Brain stem neurones with differential projection to functional subregions of the dorsal lateral geniculate complex in the cat.

Neurones (n = 194) in the ponto-mesencephalic reticular formation were identified as projecting to the region of the dorsal lateral geniculate nucleus, by their antidromic activation from this region. The projection of individual neurones was investigated by mapping of thresholds at closely spaced sites in the lateral geniculate nucleus and the dorsally located perigeniculate nucleus. Low thresholds, long latencies and stepwise antidromic latency shifts, occurring together, were taken to indicate termination. Low thresholds with short antidromic latencies without stepwise latency shifts were encountered more ventrally and ascribed to stimulation of larger fibres en route to terminal fields. The conduction velocity from the stimulation site of the large fibres to the cell bodies in the reticular formation was 0.9-3.1 m/s and to regions within the terminal fields 0.06-0.2 m/s. The considerable slowing of the conduction velocity, together with the extensive regions with low thresholds and the abundance of latency shifts, 0.3-19.5 ms, suggest arborization of very thin fibres within the terminal fields. Three groups of reticular neurones with different termination were distinguished: type A neurones (n = 32) terminate exclusively in the laminae of the lateral geniculate nucleus. Type B neurones (n = 22) terminate in the perigeniculate nucleus, the overlaying part of the reticular nucleus of thalamus and in the interlaminar plexuses in the lateral geniculate nucleus. Type C neurones (n = 27) terminate both in the lateral geniculate nucleus and the perigeniculate nucleus. The projection of individual neurones of all three types extended throughout their entire target nucleus/nuclei and is thus global with respect to the visual hemifield. On the basis of previous findings that stimulation in the reticular formation inhibits interneurones in both feed-forward and recurrent inhibitory pathways to relay cells in the lateral geniculate nucleus, it is suggested that the reticular neurones are inhibitory and that the type A neurones control the feed-forward inhibition, type B neurones the recurrent inhibition and type C neurones both inhibitory mechanisms together. Findings suggesting that different reticular neurones may be related to the interneuronal circuits of the X-, Y- and W-systems are discussed. Type A, B and C neurones were found intermingled, around the brachium conjunctivum and among its fibres from the level of the decussation to the posterior end of the locus coeruleus complex.(ABSTRACT TRUNCATED AT 400 WORDS)