Early Postnatal Changes in the Somatodendritic Morphology of Ankle Flexor Motoneurons in the Rat

Abstract

The development of locomotor function in the rat spans the first 3 postnatal weeks. We have studied morphological features of the soma and dendrites of motoneurons innervating the physiological flexor muscles of the ankle, tibialis anterior and extensor digitorum longus, by intracellular injection in vitro between the first and ninth postnatal days. We obtained serial optical sections of 96 adequately filled motoneurons in whole-mounted hemisected spinal cords by confocal microscopy, projected them onto a single plane and analysed them morphometrically. On the day after birth, the somatodendritic surfaces of most such motoneurons were covered in growth-associated spiny, thorny or hair-like appendages. 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 growth 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 their combined diameter was already significantly correlated with mean soma diameter, as in the adult cat. Thus, the critical neonatal period during which these ankle flexor motoneurons are known to change their electrophysiological properties and to be particularly sensitive to interference with neuromuscular interaction is characterized by major changes in the neuronal surface, presumably linked to synaptogenesis.

Changes in the Expression of Parvalbumin Immunoreactivity in the Lumbar Spinal Cord of the Rat following Neonatal Nerve Injury

“…The location of the TA and EDL motor pools was identified by the presence of fluorescently prelabelled motoneurons viewed under epifluorescence [Dekkers et al, 1994]. Motoneurons were found between L3 and L5 (lumbar roots) in the lumbar spinal cord and in the dorsolateral part of the ventral horn.…”

“…Prelabelled motoneurons were located at the dorsolateral limit of the ventral horn in lamina IX, which is known to be the region of the motor pool for TA/EDL muscles [Dekkers et al, 1994]. This ensured that only an identified population of motoneurons was examined in these experiments.…”

“…All surgical procedures were carried out under sterile conditions and experiments were carried out in accordance with the UK Animals and Scientific Procedures Act 1986, and associated guidelines. On the day of birth (P0), motoneurons within the TA/EDL motor pool were prelabelled by injecting the TA and EDL muscles in both hindlimbs with 0.5 Ìl 3% fast blue and 2% diamidino yellow (Illing, Germany), under halothane anaesthesia [Dekkers et al, 1994]. In some cases, 10% fluorescinated dextran amine (0.5 Ìl; molecular probes) was injected as a retrograde tracer.…”

A new stochastic tridimensional model of neonatal rat spinal motoneuron for investigating compartmentalization of neuronal conductances and their influence on firing

“…Although motoneurons innervating functionally different muscles differ in size and complexity (Lüscher and Clamann, 1992;Westbury, 1982), most studies on the neonatal rats have considered motoneurons as a homogeneous population, or have focused on motoneurons specifically innervating one group of the muscles (Dekkers et al, 1994;Westerga and Gramsbergen, 1992). While motoneurons later differ in their bimodal distribution of somatic size (at P20), and for the elongation of dendritic trees (after P16), earlier changes during the first two postnatal weeks remain elusive.…”

“…The first one was the work by Thurbon et al (1998) who have examined the passive electrical properties of ventral horn neurons in spinal cord slices from young rats (P7-15) and have provided related morphological data. The second source was the investigation by Dekkers et al (1994) who used hemisected spinal cord preparations (from which I used numerical data related to the P7-9 age group).…”