Locomotor rhythmogenesis in the isolated rat spinal cord: a phase-coupled set of symmetrical flexion-extension oscillators
The Journal of Physiology. 2007-08-01; 583(1): 115-128
DOI: 10.1113/jphysiol.2007.133413
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Juvin L(1), Simmers J, Morin D.
Author information:
(1)Université Bordeaux 1, UMR CNRS 5227, Laboratoire
Mouvement-Adaptation-Cognition bâtiment 2A, 146 rue Léo Saignat, 33076 Bordeaux,
France.
The temporal properties of limb motoneuron bursting underlying quadrupedal
locomotion were investigated in isolated spinal cord preparations (without or
with brainstem attached) taken from 0 to 4-day-old rats. When activated either
with differing combinations of N-methyl-D,L-aspartate, serotonin and dopamine, or
by electrical stimulation of the brainstem, the spinal cord generated episodes of
fictive locomotion with a constant phase relationship between cervical and lumbar
ventral root bursts. Alternation occurred between ipsi- and contra-lateral flexor
and extensor motor root bursts, and the cervical and lumbar locomotor networks
were always active in a diagonal coordination pattern that corresponded to
fictive walking. However, unlike typical locomotion in adult animals in which
extensor motoneuron bursts vary more with cycle period than flexor bursts, in the
isolated neonatal cord, an increase in fictive locomotor speed was associated
with a decrease in the durations of both extensor and flexor bursts, at cervical
and lumbar levels. To determine whether this symmetry in flexor/extensor phase
durations derived from the absence of sensory feedback that is normally provided
from the limbs during intact animal locomotion, EMG recordings were made from
hindlimb-attached spinal cords during drug-induced locomotor-like movements.
Under these conditions, the duration of extensor muscle bursts increased with
cycle period, while flexor burst durations now tended to remain constant.
Moreover, after a complete dorsal rhizotomy, this extensor dominant pattern was
replaced by flexor and extensor muscle bursts of similar duration. In vivo and in
vitro experiments were also conducted on older postnatal (P10-12) rats at an age
when body-supported adult-like locomotion occurs. Here again, characteristic
extensor-dominated burst patterns observed during intact treadmill locomotion
were replaced by symmetrical patterns during fictive locomotion expressed by the
chemically activated isolated spinal cord, further indicating that sensory inputs
are normally responsible for imposing extensor biasing on otherwise symmetrically
alternating extensor/flexor oscillators.