Changes in motoneuron excitability during postnatal life in the mouse

H-reflex recovery, H-amplitude and H/M ratio were recorded in 54 mice aged 3-12 weeks to study the motoneuron excitability and changes in it during various stages of development. The H-reflex recovery curve at 12 wk showed 3 phases: an early, relative facilitation (before 10 ms), an almost total inhibition at 10 ms and a rapid recovery thereafter. At 3 wk, however, there was only slow recovery after 10 ms and the H-reflex recovery was significantly low during the 40-100 ms period as compared to those in the other age groups, indicating that between 3 and 6 wk, there was a significant increase in the motoneuron excitability. The H-reflex amplitude also showed a significant increase during the 3-6 wk period. However, the H/M ratio did not show any significant increase either during the 3-6 wk period or thereafter. It is concluded that the H-reflex recovery at 3 wk suggests hypoexcitability of the motoneurons, possibly due to immaturity. Since there was a significant increase in the H-reflex recovery during the 3-6 wk period without any parallel increase in the H/M ratio, it is concluded that presynaptic and polysynaptic mechanisms acting on the motoneurons develop during this period. The increase in the H-reflex amplitude is possibly due to the increase in the muscle mass. The H-reflex recovery pattern at 12 wk, without the phase of late inhibition observed in man, is suggestive of less supra-spinal control mechanisms acting upon the motoneurons.