Excitatory and inhibitory influences of thalamic stimulation on pyramidal tract neurons

Abstract

Effects of single and repetitive (7/sec) thalamic stimulation on pyramidal tract (PT) neurons were investigated by laminar field potential analysis in the cat. Negative antidromic field potentials in the sensorimotor cortex, elicited by stimulation of the cerebral peduncle, and generated in fast (N 1) and slow (N 2) PT neuronal populations were used as indices. They were preceded by conditioning thalamic stimulation and changes of antidromic potentials at various depths in the cortex were plotted as functions of time intervals between the two stimuli. All cortical potentials evoked by thalamic stimulation represent either single or compound manifestations of the two elementary patterns, viz., the deep and superficial thalamocortical (T-C) responses. During the deep T-C response, both N 1 and N 2 were significantly suppressed at all levels in the cortex. Facilitatory influences on N 1 and N 2 were not seen. The superficial T-C response was associated with an initial enhancement of N 1 and N 2 and subsequent suppression. The facilitation was particularly pronounced in superficial cortical layers. On the basis of the laminar distribution of facilitation and inhibition certain conclusions regarding the location of active synaptic sites along an idealized PT neuron have been suggested: IPSPs associated with both types of T-C responses must be generated in somata as well as apical dendrites. EPSPs underlying the superficial T-C response are localized on superficial apical dendrites, while those generated during the deep T-C response are mainly on cell bodies and adjacent dendritic shafts.