Simulation of constant current injection into the B1 motoneuron in normal saline and
with octopamine. A. A current pulse of 1.6 nA with
Na at 7 μS, the normal values, depolarises the B1 motoneuron from its resting value
of -52 mV and generates four action potentials during the 1 s of simulation. B. Comparison
of the action potential generated by simulation with one recorded in a constant current
experiment in normal saline (initial resting potential -48 mV, slightly depolarised
compared to the mean B1 resting potential, -55 mV). C. The same current pulse as A,
but with
Na increased by 33% to simulate the effect of 10 μM octopamine and generates 7 action
potentials. D. A plot of the firing rate of the simulated B1 motoneuron with different
parameters reflecting octopamine action. The squares and dotted line represent the
firing rate of the normal model. The triangles show the effect of increasing the maximum
sodium conductance (
Na) by 33% simulating 10 μM octopamine, reducing the threshold and increasing the
firing rate. The crosses show the effect of modelling the increased maximum sodium
conductance plus a 0.5 nA octopamine current to simulate the depolarising effect of
a higher concentration of octopamine, 100–200 μM, [39]. A further increase in excitability
is seen. Decreasing the maximum sodium conductance reduces the number of action potentials
(diamonds). Compare these figures with the data shown in Fig. 1A.
