Abstract
This work addressed the study of subnucleus reticularis dorsalis (SRD) neurons in relation to their supraspinal input and the spinal terminating sites of their descending axons. SRD extracellular unitary recordings from anesthetized cats aimed to specifically test, 1) the rostrocaudal segmental level reached by axons of spinally projecting (SPr) neurons collateralizing or not to or through the ipsilateral nucleus reticularis gigantocellularis (NRGc), 2) whether SPr fibers bifurcate to the thalamus, and 3) the effects exerted on SRD cells by electrically stimulating the locus coeruleus, the periaqueductal grey, the nucleus raphe magnus, and the mesencephalic locomotor region. From a total of 191 SPr fibers tested to cervical 2 (Ce2), thoracic 5 (Th5) and lumbar5 (Lu5) stimulation, 81 ended between Ce2 and Th5 with 39 of them branching to or through the NRGc; 21/49 terminating between Th5 and Lu5 collateralized to or through the same nucleus, as did 34/61 reaching Lu5. The mean antidromic conduction velocity of SPr fibers slowed in the more proximal segments and increased with terminating distance along the cord. None of the 110 axons tested sent collaterals to the thalamus; instead thalamic stimulation induced long-latency polysynaptic responses in most cells but also short-latency, presumed monosynaptic, in 7.9% of the tested neurons (18/227). Antidromic and orthodromic spikes were elicited from the locus coeruleus and nucleus raphe magnus, but exclusively orthodromic responses were observed following stimulation of the periaqueductal gray or mesencephalic locomotor region. The results suggest that information from pain-and-motor-related supraspinal structures converge on SRD cells that through SPr axons having conduction velocities tuned to their length may affect rostral and caudal spinal cord neurons at fixed delays, both directly and in parallel through different descending systems. The SRD will thus play a dual functional role by simultaneously regulating dorsal horn ascending noxious information and pain-related motor responses.
Highlights
The subnucleus reticularis dorsalis (SRD), known as the dorsal reticular nucleus, is constituted exclusively by nociceptive neurons [1,2] reciprocally connected with the noxious region of the spinal dorsal horn in the rat [3,4,5,6,7,8,9]
As earlier reported [18], the cats SRD cells responding to peripheral stimuli (n = 736) had wide noxious peripheral receptive fields being driven by pinching and/or squeezing the skin and/or by intracutaneous 3.5–6 mA electrical stimulating currents applied to the plantar fore-and/or-hind pads
None of the sampled neurons showed activity related to the electrocardiogram or to the respiratory cycle, the possibility remains that some of these nonresponsive neurons were picked up from the solitary complex or the ambiguous/retroambiguous nuclei and, were excluded from further analysis
Summary
The subnucleus reticularis dorsalis (SRD), known as the dorsal reticular nucleus, is constituted exclusively by nociceptive neurons [1,2] reciprocally connected with the noxious region of the spinal dorsal horn in the rat [3,4,5,6,7,8,9]. The first aim of the present work was to elucidate this issue by electrically stimulating the spinal ipsilateral dorsolateral funiculus at cervical, thoracic and lumbar levels. A second aim was to study whether the cats SRD neurons respond antidromically to electrical stimulation of the somatosensory and/or medial thalamus This is still an unsolved issue as previous studies in felines showed controversial results related to SRD ascending projections, since whereas few and scattered cells were stained in the SRD after injecting horseradish peroxidase into the thalamus [20,21], it was latter reported that about half of neurons sampled in and around the SRD responded antidromically to stimulation of the thalamic nucleus centralis lateralis [22]
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