PROPERTIES OF SYNAPTIC ACTIVITIES AND SPIKE POTENTIALS OF NEURONS IN IMMATURE NEOCORTEX

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Abstract
Intracellular recordings were obtained from immature neurons in the sensorimotor cortex of kittens ranging in age from newborn to 1 month old. Stimulation of specific thalamocortical projections evoked long-latency (15-20 msec.) EPSP''s [excitatory post-synaptic potentials] in immature neurons which were usually succeeded by long-duration (200 - 600 msec.) IPSP''s [inhibitory post-synaptic potential]. Prolonged EPSP''s (80 - 100 msec.) unaccompanied by IPSP''s exhibited slow rise times, were of relatively large amplitude, and generally elicited single discharges in response to ventrolat-eral thalamic stimulation. Inversion of early phases of IPSP''s by transmembrane injection of hyperpolarizing currents did not alter the time course or effectiveness of residual phases of IPSP''s in depressing the direct excitability of immature neurons. Spike potential changes spontaneous activity and in association with IPSP summation during repetitive thalamic stimulation were encountered in a relatively large proportion of immature neocortical neurons. Partial responses including fast prepotentials were noted during spontaneous activity and during synaptically evoked and induced soma hyperpolarization. Some spike potential variations are considered evidence for impulse initiation in dendrites of immature cortical neurons. Possible mechanisms underlying the development of these alterations during different conditions of activation are considered in relation to the morphological features of neurons and synaptic organizations in immature neocortex.