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Digitale Medien

On the mechanism of the post-activation depression of the H-reflex in human subjects (1996)

Hultborn, H. ; Illert, M. ; Nielsen, J. ; [weitere]

Springer

Experimental brain research 108 (1996), S. 450-462

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ISSN: 1432-1106

Schlagwort(e): Spinal cord ; H-reflexes ; Ia afferents ; Reflex pathways ; Homosynaptic depression ; Human ; Cat

Quelle: Springer Online Journal Archives 1860-2000

Thema: Medizin

Notizen: 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.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF00227268

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Digitale Medien

Limitations of clearance determination using the single sample distribution volume method (1992)

Kotzerke, J. ; Burchert, W. ; Wiese, H. ; [weitere]

Springer

European journal of nuclear medicine 19 (1992), S. 19-24

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ISSN: 1619-7089

Schlagwort(e): Kidney function ; Kidney radionuclide studies ; Effective renal plasma flow (ERPF) ; Tauxe's method ; Error calculation

Quelle: Springer Online Journal Archives 1860-2000

Thema: Medizin

Notizen: Abstract To investigate the error possibly contained in the single sample distribution volume method for the determination of renal clearance, a mathematical model was applied to describe the effect of changes in distribution volume, clearance, intravascular space, intracompartmental exchange and the time point of blood sampling. The method was found to be valid only under well-defined circumstances (Topt = 45 ± 5 min, Cl = 390 ± 50 ml/min, Vd = 16.7 ± 41, α = 0.05 · V1 ml/min, V1/Vd = 0.5 ± 0.05) as shown in sample calculations. Two-compartment model-based error calculations demonstrate that this technique implies at best an uncertainty of ± 10% or more. Whilst it can be used preferably in healthy, normal-weight adults, it is not applicable, without error, under all other circumstances.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF00178303

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