Journal of neurophysiology
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1. The reliability of the propagation of action potentials (AP) through dorsal root ganglion (DRG) cells in embryonic slice cultures was investigated during repetitive stimulation at 1-20 Hz. Membrane potentials of DRG cells were recorded intracellularly while the axons were stimulated by an extracellular electrode. 2. ⋯ During repetitive stimulation, an accumulation of intracellular calcium occurred that recovered very slowly (tens of seconds) after the AP trains. 7. Computer model simulations performed in analogy to the experimental protocols produced conduction failures during repetitive stimulation only when the calcium currents during the APs were reduced. 8. From these findings it is concluded that conduction failures during repetitive stimulation are dependent on an accumulation of intracellular calcium leading to an inactivation of calcium currents, combined with small contributions of an accumulation of extracellular potassium and a summation of slow potassium conductances.
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1. Pain hypersensitivity is characterized by an increase in the response to noxious stimuli (hyperalgesia) and a reduction in threshold such that innocuous stimuli begin to elicit pain (allodynia). These sensitivity changes can be produced by an increase in excitability of dorsal horn neurons; the phenomenon of central sensitization. ⋯ The nociceptive flexion withdrawal reflex is under the control, therefore, of segmental inhibitory mechanisms mediated by glycine and GABAA receptors. Removal of this inhibition enables the reflex to be activated by low-intensity cutaneous stimuli. Given the similarities between the stimulus-response profiles of the nociceptive flexion reflex and the production of pain in man, these findings indicate that a decrease in the efficacy of spinal inhibitory circuits may contribute to the touch-evoked allodynia that occurs in pain hypersensitivity states, where A beta inputs begin to produce pain.
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1. We used the tested fiber method to record from single myelinated afferents axons ending in a chronic nerve injury site (neuroma) in the rat sciatic nerve or L4,5 dorsal root. Axons were chosen for study that fired spontaneously with a stable tonic or interrupted (bursty) autorhythmic firing pattern. 2. ⋯ The data indicate that, in chronically injured axons, the inward currents that underly electrogenicity, enable ectopic discharge, and, together with outward K+ currents, set the fundamental firing rhythm (ISI), operate primarily with the use of voltage-sensitive Na+ rather than Ca2+ channels. 8. The on-off duty cycle in bursty fibers was affected by Na+ channel ligands and also, although less so, and less consistently by, Ca2+ channel ligands. This indicates that both may play a role in the slow modulations of membrane potential that presumably underly interrupted autorhythmicity.
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1. Electromyographic recordings were made from the biceps femoris muscle through a pair of noninsulated platinum/iridium needle electrodes in male Sprague-Dawley rats artificially ventilated and anesthetized with 0.8% halothane in a N2O-O2 mixture (2/3:1/3). The animals' ventilation, heart rates, and body temperatures were continuously monitored. ⋯ The C-fiber reflex was recorded when the duration and frequency of the stimuli applied to the sural nerve varied within the 0.5- to 4-ms and 0.02- to 1-Hz ranges, respectively. It was concluded that a single 2-ms duration shock at an intensity of 1.2 times the C-fiber reflex threshold, delivered every 6 s (0.16 Hz), constituted an acceptable and optimal protocol for experiments in which the C-fiber reflex was studied as a function of time. These parameters were used throughout the subsequent experiments.(ABSTRACT TRUNCATED AT 400 WORDS)
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1. Whole-cell and single-channel patch-clamp recordings of calcium (Ca2+) currents were made in acutely dissociated neurons from the medial septum (MS) and nucleus of the diagonal band (nDB) of adult guinea pig. Barium (Ba2+) was used as the charge carrier across the Ca2+ channel and multiple channel types were identified in different cell types. 2. ⋯ Unitary amplitudes were determined at different membrane potentials with single-channel conductances calculated to be 7.8 +/- 1.2 (SD) pS. These channels were not blocked by nifedipine (10 microM) and appeared similar to T channels previously reported in both peripheral and central neurons. Ensemble averages from cell-attached patches of T channels resembled LVA currents recorded in the whole-cell configuration.(ABSTRACT TRUNCATED AT 400 WORDS)