Origin of thoracic spinal network activity during locomotor-like activity in the neonatal rat.

Lauriane Beliez, Grégory Barrière, Sandrine S. Bertrand, Jean-René Cazalets
J. Neurosci.. 2015-04-15; 35(15): 6117-6130
DOI: 10.1523/jneurosci.4145-14.2015

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Effective quadrupedal locomotor behaviors require the coordination of many
muscles in the limbs, back, neck, and tail. Because of the spinal motoneuronal
somatotopic organization, motor coordination implies interactions among distant
spinal networks. Here, we investigated some of the interactions between the
lumbar locomotor networks that control limb movements and the thoracic networks
that control the axial muscles involved in trunk movement. For this purpose, we
used an in vitro isolated newborn rat spinal cord (from T2 to sacrococcygeal)
preparation. Using extracellular ventral root recordings, we showed that, while
the thoracic cord possesses an intrinsic rhythmogenic capacity, the lumbar
circuits, if they are rhythmically active, will entrain the rhythmicity of the
thoracic circuitry. However, if the lumbar circuits are rhythmically active,
these latter circuits will entrain the rhythmicity of the thoracic circuitry.
Blocking the synaptic transmission in some thoracic areas revealed that the
lumbar locomotor network could trigger locomotor bursting in distant thoracic
segments through short and long propriospinal pathways. Patch-clamp recordings
revealed that 72% of the thoracic motoneurons (locomotor-driven motoneurons)
expressed membrane potential oscillations and spiking activity coordinated with
the locomotor activity expressed by the lumbar cord. A biphasic excitatory
(glutamatergic)/inhibitory (glycinergic) synaptic drive was recorded in thoracic
locomotor-driven motoneurons. Finally, we found evidence that part of this
locomotor drive involved a monosynaptic component coming directly from the lumbar
locomotor network. We conclude that the lumbar locomotor network plays a central
role in the generation of locomotor outputs in the thoracic cord by acting at
both the premotoneuronal and motoneuronal levels.

Copyright © 2015 the authors 0270-6474/15/356117-14$15.00/0.


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