Organization of cortical projections to the medullary subnucleus reticularis dorsalis: A retrograde and anterograde tracing study in the rat

The distribution and organization of cortical projections to the subnucleus reticularis dorsalis (SRD), the neighboring cuneate nucleus (Cu), and trig eminal nucleus caudalis (SD5C) were studied in the rat using microinjection s of wheat germ agglutinin-apo horseradish peroxidase-gold and Biotin-Dextr an. Cortical cells projecting to the caudal medulla were confined to the co ntralateral layer V with their descending axons crossing the midline at the level of pyramidal decussation. Cortical afferents to Sp5C originated from cells located mainly in the primary somatosensory cortex (S1) and the insu lar cortex, whereas cortical projections to the Cu originated mainly from t he primary motor cortex (M1), the primary and secondary somatosensory corte x (S1 and S2). The SRD received dense cortical afferents from larger, wides pread cortical areas: M1, M2, S1, S2, and the insular cortex. The existence of dense cortico-SRD connections supports the possibility of a pyramidal i nfluence over SRD neurons, which might modify nociceptive information ascen ding to the cortex itself. This proposal is consistent with the fact that S RD efferents terminate densely in thalamic areas that influence sensorimoto r cortical regions which in turn project to the SRD. Moreover, these cortic ofugal mechanisms could allow the cortex to select its own input by suppres sing or augmenting transmission of signals through SRD-hindbrain/forebrain pathways or by coordinating activities in spino-SRD-spinal circuits and thu s selecting the relevant information produced by the noxious stimulus. (C) 1999 Wiley-Liss, Inc.