In the past years we studied :
The presynaptic inhibition of Ib afferents from tendon organs during contraction
We demonstrated that during contraction of ankle extensors the different neuronal targets of Ib afferents do not get the same information : for some targets the message is filtered out by presynaptic inhibition (Zytnicki et al. 1990) whereas other targets receive an unfiltered information (Zytnicki et al. 1995). Numerous axo-axonic contacts (the morphological substrate of presynaptic inhibition) impinge on Ib collaterals, mainly on the terminal branches (Lamotte d’Incamps et al. 1998a). Some of them are active during contraction as indicated by intra-axonal recordings of primary afferent depolarizations (PADs) (Lafleur et al. 1992). Numerical simulations of a compartmental model of Ib collateral suggest that PAD is likely to be by itself responsible for presynaptic inhibition (Lamotte d’Incamps et al. 1998b). The electrotonic structure of Ib collaterals and the distribution of axo-axonic contacts on terminal branches account for the spatial processing of Ib information by presynaptic inhibition (Lamotte d’Incamps et al. 1999).
Methods : in vivo intracellular recordings of lumbar motoneurons and neurons of the dorsal spino-cerebellar tract during muscle contraction, intra-axonal recording during contraction of identified Ib afferent, intra-axonal labelling of Ib afferent, immunohistochemistry, confocal microscopy, compartmental modeling of myelinated fiber
The positive feedback elicited on homonymous motoneurons by proprioceptive input during contraction
We demonstrated that proprioceptive inputs elicited by isometric contractions of pretibial flexors induce excitatory potentials (positive feedback) in homonymous motoneurons. This positive feedback is due to the passive mechanical activation of spindle primary endings and to the lack of Ib inhibition (Brizzi et al. 2002). Contraction of Peroneus brevis elicits a similar feedback in peroneal motoneurons (Kouchtir et al. 1995). In that case spindle activation is facilitated by the action of squeletto-fusimotor axons. Mechanical activation of foot cutaneous receptors has complex actions on peroneal motoneurons (Perrier et al. 2000a) and can control the efficacy of the contraction-induced positive feedback (Perrier et al. 2000b).
Methods : in vivo intracellular recordings of motoneurons
Recent work using the dynamic clamp method in vivo :
Increasing by 50 to 100% the input conductance of a lumbar motoneuron does not change the slope of the current - frequency relationship (I-f curve). Shunting inhibition shifts the I-f curve as would a hyperpolarizing current. The shift is proportional to the conductance added at the soma with the dynamic clamp and depends on an intrinsic property of the motoneuron, its shunt potential. This quantity indicates how sensitive is the motoneuron to shunting inhibition (Brizzi et al. submitted).
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Dernière mise à jour le mercredi 18 novembre 2009