Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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Convergence between cutaneous heat nociceptors and muscles afferents was investigated by applying a phasic, conditioning electrical stimulus to the tibialis anterior muscle (a train of five 1 ms pulses over 21 ms) at varying time intervals relative to a thermal test stimulus used for evoking the withdrawal reflex in humans. The 200 ms thermal stimulus was applied on the dorsum of the foot at an intensity of two times the pain threshold. ⋯ The mean reflex onset latency of reflexes evoked by thermal stimuli alone was 354 +/- 9 ms. A facilitation of the reflex was seen when the conditioning stimulus was applied 275 ms (174 +/- 30% compared to control) and 300 ms (162 +/- 32% compared to control) after the test stimulus onset indicating sensory convergence between muscle afferents (group I-III) and cutaneous Adelta heat nociceptors arriving simultaneously at the spinal cord.
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Traumatic spinal cord injury (SCI) results not only in motor impairment, but also in chronic central neuropathic pain, which often is refractory to conventional treatment approaches. Upregulated expression of sodium channel Nav1.3 has been observed within the spinal dorsal horn neurons after SCI, and appears to contribute to neuronal hyperresponsiveness and pain-related behaviors. In this study we characterized the changes in sodium current properties within dorsal horn neurons after contusive SCI. ⋯ Small slow depolarizations below action potential threshold produced ramp currents, which were markedly enhanced by SCI (from 182 +/- 41 to 338 +/- 55 pA). The density of the noninactivating persistent sodium current was also significantly enhanced in neurons from SCI animals (from 17.4 +/- 3.2 to 27.7 +/- 4.4 pA/pF at 50-70 ms of depolarization). The increased persistent sodium current and ramp current, which are consistent with upregulation of Nav1.3 within dorsal horn neurons, suggest a basis for the hyperresponsiveness of these neurons following SCI.
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Delayed onset muscle soreness (DOMS) involves central and peripheral pain mechanisms. Referred pain patterns following stimulation of DOMS affected tissue have not been fully described. Referred pain may provide information on how central mechanisms are involved in DOMS, as referred pain is a central mechanism. ⋯ Significantly higher referred pain frequency and enlarged pain areas were found at the muscle belly and TBJ sites following injection during DOMS compared to pre-DOMS. The results indicate that muscle belly and TBJ sites are sensitised while tendon tissue per se is unaffected by DOMS. Central sensitivity changes caused by DOMS may explain the increase in referred pain frequency and enlarged pain areas.
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The operational definition of spasticity is focused on increased resistance of joints to passive rotation and the possible origin of this increased resistance in the induced tonic stretch reflex (TSR). This term is applied in the context of both cerebral and spinal injury, implying that a similar reflex mechanism underlies the two disorders. From recent studies it is clear that increased passive joint resistance in resting limbs following stroke is highly correlated with the induced TSR, but this evidence is lacking in spinal injury. ⋯ This result contrasts with our similar study of cerebral spasticity after stroke, where a comparable low frequency stretch perturbation produced clear evidence of increased TSR gain that was correlated with the hypertonia at rest. We conclude that a low frequency stretch perturbation clearly distinguished between spasticity after stroke and SCI. Spasticity in the two conditions is not equivalent and care should be taken in generalizing results between them.
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Clinical studies have demonstrated that growth hormone (GH) promotes learning and memory processes in GH-deficient (GHD) patients. In animal studies, GH also influences the N-methyl-D-aspartate (NMDA) receptor system in the hippocampus, an essential component of long-term potentiation (LTP), which is highly involved in memory acquisition. This study was designed to examine the beneficial effects of recombinant human GH (rhGH) on cognitive function in male rats with multiple hormone deficiencies resulting from hypophysectomy (Hx). ⋯ Hippocampal mRNA expression of the NMDA subunits NR1, NR2A and NR2B, insulin-like growth factor type 1 receptor (IGF-1R), and postsynaptic density protein-95 (PSD-95) was then measured in the animals by Northern blot analysis. The results suggest that there may be a relationship between the NMDA receptor subunit mRNA expression levels and learning ability, and that learning is improved by rhGH in Hx rats. Furthermore, a link between MWM performance and PSD-95 was also suggested by this study.