Fast Kinetics, High-Frequency Oscillations, and Subprimary Firing Range in Adult Mouse Spinal Motoneurons

International audienceThe fast contraction time of mouse motor units creates a unique situation where motoneurons have to fire at low frequencies to produce small forces but also at very high frequency (much higher than in cat or rat motoneurons) to reach the fusion frequency of their motor units. To understand how this problem is solved, we performed intracellular recordings of adult mouse spinal motoneurons and investigated systematically their sub-threshold properties and their discharge pattern. We show that mouse motoneurons have a much wider range of firing frequencies than cat and rat motoneurons because of three salient features. First, they have a short membrane time constant. This results in a higher cut off frequency and a higher resonance frequency, which allow mouse motoneurons to integrate inputs at higher frequencies. Second, their AHP is faster allowing the motoneurons to discharge at a higher rate. Third, motoneurons display high frequency (100–150 Hz) sub-threshold oscillations during the interspike intervals. The fast membrane kinetics greatly favors the appearance of these oscillations, creating a "sub-primary range" of firing. In this range, which has never been reported in cat and in rat spinal motoneurons, the oscillations follow the AHP and trigger spiking after a variable delay, allowing a discharge at low frequency but at the expense of an irregular rate