Propriospinal Circuitry Underlying Interlimb Coordination in Mammalian Quadrupedal Locomotion
Journal of Neuroscience. 2005-06-22; 25(25): 6025-6035
DOI: 10.1523/jneurosci.0696-05.2005
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Juvin L(1), Simmers J, Morin D.
Author information:
(1)Laboratoire de Physiologie et Physiopathologie de la Signalisation Cellulaire,
Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5543,
Equipe Neurophysiologie Adaptative des Systèmes Moteurs, 33076 Bordeaux, France.
Soon after birth, freely moving quadrupeds can express locomotor activity with
coordinated forelimb and hindlimb movements. To investigate the neural mechanisms
underlying this coordination, we used an isolated spinal cord preparation from
neonatal rats. Under bath-applied 5-HT, N-methyl-d,l-aspartate (NMA), and
dopamine (DA), the isolated cord generates fictive locomotion in which
homolateral cervicolumbar extensor motor bursts occur in phase opposition, as
does bursting in homologous (left-right) extensor motoneurons. This coordination
corresponded to a walking gait monitored with EMG recordings in the freely
behaving animal. Functional decoupling of the cervical and lumbar generators in
vitro by sucrose blockade at the thoracic cord level revealed independent
rhythmogenic capabilities with similar cycle frequencies in the two locomotor
regions. When the cord was partitioned at different thoracic levels and
5-HT/NMA/DA was applied to the more caudal compartment, the ability of the lumbar
generators to drive their cervical counterparts increased with the proportion of
chemically exposed thoracic segments. Blockade of synaptic inhibition at the
lumbar level caused synchronous bilateral lumbar rhythmicity that, surprisingly,
also was able to impose bilaterally synchronous bursting at the unblocked
cervical level. Furthermore, after a midsagittal section from spinal segments C1
to T7, and during additional blockade of cervical synaptic inhibition, the cord
exposed to 5-HT/NMA/DA continued to produce a coordinated fictive walking pattern
similar to that observed in control. Thus, in the newborn rat, a caudorostral
propriospinal excitability gradient appears to mediate interlimb coordination,
which relies more on asymmetric axial connectivity (both excitatory and
inhibitory) between the lumbar and cervical generators than on differences in
their inherent rhythmogenic capacities.