CORTICAL INTRACELLULAR POTENTIALS DURING AUGMENTING AND RECRUITING RESPONSES: II. PATTERNS OF SYNAPTIC ACTIVITIES IN PYRAMIDAL AND NONPYRAMIDAL TRACT NEURONS

Abstract

Augumenting and recruiting responses evoked in motor cortex by low-frequency (7/sec) ventrolateral (VL) and mid-line (CM) thalamic stimulation, respectively, have been studied in relation to patterns of intracellularly recorded synaptic activities of pyramidal tract neurons (PT cells) and non-PT cells. Pyramidal tract cells exhibiting EPSP''s [excitatory post-synaptic potentials] during augmentation were also synaptically influenced during recruitment. Separation of synaptic effects was encountered more fre- quently in non-PT cells than in PT cells. Differences in discharge characteristics are defined in terms of latency, magnitude, and duration of evoked EPSFs and IPSP''s [inhibitory PSP''s] during VL and CM stimulation. These differences are discussed with respect to the dual control of cortical neurons by specific and nonspecific thalamocortical projection systems. Excitatory postsynaptic potentials and/or inhibitory postsynaptic potentials have been recorded in cells in different cortical layers. In any one cell or group of cells VL or CM stimulation elicited remarkably stable PSP patterns during particular phases of augmenting or recruiting sequences. These data are considered in relation to the problem of "dendritic potentials" and the origin of components of evoked cortical responses. Alterations in PSP patterns in non-PT cells were more prominent than in PT cells during the transition from primary to augmenting responses. These changes consisted in latency shifts of initial PSP''s, attenuation of IPSP''s and/or augmentation of late EPSP''s. Enhancement of PSP''s was the major PSP alteration observed during recruitment. The intracortical synaptic events underlying augmenting and recruiting responses are discussed in the light of these findings.