Somatosensory & motor research
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Twenty-four healthy human subjects provided thresholds for their perception of pressure, sharpness, and pain. Mechanical forces were applied to the dorsal surface of the digits with flat-tipped probes of various sizes. Thresholds (expressed as force) increased with increasing probe size, as previously described. ⋯ In contrast, only 6% of subjects showed significant increases in sharpness or pressure thresholds over the same period. Thus, whereas most subjects exhibited stable pain thresholds, approximately one-fourth showed significant increases in pain threshold over time. We conclude that for evaluating regional dysesthesia or hemidysesthesia, a right-left difference in pain thresholds will provide a more sensitive and reliable measure than absolute pain threshold.
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The gate control theory of pain (Melzack and Wall, 1965) suggests that tactile stimuli can decrease the perception of pain. We have found the reverse effect: Heat at levels that induce pain can substantially suppress tactile sensitivity, independently of shifts in attention or arousal. Ten human observers were stimulated by a tonic, pain-producing heat stimulus and vibrotactile stimuli (1, 10, and 100 Hz) coincidentally presented to the right thenar eminence. ⋯ The changes are not attributable to attentional or arousal shifts, since they were not associated with changes in auditory thresholds. Furthermore, the changes occurred just below the subjects' pain thresholds (where nociceptors are presumably activated). This over-twofold diminution of vibrotactile sensitivity suggests that heat stimuli capable of inducing pain can significantly diminish taction, perhaps through a "touch gate" in a manner similar to the gate control theory proposed for pain.
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Thirty-five touch (M) neurons and 59 vibrotactile (V + M) neurons were recorded intrasomally in the trigeminal ganglion of a crotaline snake (the pit viper, Trimeresurus flavoviridis). The M neurons were excited by von Frey hair (5-10 mg) mechanical stimulation of the receptive field, and adapted slowly to a sustained stimulus. It was almost impossible to elicit 1:1 entrainment to sinusoidal movement. ⋯ Smaller myelinated and unmyelinated collaterals were given off at right angles from the descending fiber of the central axon into the TTD. They projected more densely to the rostral part than to the caudal part of the TTD. All of these data were compared with data on warm-temperature neurons, previously obtained.
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The aim of the present work was to disclose, through pharmacological activation of an isolated central nervous system maintained in vitro, spinal locomotor and respiratory-like activities inferred from an in vivo rabbit preparation. In a brainstem-spinal cord preparation in neonatal rats (0-3 days old), medullary respiratory activity occurred spontaneously in the cervical ventral roots. During 5-hydroxytryptophan (5-HTP) superfusion (0.2 mM), a slower rhythm with longer burst duration developed in the same ventral roots, with the pre-existing long-lasting slow bursting (LLSB) activity. ⋯ These results strongly suggest that the LLSB activity that originates from cervical generators belongs to the respiratory system, and not to locomotor activity. Finally, similar results in an in vivo rabbit preparation have been obtained through pharmacological activation. This preparation appears to be a suitable model for the analysis of this cervical burst generator and for the study of interactions among the different pattern generators.