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  • 1
    Electronic Resource
    Electronic Resource
    Central facilitation of Ia inhibition during tonic ankle dorsiflexion revealed after blockade of peripheral feedback (1992)
    Springer
    Experimental brain research 88 (1992), S. 651-656 
    Details
    ISSN: 1432-1106
    Keywords: Spinal cord ; Reciprocal inhibition ; Iaafferents ; Reflex pathways ; H-reflexes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Recent studies have reported that no increase of the disynaptic reciprocal inhibition can be observed during tonic voluntary dorsiflexion of the foot as compared to rest, when the size of the control H-reflex is kept constant. Other studies have, however, shown that a voluntary contraction evokes a strong and long-lasting depression of the synaptic transmission from Ia afferents to motoneurones, most likely secondary to activation of these afferents during the contraction (post-activation depression). It was thought that this effect could also interfere with the demonstration of a central facilitation of the reciprocal inhibition during movement. The amount of disynaptic Ia reciprocal inhibition from the pretibial flexors to the soleus H-reflex was therefore estimated in normal human subjects at rest and during voluntary tonic dorsiflexion before, during and after blocking the peripheral feedback from the investigated muscles. It was observed that the reciprocal inhibition measured during dorsiflexion increased during occlusion of the blood supply to the leg, reaching a maximum of inhibition after 30 min of ischaemia. After release of the ischaemia the inhibition gradually decreased to its pre-ischaemic level. It is therefore suggested that the brain facilitates transmission in the Ia disynaptic reciprocal pathway during tonic voluntary dorsiflexion of the foot, but that this facilitation is normally not observed due to a post-activation depression following the peripheral feedback activation during the movement.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00228194
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  • 2
    Electronic Resource
    Electronic Resource
    On the mechanism of the post-activation depression of the H-reflex in human subjects (1996)
    Springer
    Experimental brain research 108 (1996), S. 450-462 
    Details
    ISSN: 1432-1106
    Keywords: Spinal cord ; H-reflexes ; Ia afferents ; Reflex pathways ; Homosynaptic depression ; Human ; Cat
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract It was demonstrated that the soleus H-reflex was depressed for more than 10 s following a preceding passive dorsiflexion of the ankle joint. This depression was caused by activation of large-diameter afferents with receptors located in the leg muscles, as an ischaemic block of large-diameter fibres just below the knee joint abolished the depression, whereas a similar block just proximal to the ankle joint was ineffective. The depression of the H-reflex was not caused by changes in motoneuronal excitability, as motor-evoked potentials by magnetic brain stimulation were not depressed by the same passive dorsiflexion. Therefore it was concluded that the long-lasting depression is due to mechanisms acting at presynaptic level. The transmission of the monosynaptic Ia excitation from the femoral nerve to soleus motoneurones was not depressed by the ankle dorsiflexion. The depression thus seems to be confined to those afferents that were activated by the conditioning dorsiflexion. In parallel experiments on decerebrate cats, more invasive methods have complemented the indirect techniques used in the experiments on human subjects. A similar long-lasting depression of triceps surae monosynaptic reflexes was evoked by a preceding conditioning stimulation of the triceps surae Ia afferents. This depression was accompanied by a reduction of the monosynaptic Ia excitatory postsynaptic potential recorded intracellularly in triceps surae motoneurones, but not by changes in the input resistance or membrane potential in the motoneurones. Stimulation of separate branches within the triceps surae nerve demonstrated that the depression is confined to those afferents that were activated by the conditioning stimulus. This long-lasting depression was not accompanied by a dorsal root potential. It is concluded that the long-lasting depression is probably caused by a presynaptic effect, but different from the “classical” GABAergic presynaptic inhibition which is widely distributed among afferent fibres and accompanied by dorsal root potentials. It is more probably related to the phenomenon of a reduced transmitter release from previously activated fibres, i.e. a homosynaptic post-activation depression. The consequences of this post-activation depression for the interpretation of results on spinal mechanisms during voluntary movements in man are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00227268
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  • 3
    Electronic Resource
    Electronic Resource
    Segmental reflexes and ankle joint stiffness during co-contraction of antagonistic ankle muscles in man (1994)
    Springer
    Experimental brain research 102 (1994), S. 350-358 
    Details
    ISSN: 1432-1106
    Keywords: Co-contraction ; Motoneurones ; Musculus soleus ; Stiffness ; Reflex pathways ; H-reflex ; Stretch reflex ; Human
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The size of soleus H-reflexes and short-latency stretch reflexes was measured at different levels of plantar flexion or co-contraction (simultaneous activation of dorsi- and plantar flexors) in seven healthy subjects. In four of seven subjects the short-latency stretc reflex was smaller during weak co-contraction than during isolated plantar flexion at matched background electromyogram (EMG) levels in the soleus muscle. In three of these four subjects the stretch reflex was larger during strong co-contraction than during plantar flexion, whereas it had the same size during the two tasks in the last subject. In the remaining subjects the stretch reflex either had the same size or was larger at all levels of co-contraction than at similar levels of plantar flexion. In contrast, the H-reflex was found to decrease with co-contraction at all contraction levels in all subjects. The decrease in the reflexes during weak co-contraction might be caused by presynaptic inhibition of Ia afferents. It is unclear why only the H-reflex decreased during strong co-contraction. The stiffness of the ankle joint was measured from the torque increment following the stretch of the plantar flexors divided by the stretch amplitude. In all subjects the total stiffness of the ankle joint was larger during strong co-contraction than during plantar flexion of similar strength. The stiffness was smaller during weak co-contraction than during weak plantar flexion in three out of seven subjects. The medial gastrocnemius muscle was more active at a given level of soleus activity during the co-contraction task than during the isolated plantar flexion task. It is suggested that the increase in the stiffness during co-contraction as compared to isolated plantar flexion was mainly due to the mechanical contribution of the activity in the tibialis anterior and medial gastrocnemius muscles. The decrease in stiffness during weak co-contraction was, in contrast, most likely mainly caused by modulation of reflex stiffness.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00227521
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  • 4
    Electronic Resource
    Electronic Resource
    H-reflexes are smaller in dancers from The Royal Danish Ballet than in well-trained athletes (1993)
    Springer
    European journal of applied physiology 66 (1993), S. 116-121 
    Details
    ISSN: 1439-6327
    Keywords: Ballet dancers ; H-reflexes ; Ia afferents ; Reciprocal inhibition ; Reflex pathways ; Spinal cord ; Training
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The size of the maximalH-reflex (H max) was measured at rest and expressed as a percentage of the maximalM-response (M max) in 17 untrained subjects, 27 moderately trained subjects, 19 well-trained subjects and 7 dancers from the Royal Danish Ballet. TheH max/M max was significantly larger in the moderately and well-trained subjects than in the untrained subjects but smaller in the ballet dancers. It is therefore suggested that both the amount and the type of habitual activity may influence the excitability of spinal reflexes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01427051
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    Paper (German National Licenses)
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