Forelimb locomotor generators and quadrupedal

locomotion in the neonatal rat

Be¬range¡re Ballion, Didier Morin and Denise Viala European Journal of Neuroscience, Vol. 14, pp. 1727±1738, 2001

The spinal localization of the forelimb locomotor generators and their interactions with other spinal segments were investigated

on in vitro brainstem±spinal cord preparations of new-born rats. Superfusion of the cervicothoracic cord (C1±T4) with high K+/low

Mg2+ artiÆcial cerebrospinal Øuid (aCSF) evoked rhythmic motor root activity that was limited to low cervical (C7, C8) and high

thoracic (T1) spinal levels. This activity consisted of synchronous, homolateral bursts and a typical alternating bilateral pattern.

Rhythmic activity with similar locomotor-like characteristics could be induced with either serotonin (5-HT, 5 mM), N-methyl-Daspartate

(NMDA, 5 mM), kainate (10 mM) or a `cocktail' of 5-HT (5 mM) and NMDA (5 mM). During 5-HT/NMDA perfusion of the

cervicothoracic cord, induced bursting was no longer restricted to C7±T1 levels, but also occurred at cervical C3±C5 levels and

with C5±C8 homolateral alternation. Spinal transections between C6 and C7 cervical segments did not abolish rhythmic activity in

C7±T1, but suppressed locomotor-like rhythmicity at C3±C5 levels. Reduced regions comprising the C7±C8 or C8±T1 segments

maintained rhythmicity. Superfusion of the whole cord with 5-HT/NMDA induced ventral root bursting with similar frequencies at

all recorded segments (cervical, thoracic and lumbar). After isolation, the T3±T10 cord was unable to sustain any rhythmic activity

while cervical and lumbar segmental levels continued to burst, albeit at different frequencies. We also found that the faster caudal

and the slower rostral locomotor generators interact to produce coordinated locomotor-like activity in all segments of the intact

spinal cord. In conclusion, C7±T1 spinal levels display a strong motor rhythmogenic ability; with the lumbar generators, they

contribute to coordinated rhythmic activity along the entire spinal cord of a quadrupedal locomoting mammal.


FIG. 1. Experimental set-up for ventral root recordings from brainstem±

spinal cord preparations of new-born rats. The bath was partitioned into

three parts (`1', `2', and `3') with Plexiglas bridges, allowing the perfusion

of different aCSF solutions into each compartment.

FIG. 2. Rhythmic motor activity recorded from various cervical and thoracic segments in different brainstem±spinal cord preparations superfused with a high

K+/low Mg2+ aCSF. (A±E) Left (l) and right (r) integrated (Ú) and raw ventral root recordings from cervical (C6±C8) and thoracic (T1±T2) spinal levels. Stars

indicate Æctive respiratory bursts. (F) Homolateral, locomotor-like activity recorded from right C7, C8 and T1 ventral roots. The strength of coupling between root activities is shown by circular representations (C7 vs. C8 and C8 vs. T1) of 60 random cycles taken in episodes of rhythmicity from four preparations.


FIG. 8. Demonstration of reciprocal actions between the cervicothoracic and lumbosacral locomotor generators. (A and B) Simultaneous recordings at

indicated cervical and lumbar levels of preparations shown in corresponding schematic. Effect of bath application of 5-HT (10 mM)/NMDA (10 mM) either at

thoracolumbar level (grey area) in A or cervicothoracic level (grey area) in B. Phase values (n = 75 from Æve preparations in A; n = 45 from three

preparations in B) were plotted on circular representations.