Non-stationary Noise Analysis > Introduction
The non-stationary noise module analyses the random fluctuations in the decay of ion channel currents, providing an estimate of single-channel current and total number of channels in the fluctuating population. For a cell containing a population of n ion channels, each capable of passing a current i, the mean whole cell current, Im(t) is,
1
where p(t) is the probability of a channel being open at time t. The variance, 2(t), of the current fluctuations, at time t, about this mean is,
2
These two equations can be combined to provide a relationship between 2(t) and Im(t),
3
The single-channel current, i, and number of channels, n, can thus be calculated by fitting the above parabolic function to a plot of 2(t) vs Im(t) during a current transient where p(t) is changing.
Im(t) can be computed as the average current of a series of transient current records, repeated M times all evoked by the same stimulus,
4
The variance, 2(t), at each sample point, t, can similarly be computed from
5
The method was developed by Sigworth (1981) for voltage-activated Na currents. It has also been used to study the fluctuations during the rapidly desensitising currents induced by high concentrations of acetylcholine (Dilger & Brett, 1990). With modification it can also been applied to synaptic currents. The basic non-stationary variance approach assumes that the only source of variance arises from the fluctuations of the ion channels that carry the current. However, synaptic current amplitude can fluctuate due to both ion channels and quantal size/content variation. Traynelis et al (1993) found a way round this problem by scaling the amplitude of the average current to the peak amplitude of each signal before the subtraction in Eqn. 5, thus compensating for the quantal variation. This approach does have limitations and it is worth reading De Koninck & Mody (1994) if considering using the scaling approach.