EARLY POSTNATAL CHANGES IN THE SOMATODENDRITIC MORPHOLOGY OF ANKLE FLEXOR MOTONEURONS IN THE RAT

The development of locomotor function in the rat spans the first 3 pos tnatal weeks. We have studied morphological features of the soma and d endrites of motoneurons innervating the physiological flexor muscles o f the ankle, tibialis anterior and extensor digitorum longus, by intra cellular injection in vitro between the first and ninth postnatal days . We obtained serial optical sections of 96 adequately filled motoneur ons in whole-mounted hemisected spinal cords by confocal microscopy, p rojected them onto a single plane and analysed them morphometrically. On the day after birth, the somatodendritic surfaces of most such moto neurons were covered in growth-associated spiny, thorny or hair-like a ppendages. These had disappeared from the soma by the fourth postnatal day and from most proximal dendrites by day 7, but were still common distally on day 9. During this period there was little or no net growt h of either the soma (which was still much smaller than in the adult) or the dendritic tree. A dorsal dendritic bias was present and 'sprays ' of long, loosely bundled dorsal dendrites were often seen. The mean number of primary dendrites remained constant at about eight, and thei r combined diameter was already significantly correlated with mean som a diameter, as in the adult cat. Thus, the critical neonatal period du ring which these ankle flexor motoneurons are known to change their el ectrophysiological properties and to be particularly sensitive to inte rference with neuromuscular interaction is characterized by major chan ges in the neuronal surface, presumably linked to synaptogenesis.