Electrophysiological Study of Supraspinal Input and Spinal Output of Cat's Subnucleus Reticularis Dorsalis (SRD) Neurons

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.

Methods and strategies of tDCS for the treatment of pain: current status and future directions

“…One possible target is to enhance the endogenous pain modulation system before pain arises. The endogenous pain modulation system in healthy population and acute pain conditions have the ability to deal with nociceptor stimuli via supraspinal structures as primary motor and sensory cortices, the thalamus, the cingulate cortex, the periaqueductal gray, the rostral ventromedial medulla and the connection with the subnucleus reticularis dorsalis and projections to the spinal cord [177][178][179][180][181]. However, this pathway is impaired in chronic pain leading to an unbalance on the endogenous pain pathway.…”

Reticular Formation and Pain: The Past and the Future

“…The DRt is also a major integrative relay for descending nociceptive modulation from several brain areas including the cortex (Zhang et al, 2005 ), the hypothalamus (Amorim et al, 2015 ) and the brainstem (Martins et al, 2013 ; Velo et al, 2013 ; Leiras et al, 2016 ). The DRt was shown to relay descending pain facilitation from the anterior cingulate cortex (ACC; Zhang et al, 2005 ).…”

The pain conductor: brainstem modulation in acute and chronic pain

“…These regions include the periaqueductal gray (PAG) in the midbrain; the parabrachial complex (PB) and locus coeruleus (LC) in the pons; and the rostral ventromedial medulla (RVM) and subnucleus reticularis dorsalis (SRD) in the medulla. Neuroanatomical tract tracing revealed that each of these brainstem regions projects directly to, or receives projections from, both the DH and SpV [4][5][6][7][8]. Modern brain imaging has revealed that these pathways are likely conserved in humans, and with the advent of ultra-high field magnetic resonance imaging (MRI), we have begun to better define their contributions to pain modulation in both experimental and clinical settings, as well start to robustly characterize the manner in which these brainstem sites interact with one another [9 && ].…”