Conditioning the stimulation of group I afferents from flexor muscles depresses the monosynaptic excitatory postsynaptic potentials (EPSPs) elicited in spinal motoneurons by the stimulation of muscle spindle afferents (Ia afferents), without changing the membrane properties of the motoneurons or the time course of the Ia-EPSPs. The chapter determines the extent to which the depression of Ia EPSPs is associated with changes in motoneuron-membrane properties. The presynaptic and postsynaptic inhibitions are mutually exclusive. There is now good evidence that presynaptic inhibition is mediated by gamma-aminobutyric acid (GABA)ergic interneurons that make axo-axonic synapses with the intraspinal terminals of the afferent fibers. These GABAergic interneurons also have monosynaptic inhibitory connections with motoneurons. Direct evidence for the existence of GABAergic boutons that make synapses with identified muscle-spindle afferents as well as with the postsynaptic neurons is available. © 1994, Elsevier Science Publishers, B.V.
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