The Journal of Neurophysiology Vol. 77 No. 1 January 1997, pp. 200-206
Copyright ©1997 The American Physiological Society

Locomotor Rhythm Evoked by Ventrolateral Funiculus Stimulation in the Neonatal Rat Spinal Cord In Vitro

David S. K. Magnuson and Tammy C. Trinder

Department of Physiology, University of Manitoba, Winnipeg, R3E 0W3 Manitoba, Canada

Magnuson, David S. K. and Tammy C. Trinder. Locomotor rhythm evoked by ventrolateral funiculus stimulation in the neonatal rat spinal cord in vitro. J. Neurophysiol. 77: 200-206, 1997. Spinal cords from 2- to 8-day-old rats, maintained in vitro, were used to investigate the effects of discrete electrical stimuli applied to the ventrolateral funiculus (VLF) on motor neuron activity recorded from the lumbar ventral roots. Short trains of stimuli (1-3 s) delivered to one VLF in the low cervical region elicited rhythmic activity that persisted for up to 30 s. Responses consisted of short periods of activity (1-5 s) occurring simultaneously in the ipsilateral L₅ and contralateral L₃ ventral roots that alternated with similar bursts of activity in the ipsilateral L₃ ventral root, a pattern consistent with locomotion. The rhythmicity of the ventral root responses to VLF stimulation was not affected by midsagittal sectioning of the preparation rostral to T₁₀ and/or caudal to L₄. Midsagittal sectioning of the lower thoracic or upper lumbar segments, however, disrupted the rhythmicity of the ventral root responses, leaving only long-duration simultaneous activation of the ipsilateral roots following VLF stimulus trains. The minimum lesion that effectively abolished the rhythmicity was one that divided only the L₂ and L₃ segments. In preparations rendered arrhythmic to VLF stimulation by an L₂/L₃ midsagittal lesion, rhythmicity could still be induced by N-methyl-D-aspartate (NMDA; 2-5 µM) and serotonin (5-HT; 20-50 µM), a drug combination commonly used to induce locomotor-like rhythmicity and air-stepping in vitro. Field potentials recorded following single stimuli delivered to the VLF revealed short-latency, large-amplitude responses in the ventral horn and intermediate gray both ipsilateral and contralateral to the stimulus site at T₁₂ and L₂. These observations suggest that 1) the discrete pathway under study may be an important descending locomotor command pathway and 2) this pathway has a strong bilateral projection in the lower thoracic and upper lumbar segments that is crucial for the initiation of VLF-induced rhythmic motor output. The induction of rhythmicity by NMDA/5-HT in an L₂/L₃-lesioned preparation suggests that these two rhythmogenic mechanisms may represent different levels within the circuitry that comprises the central pattern generator for locomotion. The rhythmic activity resulting from VLF stimulation is dependent on a bilateral projection that can be bypassed by the generalized excitation and subsequent rhythmicity that results from bath application of the NMDA/5-HT combination.

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