Muscle & nerve
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Laser pulses selectively excite mechano-thermal nociceptors and evoke brain potentials that may reveal small-fiber dysfunction. We applied CO2-laser pulses to the perioral and supraorbital regions and recorded the scalp laser-evoked potentials (LEPs) and reflex responses in the orbicularis oculi, masticatory, and neck muscles in 30 controls and 10 patients with facial sensory disturbances. Low-intensity pulses readily evoked scalp potentials consisting of a negative component with a latency of 165 ms followed by a positive component at 250 ms. ⋯ Although only high-intensity pulses evoked reflex responses, some subjects showed--even to low-intensity pulses--an orbicularis oculi (blink-like) response that markedly contaminated the scalp recording. Scalp LEPs were abnormal in patients with hypalgesia and normal trigeminal reflexes and normal in patients with normal pain sensitivity and abnormal trigeminal reflexes. Possibly because of the high receptor density in this area and the short conduction distance, laser stimulation of the trigeminal territory yields low-threshold and large LEPs, which are useful for detecting dysfunction in peripheral and central pain pathways.
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Several families of voltage-gated potassium channels (Kv), including a spectrum of subtypes, are involved in regulating and modifying the integration and transmission of electrical signals in the nervous system. However, the specific patterns of Kv expression in normal or injured dorsal root ganglion (DRG) neurons have not been studied. Previous studies have examined the expression of voltage-gated sodium channels in DRG neurons, and also the selective up- and downregulation of several of these channels following axonal injury to the DRG neurons. ⋯ Kv1.4 and 1.6 immunostaining were not altered by axotomy, and Kv1.5 immunoreactivity was low in both control and axotomized DRG neurons. These results provide molecular correlates for the expression of multiple K+ currents in normal DRG neurons and indicate that, in relation to changes in sodium channel expression, there are decreases in specific potassium channels following axotomy in these cells. The alterations in K+ and Na+ channel expression following axonal injury may lead to changes in electrical excitability of the DRG neurons, and might contribute to chronic pain syndromes.