POSTNATAL CHANGES IN THE DORSAL-ROOT REFLEX IN THE ISOLATED SPINAL-CORD OF THE HAMSTER, MESOCRICETUS-AURATUS

Spontaneous activity has been demonstrated in the lumbar dorsal roots of isolated spinal cord preparations taken from animals ranging in age from 2 to 65 days. Peaks of activity were recorded at 2 and 5 weeks o f age, with mean firing frequencies of 33 Hz and 28 Hz respectively. T he firing frequency in weeks 3 and 4 was lower (15 Hz) as was the freq uency in cords taken from animals older than 6 weeks. The pattern of t he spontaneous dorsal root activity changed during the first 5 weeks o f life. In cords taken from animals less than 10 days old, the roots f ired single action potentials, producing a single broad peak in Inter Spike Interval plots (ISI). Dorsal root recordings made from cords tak en from animals in weeks 2 and 3 of life exhibited both single spikes and bursts of action potentials. By the end of the third week of life, individual spike activity had declined and the bursts of action poten tials characteristic of the adult pattern had become dominant, produci ng a bimodal ISI plot. Cross correlation analysis of dorsal root and d orsal horn activity in lumbar segments up to five segments apart, reve aled an increasing degree of correlation developing over the first 4 w eeks of postnatal life. Dorsal horn responses to dorsal root stimulati on in cords taken from young animals were prolonged, lasting in excess of 250 msec. In the third week of life, the duration of the excitator y component of the response was reduced to approximately 50 msec by th e development of an inhibitory phase.