Abstract
Experiments were carried out on unanesthetized fetal sheep, 30–150 days gestational age; d-c recording showed a positive resting steady potential between the pial surface of the cortex and a reference electrode at the earliest age studied (30 days). At this time no other electrophysiological cortical phenomena could be observed. There was a slight increase of the steady potential values during the gestation period. Locally applied KCl caused a change of the steady potential from positive to negative values. The magnitude of this KCl shift was of the same order as in the adult. The steady potential could be driven into a longlasting negativity by repetitive cortical stimulation. This shift in the steady potential base line could be elicited at an age when direct cortical responses were still absent. At an age when the response could be obtained repetitive stimulation in a few experiments induced no shift or even a positive one. The first detectable direct cortical response was obtained in the middle of the gestation period and consisted of an early negative deflection (the primary potential, duration 10 msec) followed by a second negative deflection (the slow negativity, summit latency 50 msec and duration 100 msec). The response was essentially the same as that found in adult sheep and there was no obvious change of the configuration with fetal age. The stimulus threshold decreased during the gestation period to about 10% of the earliest values. Repetitive stimulation blocked the primary potential and facilitated the slow negativity in most cases. On the basis of the morphological cortical development, it is proposed that the steady potential does not depend on axially-polarized structures but is related to basic biophysical and biochemical processes and is essentially independent of later developing electrophysiological correlates to neuronal activity. The two components of the direct cortical response appear to be generated by different structures or mechanisms with different rate of maturation.
Published Version
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