Differential maturation of motoneurons innervating ankle flexor and extensor muscles in the neonatal rat

The first postnatal week is a critical period for the development of postur e in the rat. The use of ankle extensor muscles in postural reactions incre ases during this period. Changes in excitability of motoneurons are probabl y an important factor underlying this maturation. The aim of this study was to identify whether variations in the maturation exist between motor pools innervating antagonistic muscles. Intracellular recordings in the in vitro brain stem-spinal cord preparation of neonatal rats (from postnatal day 0- 5) were used to examine the developmental changes in excitability of motone urons innervating the ankle flexors (F-MNs) and:the antigravity ankle exten sors (E-MNs). No significant difference in resting potential, action potent ial threshold, input resistance or rheobase was observed at birth. The age- related increase in rheobase was more pronounced for F-MNs than for E-MNs. The development of discharge properties of E-MNs lagged behind that of F-MN s. More F-MNs than E-MNs were able to,fire repetitively in response to curr ent injection at birth. F-MNs discharged at a higher frequency than E-MNs a t all ages. Differences in the duration of action potential afterhyperpolar ization accounted, at least partly, for the differences in discharge freque ncy between E-MNs and F-MNs at birth, and for the age-related increase in f iring rate. These results suggest that E-MNs are more immature at birth tha n F-MNs and that there is a differential development of motoneurons innerva ting antagonistic muscles. This may be a critical factor in the development of posture and locomotion.