Thalamocortical and intrathalamic interactions during slow repetitive stimulation of n. centralis lateralis.

Extracellular single unit activity was recorded simultaneously in cortex (anterior part of the middle suprasylvian gyrus, MSSG) and thalamus (n. ventralis anterior, VA; n. lateralis posterior, LP) during repetitive low frequency stimulation (RLFS) of n. centralis lateralis (CL) in lightly anesthetized cats. Such stimulation induced typical recruiting responses in the cortical EEG consisting of long-latency, surface-negative waves reversing in polarity at 0.1-2mm depth. These cortical EEG responses were associated with long-latency (8-20 ms) action potential (AP) discharges of cortical neurons appearing with the 2nd stimulus of the train. The number of AP discharges and response latency increased as the train of CL stimuli progressed. In 12 of these neurons there was a short-latency (up to 2-5 ms) response which, however, did not show incremental features during RLFS. Thalamic neurons in VA usually responded to the first stimulus within a train of RLFS of CL, while LP neurons responded only to the 2nd or 3rd stimulus. Peristimulus time histograms (PSTHs) of AP discharges in VA showed an increase in both number and latency of APs as the train of stimuli progressed. This was also observed in those thalamic neurons in LP which changed their firing during RLFS of CL. The peak of firing of 20 VA neurons preceded, and that of 7 followed that of the MSSG neurons, while 6 VA and MSSG neuronal pairs reached their peak firing simultaneous; peak firing of 29 LP neurons preceded, and that of 21 followed the firing peak of MSSG neurons. The thresholds of the incremental responses of MSSG, VA and LP neurons to progressively increased intensity of RLFS of CL were different: MSSG and VA neurons changed their firing pattern at an intensity incapable of modifying the activity of LP neurons. When stimulation intensity was increased to a level sufficient to change the responses of both neurons of a given pair (either MSSG-VA or MSSG-LP) the time sequence of involvement in the incremental process was in the following order: VA, MSSG, LP. Within a range of RLFS extending from 2-20 Hz, stimulation at 6.6 Hz (150 ms interstimulus intervals) induced the most prominent incremental responses both in the cortex and thalamus, i.e. response increases were largest when each subsequent stimulus occurred 50-130 ms before the expected rebound excitation following the preceding response.(ABSTRACT TRUNCATED AT 400 WORDS)