Individual Neurons - Computational Explorations in Cognitive Neuroscience - page 50

the exact value of excitatory input required to just reach
threshold, showing your math (note that: g l is always
1 because the leak channels are always open; g e is 1
when the input is on; inhibition is not present here and
can be ignored). Does this agree with your empirically
determined value? (Hint: It should!)

of g_bar_e that gives essentially the same activation
value as the default parameters. (c) Then use the same
approach as in question 2.2 to solve for the exact value
of g_bar_e that will compensate for this change in
e_rev_e (use .256 for the membrane potential under
the default parameters, and show your math).

You can also play around with the value of the leak
conductance, g_bar_l , which controls the size of the
leak current.

At this point, you should have a good idea about how
the conductance and reversal potential parameters influ-
ence the resulting membrane potential. Just to demon-
strate that it does not make a real difference in the be-
havior of the unit (and to satisfy the neuroscientists out
there), we can switch from using the normalized 0 ￿ 1
reversal potential values to the biologically based ones.

Press the Defaults button on the control panel to
restore the default parameters, and then see what hap-
pens when you increase or decrease the leak.

Question 2.3 (a) How does the response of the unit
change when you change g_bar_l ?Why? (b) How
does this differ from changes to g_bar_e ? (c) Use
the same technique you used in the previous question to
compute the exact amount of leak current necessary to
put the membrane potential exactly at threshold when
the g_bar_e value is at the default of .4 (show your
math).

Click the UseBioVm button to switch to using the
biological values, and then click Run .
The new GraphLog display should appear similar to
the previous one, except that now the membrane poten-
tial is plotted on a scale from ￿90 to +55 , instead of

,andthe I_net is also on a larger scale. We
changed the scaling (gain) of the activation function so
that it is identical to that used before.

Click the UseStdVm button to switch to back to using
the normalized values.
If you are adventurous, you can look into the “guts”
of the simulation to see exactly what we did to switch
from normalized to biological parameters. The way a
unit behaves in the simulator is controlled by something
called the UnitSpec ( unit specifications ).

Press Defaults to restore the default parameters.
Now that we have a sense of how the unit responds
to different currents, and computes a resulting mem-
brane potential that reflects the balance of these cur-
rents, we can explore the role of the reversal potentials
( e_rev_e and e_rev_l ).

Select View on the control panel, and then
pick UNIT_SPECS . Two complex edit dialog will ap-
pear, one named UnitSpec_0 and the other named
BioUnitSpec_0 . Arrange the two edit dialogs side-by-
side.
Going down the parameters from top to bottom, you
can see which have different values, and hopefully make
sense of the differences. You can also click on the pa-
rameter label for a helpful comment or explanation of
the parameter. The values you manipulated in the con-
trol panel in the previous exercises are just a small sub-
set of those in the full UnitSpec — see if you can locate
them there.

Question 2.4 (a) What happens to the unit's activity if
you change the leak reversal potential e_rev_l from
.15 to 0? (b) What about when you increase it to .2?
For both questions, explain the results, taking note of
what happens before the input goes on as well as what
happens while it is on. (c) What can you conclude
about the relationship between the resting potential and
the leak reversal potential?

Press Defaults to restore the default parameters.

Question 2.5 (a) What happens to the unit's activity if
you change the excitatory reversal potential e_rev_e
from 1 to .5? Why does this happen? (b) Can you com-
pensate for this by changing the value of g_bar_e ?To
two decimal places, use the simulator to find the value

Press Cancel on the unit specs before continu-
ing, which will close the edit dialog and not apply any
changes you might have accidentally made.