Ontogenetic studies of the primary evoked response and strychnine spike in motor-sensory cortex of kittens

The development of the primary evoked potential in the motor-sensory cerebral cortex and the corticofugal reflex discharge in the medullary pyramids were studied in the first 2 weeks of postnatal life in kittens. In α-chloralose-anesthetized, paralyzed kittens at 1 day of age, the primary evoked potential produced by electrical stimulation of the contralateral forepaw was usually a purely negative response of 50-ms latency, poor frequency-following capability, and slow time course. During the first 2 weeks, the response became positive-negative as the latency shortened to 24 ms. During the same period, the frequency-following capability improved and the duration of the potential decreased. The cortical responses to stimulation of the other three paws were smaller, longer-latency negative potentials, but reliably evoked even in the youngest animals. The corticofugal reflex was not seen in any animal less than 14 days of age and was seldom seen in 14-day-old kittens. Topical application of strychnine enhanced the amplitude of the cortical response to stimulation of the contralateral forepaw, but had no effect on responses to stimulation of the other three paws. No effect on response latency was seen. The strychnine effect developed in about 15 s as in adults, but lasted two to four times longer. Both positive and negative components of the primary response were enhanced by strychnine, and, in animals that had no positivity in control responses, one was induced, remaining in the record for at least 11 h. As far as can be determined from these studies, the effect of topical strychnine is the same in neonatal and adult cat cerebral cortex.