2. Intracellular recordings were made from a mouse neuromuscular junction. The n

Question

2. Intracellular recordings were made from a mouse neuromuscular junction. The nerve supply was stimulated 150 times at 1Hz. The mean size of the EPP evoked was 1.00 mV. Five of the stimuli evoked no response (i.e. there were 5 'failures'). a) What was the mean quantal content at this neuromuscular junction? b) What do you predict for the quantal size, the amplitude of the uniquantal event (MEPP)? c) How many of the EPPs would you predict to have quantal contents of 1, 2, 3, and 4 quanta? d) What do you predict would be the standard deviation of the EPP amplitudes?

Explanation / Answer

a.To calculate mean quantal content use method of failures:

m=ln(Trials/Failures)

m=ln(150/5)

ln(30)

=3.40

b.The relation between m and MEPP is:

m=EPP/MEPP (EPP-END PLATE POTENTIALS,MEPP-MINIATURE END PLATE POTENTIAL)

MEPP=EPP/m

MEPP=1/3.4

MEPP=0.29mV

c.P(x)=m.exp(-m)/x!

Substituting for x

P(1)=0.1134

P(2)=0.1928

P(3)=0.2186

P(4)=0.1859

In 150 trials would therefore expect:

1:150*0.1134=17

2:=150*0.1928=29

3=150*0.2186=33

4=150*0.1859=28

d.The mean amplitude of the MEPPs is such that most wouldbeburied in the baseline noise in the recording,biasing the measured mean to higher values.This wouldin turn lead to an underestimate of quantal content by the direct Method.Many uniquantal evoked responses might also be buried in the noise,leading to an overestimate of the number of failures and therefore an underestimate of quantal content.This increased noise will also affect the accuracy of measurement of the evoked responses and therefore the variance of EPP amplitudes but it is hard to predict whether this will underestimate or overestimate quantal content.

  

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