The motor output and behavior produced by rhythmogenic sacrocaudal networks in spinal cords of neonatal rats

The characteristics of the rhythmic motor output and behavior produced by intrinsic sacrocaudal networks were studied in isolated tail-spinal cord preparations of neonatal rats. An alternating left-right rhythm could be induced in the sacral cord by stimulus trains applied to sacrocaudal afferents at various intensities. Strengthening the stimulation intensity enhanced the rhythmic efferent firing and accelerated the rhythm by < or =30%. High stimulation intensities induced tonic excitation or inhibition and thereby perturbed the rhythm. Increasing the stimulation frequency from 1 to 10 Hz decreased the cycle time of the rhythm by 36%. The rhythm was blocked during prolonged afferent stimulation but could be restored by stimulation of contralateral afferents. Sacrocaudal afferent activation produced ventroflexion accompanied by either low- or high-amplitude rhythmic abduction of the tail. The low-amplitude abductions were produced by alternating flexor bursts during long stimulus trains. The activity of abductors and extensors was substantially reduced during these trains, their recruitment lagged after that of the flexors, and their activity bursts were much shorter. It is suggested that tail extensor/abductor motoneurons were suppressed during the stimulus train by inhibitory afferent projections. The high-amplitude abductions appeared after cessation of stimulus trains. Alternating left-right activation of the tail muscles, and coactivation of the principal muscles on each side of the tail were observed during these abductions. It is suggested that flexors and extensors assist the abductors to produce the high-amplitude abductions. This suggestion is supported by the finding that tail abduction could be produced by direct unilateral stimulation of any of the principal tail muscles. The relevance of the findings described in the preceding text to the use of regional sacral circuits in generation of stereotypic motor behaviors and to future studies of rhythmogenic sacrocaudal networks is discussed.