Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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Effects of persistent temporomandibular joint (TMJ) inflammation on nociceptive responses of remote bodily areas of the rat were investigated. Monoarthritis of the TMJ region was evoked by the injection of complete Freund's adjuvant (CFA) into the left TMJ region. Rats without injection of CFA into the TMJ region served as controls (non-CFA group). ⋯ The number of Fos-positive neurons in the lumbar dorsal horn ipsilateral to the formalin injection at 1 and 7 days after CFA injection into the TMJ were similar to those in the non-CFA group; however, those were significantly increased in the laminae I-II and V-VI of the lumbar dorsal horn at 14 days after CFA injection. TMJ inflammation for 7 and 14 days alone produced a small number of Fos-expressing neurons in the lumbar dorsal horn. These results provide evidence that persistent unilateral inflammation of the TMJ region causes an increase in behavioral hyperalgesia of the hindpaw, which is attributed to the modulation of neural activities, in part, in the lumbar dorsal horn, likely mediated by supraspinal neural mechanisms.
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Several recent studies with transcranial magnetic stimulation (TMS) have demonstrated changes in motor evoked potentials (MEPs) in human limb muscles following modulation of sensory afferent inputs, but little is known about the regulation of the human tongue motor control. To test the effect of local anesthesia (LA) of the lingual nerve and topical application of capsaicin stimulation on tongue MEPs. Fourteen volunteers participated (21-30 years) in two randomized sessions; before, during a nerve block of the lingual nerve or topical capsaicin application (30 microl 5%) on the tongue, and after anesthesia or pain had subsided. ⋯ LA did not have any effect on FDI MEPs but was associated with a significant facilitation of tongue MEPs at T+50% and T+60% about 50 min after the nerve block in the recovery phase. Also in this condition, the direct motor responses evoked by hypoglossal nerve stimulation remained constant. No direct effect of a strong burning sensation could be shown on peripheral or central corticomotor pathways to the relaxed tongue musculature, however, LA of the lingual nerve (cranial nerve V) seems able to induce a delayed change in corticomotor control of tongue musculature (cranial nerve XII) possibly related to unmasking effects at the cortical level but not completely excluding excitability changes at the brain stem level.
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Noxious low-frequency stimulation (LFS) of presynaptic nerve fibers induces long-term depression (LTD) of synaptic transmission. In vitro studies suggest a sole homosynaptic effect. Consequently, the present study addressed the hypothesis that LTD of craniofacial nociception in man is mediated by a homosynaptic mechanism. ⋯ Pain perception decreased after ipsilateral LFS but not after contralateral LFS. The bilateral effect of noxious LFS on the BR provides evidence for heterosynaptic LTD based on bilateral projections of supraorbital nerve afferents onto spinal trigeminal nuclei. The divergent effect on pain perception may be due to a preferential contralateral projection of nociceptive afferents onto reflex interneurons but not onto trigeminothalamic projection neurons.
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The central processes occurring during fatiguing exercise are not well understood, however transcranial magnetic stimulation (TMS) studies have reported increases both in corticomotor excitability, as measured by the motor-evoked potential (MEP) amplitude, and in long-interval intracortical inhibition, as measured by the duration of the post-MEP silent period. To determine whether short-interval cortical inhibition (SICI) is modulated by fatiguing exercise, we used single and paired-pulse TMS to measure MEP amplitude and SICI for the first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles of the hand during, and for 20 min after, a 10-min intermittent maximal voluntary abduction of the index finger designed to fatigue the FDI muscle. For the FDI, the index of SICI increased at the onset of exercise (from 0.25+/-0.05 to 0.55+/-0.11, P < 0.05) and then decreased progressively as force declined. ⋯ MEP amplitude for both the FDI and ADM increased above baseline during exercise and then decreased below baseline during the recovery period. These results demonstrate that there are significant changes in SICI during and after a fatiguing exercise protocol that are isolated to the representation of the fatigued muscle. The inter-relationship between the changes in excitation and inhibition suggests the presence of a measured and adaptive process of modulation in central excitation and inhibition acting to increase corticomotor drive to the exercising muscle as fatigue is developing.