Regional localization of patterned spontaneous discharges during maturation in culture of fetal mouse medulla and spinal cord explants.

Development and regional localization of patterned spontaneous bioelectric activities, recorded extracellularly, were studied in cultures of frontal sections of spinal cord and medulla, explanted from 13- to 15-day-old fetal mice. Spontaneous single- and multi-unit tonic and phasic spike discharges were recorded after 4-21 days in vitro. Stereotyped slow waves with positive or negative polarity, and durations up to several seconds, often occurred concomitant with spike burst discharges. Microelectrode mapping demonstrated regional localization of spontaneous activity patterns in these cross-sectional central nervous system explants, which were oriented so that dorsal and ventral borders could be regularly identified. Phasic discharges were widely distributed throughout the explants, whereas tonic discharges were located primarily in the dorsal region of the medulla; negative slow waves were generated in central and ventral regions of spinal cord and medulla explants, whereas positivities were located in the peripheral regions of the explants. Phasic discharges were often synchronous when recorded at two symmetrical regions of an oriented explant, as early as 4 days in vitro. The major developmental change in spontaneous activities of these explants was a progressive shortening of both burst and cycle durations of the phasic discharges.