[Characteristics of monosynaptic connections between neurons of the callosal system and specific thalamic nuclei]

In the sensorimotor cortex of awake rabbits monosynaptic afferent and efferent connections of callosal neurons (CN) with ipsilateral specific thalamic nuclei (ventral posterolateral, ventral posteromedial, ventral lateral and anteroventral nuclei) were analyzed by electrophysiological method and compared with those of target-units of callosal fibres (TU). It was demonstrated that CN and TU differed in their monosynaptic inputs from the thalamus and in their axons projecting to these structures and/or being a part of the pyramidal tract. These differences were the following: a greater portion of TU had the descending projections (54%) as compared with CN (14%); 22% of TU responded by monosynaptic action potentials to the stimulation of specific thalamic nuclei, while CN never manifested such responses. TU could project to the thalamus through the main axon stem as well as through the collaterals of the pyramidal tract axons. It is supposed that the discovered differences evidence for the much greater convergence of the thalamic relay neuron effect on TU, thus revealing the mechanism underlying the differences both in receptive field properties and in the background and evoked activity of CN and neurons synaptically excited in response to the transcallosal stimulation.