From the statistical standpoint, a discharge-interval-time-series of single NMU consists of several kinds of fluctuations with different periods. The repetition of the arithmetical moving average method can divide the original series into those of two types. They are the slow undulation, the series of moving averages and the irregular fluctuation, the series of deviations from the moving averages. Applying WHITTAKER'S periodgram analysis to the series which remains after the elimination of the spiky fluctuation from the irregular fluctuation, the harmonic wave component (or the H-type fluctuation) is extracted. And, on the contrary, the spiky component (or the R-type fluctuation) remains after elimination the harmonic wave by means of the moving average method (chart 1). The same time-series discussed in the preceeding report were analysed again by the above-mentioned methods and the peculiarities of the subdivided elementary fluctuations were asserted. 1) The average amplitude A of the slow undulation widens in proportion to the mean value τ^^- of the original time-series in all of the animals observed. In the dog, however, the A has a settled value until the τ^^- exceeds 70∼80 ms and thus the τ^^--A curve has a horizontal part (Chart 2∼3). 2) The H-type fluctuation generally consists of two harmonic wave components of different periods P and therefore, the τ^^--P points distribute along two different regression curves. The one is called the C-curve and it is supposed to represent the period of the sway of activity in the motor area of the cerebral cortex. The other is called the S-curve which represents the periodic fluctuation in the subcortical nervous system. The period of each grows with the increase of the τ^^-, but has a certain maximal value (Chart 4∼7). It is assumed that the increase of the cortex activity (or corticalization) in the control of muscular contraction brings an increase of inclination of the C-curve and the reinforcement of the lower motor neuron system (or spinalization) results in the lowering of the maximal values of both curves. Every conditioning causes the spinalization in the rabbit and goat, the lifting of the maximal values in the C- and S-curve in the dog. 3) The period and decrement of the R-type fluctuation calculated by means of the correlogram analysis show the relationship between the adjoining motor impulses sent out from the spinal cord. In rabbits and goats no change is manifested in any of the various standing postures. But in the dog the seesaw movement gives rise not only to the prolongation of the period, but also to decrease regularity in the adjacent discharge intervals in the long muscles passing over several joints (Chart 9). Therefore, it is supposed that the rabbit and goat always maintain their erect position by the activation of the lower motor neuron system, but in the dog the cortical nervous system gives rise to phasic contraction of the essential muscles for the prevention of disturbance of the standing posture.

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