Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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Brief radiant heat pulses, generated by a CO2 laser, were used to activate slowly conducting afferents in the hairy skin in man. In order to isolate C-fibre responses a preferential A-fibre block was applied by pressure to the radial nerve at the wrist. Stimulus estimation and evoked cerebral potentials (EP), as well as reaction times, motor and sudomotor activity were recorded in response to each stimulus. ⋯ Latency corrected averaging with a modified Woody filter yielded a grand mean ultralate EP (N1050/P1250), the shape of which was surprisingly similar to the late EP (N240/P370). The similarity of these components indicates that both EPs may be secondary responses to afferent input into neural centers, onto which myelinated and unmyelinated fibres converge. Such convergence may also explain through the known mechanisms of short term habituation and selective attention, why ultralate EPs are not reliably present without peripheral nerve block.
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Responses were recorded from 160 ascending tract cells in segments L4 to L6 of the spinal cord in chloralose anaesthetized, spinalized cats. The tract cells were identified by antidromic activation following stimulation of pathways in the lateral and ventral funiculi at the level of the spinal cord transection at the thoracolumbar junction. Axonal conduction velocities ranged from 9 to 114 m/s. ⋯ The deep receptive fields were chiefly in the muscles of the thigh and/or leg. For 6 "dj" cells, the receptive fields included not only the knee joint but also deep fields like those of "sdj" cells. Cutaneous receptive fields were classified as "low threshold" (cells excited best by innocuous intensities of mechanical stimulation), "wide dynamic range" (cells activated by weak mechanical stimuli, but the best responses were to noxious stimuli) or high threshold (innocuous stimuli had little effect, but noxious mechanical stimuli produced a vigorous discharge).(ABSTRACT TRUNCATED AT 400 WORDS)
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This study analyzed neuronal encoding and response thresholds to thermal stimuli at the ventro-basal (V. B.) thalamus level during a hyperalgesic inflammation induced by intra-plantar injection of carrageenin in the rat. The threshold and the encoding capacity of the cells were studied during two phases of the inflammatory process, namely the "acute" phase (the first two hours following the injection), and "sub-acute" phase 24 h after). ⋯ By contrast to the acute phase, the two groups of neurones exhibited only a weak ability to encode the stimulus intensity especially when the stimulus was applied to the inflamed paw. Both peripheral and central mechanisms are likely to be involved in the modifications of response threshold and encoding capacity at the VB thalamus level seen in these conditions of hyperalgesic inflammation. The differential time course of the responses to a liminal or to a supra-liminal temperature during the inflammation, are discussed in reference to some of the mismatches occurring in clinical situations of hyperalgesia.
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The functional role of opposing muscles in the production of isometric force trajectories was studied in six adult subjects producing impulses and steps of elbow flexor force, with different rise times and amplitudes. Rapidly rising forces were invariably associated with an alternating pattern of EMG activity in agonist and antagonist muscles: an agonist burst (AG1) initiated the development of force in the desired direction while a reciprocal burst in the antagonist (ANT-R) led to the deceleration of the force trajectory prior to the peak force. The temporal pattern of agonist and antagonist activation was dependent on force rise time. ⋯ Rather it served to truncate the rising force when very brief rise times were required, thus compensating for the low-pass filter properties of the agonist muscle. Subjects were able to voluntarily suppress ANT-R in rapidly accelerated force trajectories, indicating that the linkage between the commands controlling agonist and antagonist is not obligatory; however AG1 was then prolonged. Our findings emphasize that neuronal commands to opposing muscles acting at a joint must be adapted to constraints imposed by the properties of the neuromuscular plant.
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The present report examines the control strategy adopted by subjects to modulate the amplitude of transient force responses aimed to a target. Previous studies (Freund and Budingen 1978; Ghez and Vicario 1978) suggest that subjects modulate the rate of rise of force while maintaining force rise time at a near-constant value, independent of peak force. Such studies, however, have examined only the most rapid responses where force rise time could have been at a physiological limit. ⋯ Thus, when subjects were attempting to be as accurate as possible, they more consistently regulated force rise time around a constant value. This pulse height control policy allows responses of different amplitudes to be produced by proportional scaling of a stereotyped waveform. We conclude that a pulse height control policy with regulation of force rise time is a strategy adopted by subjects to simplify accurate control of response amplitude.