Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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The influence of stimulus temperature rise rate (2.5 degrees C/s, 5.0 degrees C/s, and 10.0 degrees C/s), adapting (baseline) temperature (25 degrees C, 30 degrees C, and 35 degrees C), and duration of peak stimulus temperature (1.0 s, 2.5 s, 5.0 s, and 10.0 s) on responses evoked by noxious heat stimuli of suprathreshold intensity was studied in wide dynamic range (WDR) neurons of the rat spinal dorsal horn. The spinal neuronal responses were compared with human psychophysical data obtained using the same stimuli. Noxious heat stimuli with a peak temperature of 54 degrees C were applied with a contact thermostimulator to the glabrous skin of the hindfoot in rats or to the palmar skin in humans. ⋯ The changes in the total impulse counts evoked by varying supraliminal heat stimuli in spinal dorsal horn WDR neurons corresponded well with the changes in pain magnitude estimates in humans. Also, the changes in spinal neuronal response onset latencies were accompanied by corresponding changes in onset latencies of human pain reactions but not with pain magnitude estimates. The effect of spinalization indicated that descending pathways control not only the response magnitude in the spinal dorsal horn WDR neurons but also the temporal characteristics of the spinal neuronal response.
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Anticipatory postural adjustments (APA) are needed to perform a movement without perturbing posture. We investigated the development of APA in 3- to 4-year-old children during a bimanual load-lifting task. The task required maintaining a stable elbow position despite imposed or voluntary unloading of the forearm. ⋯ In addition, children showed high intra-individual variability in the voluntary situation, revealed by the coexistence of both adult-like and immature patterns in kinematic and electromyographic data. In conclusion, the present study reports that APA, associated with a bimanual load-lifting task, are still being set up in 3- to 4-year-old-children. The intra-individual variability should decrease with age and be associated with a progressive mastering of the timing parameters characterizing APA.
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In 11 healthy subjects motor-evoked potentials (MEPs) and silent periods (SPs) were measured in the right first dorsal interosseus (FDI) and abductor pollicis brevis muscles (APB): (1) when transcranial magnetic cortex stimulation (TMS) was applied at tonic isometric contraction of 20% of maximum force, (2) when TMS was applied during tactile exploration of a small object in the hand, (3) when TMS was applied during visually guided goal-directed isometric ramp and hold finger flexion movements, and (4) when at tonic isometric contraction peripheral electrical stimulation (PES) of the median nerve was delivered at various intervals between PES and TMS. Of the natural motor tasks, duration of SPs of small hand muscles was longest during tactile exploration (APB 205+/-42 ms; FDI 213+/-47 ms). SP duration at tonic isometric contraction amounted to 172+/-35 ms in APB and 178+/-31 ms in FDI, respectively. ⋯ Amplitudes of MEPs did not change significantly in any condition. Inhibitory control of motor output circuitries seems to be distinctly modulated by peripheral somatosensory and visual afferent information. We conclude that somatosensory information has privileged access to inhibitory interneuronal circuits within the primary motor cortex.
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Clinical Trial
Recordings of polymodal single c-fiber nociceptive afferents following mechanical and argon-laser heat stimulation of human skin.
Recordings were made in the peroneal nerve of healthy volunteer subjects from C-mechano-heat (CMH) nociceptors (n=25) with their receptive fields in the skin on the dorsum of the foot. The investigation focused on afferent single C-fiber activity induced by short (200 ms) high-intensity argon-laser light pulses projected to localized spots of the skin. Cutaneous heat stimulation with the argon laser, 2-3 times the activation threshold, induced inter-burst spike frequencies in the nerve, reaching 50 Hz, while mechanical stimulation 10-20 times threshold only evoked frequencies reaching 10 Hz. ⋯ However, the magnitude of the reported pain sensation to comparable degrees of C-fiber activation showed a high variability between different subjects. A fairly good subjective estimate of the afferent-fiber activation was observed when skin spots from 3- down to 1-mm diameter were stimulated. In a few individuals, no painful sensation was reported when the stimulated spots were reduced to 1-mm diameter, despite the occurrence of multiple spikes in single C-fiber afferents, amplifying the importance of spatial summation in the perception of pain.
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Interactions between proprioceptive and vestibular inputs contributing to the generation of balance corrections may vary across muscles depending on the availability of sensory information at centres initiating and modulating muscle synergies, and the efficacy with which the muscle action can prevent a fall. Information which is not available from one sensory system may be obtained by switching to another. Alternatively, interactions between sensory systems and the muscle to which this interaction is targeted may be fixed during neural development and not switchable. ⋯ The disparity between the amplitudes of stretch reflex and automatic balance-correcting responses in triceps surae and the insignificant alteration in the timing of balance-correcting responses in these muscles with nulled ankle inputs indicates that ankle inputs do not trigger balance corrections. Furthermore, modulation of balance corrections normally performed by vestibular inputs in some but not all muscles is not achieved by switching to another sensory system on vestibular loss. We postulate that a confluence of trunk and upper-leg proprioceptive input establishes the basic timing of automatic, triggered balance corrections which is then preferentially weighted by vestibular modulation in muscles that prevent falling. (ABSTRACT TRUNCATED)