Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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Integration of tactile afferent signals with motor commands is crucial for the performance of purposeful movements such as during manipulation of an object in the hand. To study the somatotopic organization of sensorimotor integration we applied electrical peripheral conditioning stimuli to a digit located near (homotopic stimulation) or distant from (heterotopic stimulation) relaxed or isometrically contracted intrinsic hand muscles at variable time intervals prior to transcranial magnetic stimulation (TMS). Cutaneous stimulation has previously been shown to modulate the amplitude of the motor evoked potential (MEP) and to shorten the duration of the silent period (SP) evoked by TMS. ⋯ In contrast to the SP duration, the MEP size in voluntarily contracted target muscles was unaffected by the location of the conditioning stimulus. The somatotopic gradient of SP shortening was abolished when the two target muscles were simultaneously activated isometrically. Together, our findings suggest that somatotopy of input-output relationships is implemented at both a spinal and a cortical level in the human central nervous system and may also depend on the motor task involved.
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Saccadic accuracy, measured as the ratio of the size of a saccade to the size of the target step that elicits it, i.e., saccade gain, can be altered by jumping the target surreptitiously during the targeting saccade. The gain change produced by this paradigm does not generalize or transfer to saccades of all sizes. Instead, the amount of transfer decreases the more the tested saccade differs in amplitude and direction from that adapted. ⋯ Thus, the mechanism that adapts saccade size can support a robust gain increase for saccades of one size while simultaneously supporting a robust gain decrease for saccades only 13 degrees larger. Furthermore, the presence during adaptation of a non-adapted target step with a size intermediate to the two adapting steps reestablishes a nearly normal gain within only 6.5 degrees of a robust gain increase and decrease. These data indicate that saccadic gain adaptation can set very different gain states for saccades with rather similar vectors.
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The purpose of this study was to measure the changes in levels of nerve growth factor (NGF) in dorsal root ganglia (DRG) and spinal nerves with the aim of investigating the role of NGF in a rat neuropathic pain model. Nerve injuries were made by tight ligation of the left L5 and L6 spinal nerves using 6-0 silk thread in male Sprague-Dawley rats. Before surgery and 1, 3, 5, 7, and 14 days after surgery, tissue samples collected included the L3-6 DRGs bilaterally, segments of the ipsilateral L5-6 spinal nerves proximal and distal to ligation sites, and corresponding sites of the contralateral L3-6 and the ipsilateral L3-4 spinal nerves. ⋯ Unlesioned sites did not show any significant changes in NGF levels. The increase of NGF in distal segments of injured spinal nerves may be due to an accumulation of retrogradely transported NGF. The maintenance of NGF levels in the DRGs that had lost peripheral connections may reflect local synthesis after nerve injury.