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.

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+ artificial cerebrospinal fluid (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 microm), N-methyl-d-aspartate (NMDA, 5 microm), kainate (10 microm) or a "cocktail" of 5-HT (5 microm) and NMDA (5 microm). 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.

A 1:1 coordination between locomotor and respiratory movements has been described in various mammalian species during fast locomotion, and several mechanisms underlying such interactions have been proposed. Here we use an isolated brainstem-spinal cord preparation of the neonatal rat to determine the origin of this coupling, which could derive either from a direct interaction between the central locomotor- and respiratory-generating networks themselves or from an indirect influence via a peripheral mechanism. We demonstrate that during fictive locomotion induced by pharmacological activation of the lumbar locomotor generators, a concomitant increase in spontaneous respiratory rate occurs without any evident form of phase coupling. In contrast, respiratory motor activity can be fully entrained (1:1 coupling) over a range of periodic electrical stimulation applied to low-threshold sensory pathways originating from hindlimb muscles. Our results provide strong support for the existence of pathways between lumbar proprioceptive afferents, medullary respiratory networks, and phrenic motoneurons that could provide the basis of the locomotor-respiratory coupling in many animals. Thus a peripheral sensory system involved in a well defined rhythmic motor function can be responsible for the tight functional interaction between two otherwise independent motor behaviors.