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In rat spinal cord, slice repetitive electrical stimulation of the dorsal root at an intensity that activates C-fibers evokes a slow-to-develop and prolonged (30–50 s) change in light transmittance (OISDR) in the superficial part of the ipsilateral dorsal horn (DHs). Inhibition of astrocyte metabolism [by bath-applied 400 µM fluoroacetate and 200 µM glutamine (FAc + Gln)] or interference with glial and neuronal K+ transport [by 100 µM 4-aminopyridine (4-AP)] leads to dissociation of the OISDR and the postsynaptic DHs response to a single-pulse, constant-current dorsal root stimulus (P-PSPDR). The OISDR decreases under FAc+Gln, whereas the P-PSPDR remains unaltered; under 4-AP, the P-PSPDR increases, but the OISDR decreases. In contrast, both the OISDR and P-PSPDR increase when K+o is elevated to 8 mM. These observations from slices from normal subjects are interpreted to indicate that the OISDR mainly reflects cell volume and light scattering changes associated with DHs astrocyte uptake of K+ and glutamate (GLU). In slices from subjects that received an intracutaneous injection of formalin 3–5 days earlier, both the OISDR and the response of the DHs ipsilateral to the injection site to 100-ms local application (via puffer pipette) of 15 mM K+ or 100 µM GLU were profoundly reduced, and the normally exquisite sensitivity of the DHs to elevated K+o is decreased. Considered collectively, the observations raise the possibility that impaired regulation of DHs K o and GLUâ‚€ may contribute to initiation and maintenance of the CNS pain circuit and sensorimotor abnormalities that develop following intracutaneous formalin injection. [ABSTRACT FROM AUTHOR]

Three types of experiment were carried out on anesthetized monkeys and cats. In the first, spike discharge activity of rapidly adapting (RA) SI neurons was recorded extracellularly during the application of different frequencies of vibrotactile stimulation to the receptive field (RF). The second used the same stimulus conditions to study the response of RA-I (RA) cutaneous mechanoreceptive afferents. The third used optical intrinsic signal (OIS) imaging and extracellular neurophysiological recording methods together, in the same sessions, to evaluate the relationship between the SI optical and RA neuron spike train responses to low- vs high-frequency stimulation of the same skin site. RA afferent entrainment was high at all frequencies of stimulation. In contrast, SI RA neuron entrainment was much lower on average, and was strongly frequency-dependent, declining in near-linear fashion from 6 to 200 Hz. Even at 200 Hz, however, unambiguous frequencyfollowing responses were present in the spike train activity of some SI RA neurons. These entrainment results support the "periodicity hypothesis" of Mountcastle et al. (J Neurophysiol 32: 452-484, 1969) that the capacity to discriminate stimulus frequency over the range 5-50 Hz is attributable to the ability of SI RA pyramidal neurons to discharge action potentials in consistent temporal relationship to stimulus motion, and raise the possibility that perceptual frequency discriminative capacity at frequencies between 50 and 200 Hz might be accounted for in the same way. An increase in vibrotactile stimulus frequency within the range 6-200 Hz consistently resulted in an increase in RA afferent mean spike firing rate (M FR). SI RA neuron M FR also increased as frequency increased between 6 and 50 Hz, but declined as stimulus frequency was increased over the range 50-200 Hz. At stimulus frequencies > 100 Hz, and at positions in the RF other than the receptive field center (RF[sub center]), SI RA neuron MFR declined sharply within 0.5-2s of stimulus onset and rebounded transiently upon stimulus termination. In contrast, when the stimulus was applied to the RF[sub center], MFR increased with increasing frequency and tended to remain well maintained throughout the period of high-frequency stimulation. The evidence obtained in "combined" OIS imaging and extracellular microelectrode recording experiments suggests that SI RA neurons with an RF[sub center] that corresponds to the stimulated skin site occupy small foci within the much larger SI region activated by same-site cutaneous flutter stimulation, while for the RA neurons located elsewhere in the large SI region activated by a flutter stimulus, the stimulus site and RF[sub center] are different. [ABSTRACT FROM AUTHOR]

Spike discharge activity was recorded from low-threshold, rapidly adapting, skin mechanoreceptive afferents (RA afferents) dissected from the median (forelimb) or tibial (hindlimb) nerves in anesthetized monkeys and cats. The spike activity was evoked by delivery of controlled sinusoidal vertical skin displacement ("flutter") stimuli to the receptive field (RF). The stimuli (15-30 Hz; 30-400 mum peak-to-peak amplitude; duration 0.8-15 s) were superimposed on a static skin indentation (0.5-1.0 mm) which was either maintained continuously throughout the run or applied trial-by-trial. The neural activity and the analog signal of the position of the stimulator probe were digitized at 10 kHz resolution and stored for off-line analysis. The main goal was to determine whether changes in the RA afferent response to skin flutter stimulation may be responsible for the enhanced capacity to discriminate stimulus frequency that accompanies a relatively brief (approximately equal to 1 min) pre-exposure to such stimulation in humans. To this end, the spike train data were evaluated using methods that enabled independent measurement of entrainment and responsivity. Responsivity (response intensity) was measured as the average number of spikes/stimulus cycle, while entrainment (the degree to which evoked spike train activity is phase-locked to the stimulus) was quantitatively assessed using statistical techniques developed for the analysis of "circular" (directional) data, supplemented by methods based on the calculation of power spectra from point process data. The methods are demonstrated to enable quantification of RA afferent entrainment over a range of stimulus durations and amplitudes substantially greater than reported in previous studies. While RA afferent responsivity was found to decline to a minor extent (10-20%) both across and within stimulus trials, entrainment remained consistently high and stable, and exhibited no temporal trends or dependence on any other measured factor. The average phase angle of the entrained RA afferent response also remained stable both within and across trials, showing only a tendency to increase slightly during the initial 100-500 ms after stimulus onset. The results imply that the improved capacity to discriminate stimulus frequency that develops in response to an exposure to cutaneous flutter stimulation is not attributable to a change in RA afferent entrainment per se. [ABSTRACT FROM AUTHOR]

The influence of stimulus velocity and traverse length on a subject's ability to indicate direction of brush movement across perioral skin was determined using a forced-choice procedure. The data show that correct identification of brush direction increases with traverse length and is optimal for velocities between 3 and 25 cm/sec. [ABSTRACT FROM AUTHOR]