Journal of neurophysiology
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Extended suprathreshold vibratory stimulation applied to the skin results in a desensitization of cutaneous mechanoreceptive afferents. In a companion paper, we describe the dependence of the threshold shift on the parameters of the adapting stimulus and discuss neural mechanisms underlying afferent adaptation. Here we describe the time-course of afferent adaptation and recovery. ⋯ We also showed that threshold adaptation is accompanied by a shift in the timing of the spikes within individual cycles of the adapting stimulus (i.e., a shift in the impulse phase). We invoked an integrate-and-fire model to explore possible mechanisms underlying afferent adaptation. Finally, we found that the time-course of afferent adaptation is more rapid than that of its psychophysical counterpart, as is the time-course of recovery from adaptation, suggesting that central factors play a role in the psychophysical phenomenon.
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Comparative Study Clinical Trial
Evoked H-reflex and V-wave responses during maximal isometric, concentric, and eccentric muscle contraction.
This study was designed to investigate the modulations of H-reflex and V-wave responses during passive and maximal active dynamic actions. Experiments were performed on 16 healthy males [age: 24 +/- 4 (SD) yr]. Maximal H-reflexes (Hmax) and M-waves (MmaxR) were evoked at the same muscle length during passive isometric, shortening and lengthening actions and during maximal voluntary isometric, concentric, and eccentric plantar-flexion. ⋯ No difference was observed between V/MmaxA and Hsup/MmaxA ratios during eccentric efforts. The H-reflex modulations, present during lengthening actions, were mainly attributed to presynaptic inhibition of Ia afferents and to homosynaptic postactivation depression. Results on V wave and H reflex suggest that during eccentric MVC, the spinal loop is specifically modulated by the supra-spinal centers and/or neural mechanisms at spinal level.
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Lamina I is a sensory relay region containing projection cells and local interneurons involved in thermal and nociceptive signaling. These neurons differ in morphology, sensory response modality, and firing characteristics. We examined intrinsic properties of mouse lamina I GABAergic neurons expressing enhanced green fluorescent protein (EGFP). ⋯ Single spike cells were less excitable with lower membrane resistivity and higher rheobase. Most fusiform cells (64%) fired tonically while most multipolar cells (56%) fired single spikes. In summary, lamina I inhibitory interneurons are functionally divisible into at least two major groups both of which presumably function to limit excitatory transmission.
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Comparative Study
Kinematic and dynamic synergies of human precision-grip movements.
We analyzed the adaptability of human thumb and index finger movement kinematics and dynamics to variations of precision grip aperture and movement velocity. Six subjects performed precision grip opening and closing movements under different conditions of movement velocity and movement aperture (thumb and index finger tip-to-tip distance). Angular motion of the thumb and index finger joints was recorded with a CyberGlove and a three-dimensional biomechanical model was used for solving the inverse dynamics problem during precision grip movements, i.e., for calculating joint torques from experimentally obtained angular variations. ⋯ However, the adaptation to large aperture, achieved by an increase of the relative contribution of the proximal joints, was subject-invariant. At the level of movement dynamics, the adaptation of thumb-index finger movements to task constraints was similar among all subjects and required the linear scaling of joint torques, the synchronization of joint torques under high velocity conditions, and a flexible redistribution of joint torques between the proximal joint of the thumb and that of the index finger. This work represents one of the first attempts at calculating the joint torques during human precision-grip movements and indicates that the dynamic synergies seem to be remarkably simple compared with the synergies found for movement kinematics.