The dynamic response of cat alpha-motoneurones investigated by intracellular injection of sinusoidal currents.

Sine-wave currents intracellularly injected into spinal alpha-motoneurones were found to modulate sinusoidally the regular rhythmic firing (carrier frequency) evoked by a current step. Cycle histograms of the instantaneous frequency could be accurately fitted by sinusoidal functions. Those functions were treated as the cell output. For a given modulation frequency between 2 and 14-18 Hz, the amplitude of the cell output was linearly related to the amplitude of the sine-wave current, all over a wide range of current intensities. The sensitivity (gain) and the phase relationships were estimated by varying the modulation frequency of a given sine-wave. When modulation frequency varied from 1-2 Hz to 14-18 Hz, there was a progressive increase of the gain and a phase advance. The experimental gain curve closely conformed to the response of an ideal linear transducer sensitive to both the intensity and the velocity of the input. The phase advance was instead less than that predicted by the model. No "carrier dependent" variations of gain and phase were detected. Differences among motoneurones regarded both the static gain and the "corner frequency" (a measure of the dynamic sensitivity). In 10 motoneurones, the corner frequency ranged between 5 and 10 Hz.