Some synaptic inputs and ascending projections of lateralis posterior thalamic neurons

(1) Some synaptic inputs and telencephalic targets of lateralis intermedius-lateralis posterior (LI-LP) thalamic neurons have been investigated in the ence´phale isole´cat by recording extracellular unit discharges and focal slow waves following stimulation of cortical areas 5 and 7, center median (CM) thalamic nucleus and mesencephalic reticular formation (RF). (2) Orthodromic and/or antidromic discharges in LI-LP neurons following anterior suprasylvian stimulation are electrophysiological evidence of thalamocortical reciprocal pathway in this “association” system. A few neurons were backfired from several cortical stimulated foci at different latencies, thus suggesting the existence of LI-LP output cells with branching axons. The numerous cases of cortically elicited synaptic excitation in LI-LP cells, taken together with evidence of an unexpectedly high proportion of corticothalamic neurons found in parallel experiments of this laboratory on areas 5 and 7, support the suggestion that complex functions of the “association” cortex partially depend on its downstream connections with thalamic integrative structures. (3) Monosynaptic and oligosynaptic excitatory projections link the brain stem reticular core with the LI-LP nuclear group. For some neurons, this is a high-security pathway as shown by the ability of thalamic cells to follow at short latencies fast repetitive reticular shocks. The RF-elicited phasic excitation is followed by a period of suppressed spontaneous and evoked discharge. The excitatory projections from the upper brain stem RF to the LI-LP nuclei are discussed in relation to disfacilitatory phenomena in thalamocortical systems due to withdrawal of reticulofugal ascending activation at sleep onset.