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 trigeminal nucleus caudalis (Sp5C) were studied in the rat using microinjections of wheat germ agglutinin-apo horseradish peroxidase-gold and Biotin-Dextran. Cortical cells projecting to the caudal medulla were confined to the contralateral 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 insular cortex, whereas cortical projections to the Cu originated mainly from the primary motor cortex (M1), the primary and secondary somatosensory cortex (S1 and S2). The SRD received dense cortical afferents from larger, widespread cortical areas: M1, M2, S1, S2, and the insular cortex. The existence of dense cortico-SRD connections supports the possibility of a pyramidal influence over SRD neurons, which might modify nociceptive information ascending to the cortex itself. This proposal is consistent with the fact that SRD efferents terminate densely in thalamic areas that influence sensorimotor cortical regions which in turn project to the SRD. Moreover, these corticofugal mechanisms could allow the cortex to select its own input by suppressing or augmenting transmission of signals through SRD-hindbrain/forebrain pathways or by coordinating activities in spino-SRD-spinal circuits and thus selecting the relevant information produced by the noxious stimulus.