Regionalization and intersegmental coordination of rhythm-generating networks in the spinal cord of the chick embryo

Abstract

We have examined the regionalization and coordination of rhythm-generating networks in the isolated spinal cord of the chick embryo between embryonic days 9 and 13, by recording the pattern of rhythmic activity recorded from muscle nerves and ventral roots following a variety of lesions. We found that the capacity for rhythmic activity is distributed along the rostrocaudal axis of the cord but can be expressed in a single, isolated segment. Specializations within the lumbosacral cord were investigated by isolating particular regions and recording their motor output. The rostral part of the lumbosacral cord generates more cycles than the caudal part, and this difference becomes more pronounced with development. In the unlesioned cord, motoneuron activity is synchronized along the rostrocaudal axis. Lesion experiments revealed that the synchronization of motoneuron activity and the synaptic drive to caudal motoneurons is mediated in part by propriospinal pathways traveling in the ventrolateral white matter tracts and by synaptic interactions within the gray matter. The dorsal fiber tracts may also be involved but their effects appear to be weak. Lesions in dorsal-ventral and mediolateral planes were used to localize regions critical for rhythmogenesis and for the alternation of flexor and extensor motoneurons. Rhythmic activity with alternation persisted in spinal cords in which the dorsal and medial half had been removed. Severe medial or dorsal lesions, resulting in a thin strip of lateral or ventral gray matter, altered the phasing of motoneuron activity from alternating to synchronous without effects on cycle timing. These results suggest that the critical neural components for alternation are located close to and dorsomedial to the lateral motor column, and that the capacity for rhythmogenesis is distributed widely throughout the ventral gray matter and is not localized to specific nuclei.

Contribution of NMDA and non–NMDA glutamate receptors to locomotor pattern generation in the neonatal rat spinal cord

“…example in figure 1 a). The slower changes in root polarization level mainly corresponded to slowly summating depolarization within the motoneurone population (Ho & O'Donovan 1993 ;Bracci et al 1996 b).…”

Modeling Spontaneous Activity in the Developing Spinal Cord Using Activity-Dependent Variations of Intracellular Chloride

“…The firing rate-like framework of our model provides a plausible approximation to the actual network behavior because spontaneous activity generated by the spinal cord does not depend on the details of network architecture (Ho and O'Donovan, 1993;Chub and O'Donovan, 1998). Furthermore, the model does not require details of the connectivity or biophysical properties of individual spinal neurons (Tabak et al, 2000(Tabak et al, , 2001.…”

Selective Fasciculation and Divergent Pathfinding Decisions of Embryonic Chick Motor Axons Projecting to Fast and Slow Muscle Regions

“…As described previously (Landmesser and O'Donovan, 1984a;O'Donovan, 1989;Ho and O'Donovan, 1993;Sernagor et al, 1995;Sholomenko and O'Donovan, 1995), stimulation of descending input with a single electrical shock to the thoracic cord activates a central pattern generator in the lumbosacral cord, setting off a series of highly stereotyped motoneuron bursts (or cycles) characteristic for each muscle. Figure 9A shows the EMG to one such muscle, the AITIB, after a single stimulus to the cord.…”

“…The different activity patterns recorded from the fast and slow regions of the AITIB and IFIB indicate that these fast and slow motoneurons are differentially activated by the central pattern generator (CPG) (O'Donovan, 1989), presumably via differential innervation by local interneurons (O'Donovan, 1989;Ho and O'Donovan, 1993;Sernagor et al, 1995). Because fast-and slow-projecting motoneurons were mixed within the motor pool, selective connectivity suggests the recognition of specific molecular cues on the motoneuron cell bodies/dendrites by local interneurons.…”