There are fewer ACHRs, thus less EPP and less safety factor. With LEMS, p is decreased because of less calcium influx because of the antibodies to the calcium channels. This results in a lower m.
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Describe the appearance of EPPs and action potentials with presynaptic slow RNS.
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They typically start below threshold and subsequently decline further until mobilization stores kick in, causing an increase in EPPs, potentially back above baseline.
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What is the typical frequency of maximally contracting musscle during exercise?
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30-50 Hz.
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Describe posttetanic facilitation.
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After about 10 s of exercise/tetanic activity, there is an increase in the EPP, typically above baseline.
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Describe posttetanic exhaustion.
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After about a minute of exercise/tetanic activity, there is a prolonged decline in the EPP for several minutes.
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Describe pseudofacilitation.
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Increase in CMAP amplitude following 10 seconds of maximal contraction. No increase in number of MUAPs recruited, but increase in amplitude due to increased synchronicity.
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Describe the effect of temperature with function in myasthenia gravis.
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Increased temperature usually makes symptoms worse (and colder temps better). Reason not certain but likely because of relatively increased (and decreased) activity of acetylcholinestherase.
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What distribution of muscles does MG clinically affect most?
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ocular, bulbar, and proximal extremity.
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What proximal nerve does Preston and Shapiro recommend stimulating (and recording) for MG?
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The spinal accessory with recording over the trapezius.
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How do you calculate amplitude decrement and what value is considered abnormal?
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(baseline CMAP - lowest CMAP) / baseline CMAP. 10% or greater decrement is considered abnormal.
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When calculating posttetanic facilitation, what does an increase of 40% tell you? 100%?
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40% doesn't tell you much as this can happen in normal individuals because of increased synchronicity. 100% indicates a presynaptic NMJ disorder.
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What are three types of disorders that can show a decrement with slow RNS?
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Nerve damage with newly sprouted, unstable nerves. Some alpha motor neuron diseases. Certain myopathies, especially metabolic (e.g. McArdle's).
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