[Transformation of frequency-modulated afferent activity by spinal motor neurons in the cat]

Transformation of the frequency-modulated afferent activity into postsynaptic shifts of the membrane potential in the spinal motoneurons and then into firing rates of evoked activity was studied in anesthetized cats. The phase of membrane potential oscillations evoked by activation of excitatory monosynaptic inputs practically coincided with the phase of a modulation signal in a wide frequency range. Phase lags of the oscillation were observed only at high frequencies, but even at 10.0 Hz the lag did not exceed 30-45 degrees. At the same time changes in the firing rates of the evoked activity always led the modulation signal. Transformation properties of the spinal motoneurons are discussed from the standpoint of nonlinear dynamic sensitivity of the central neurons.