Assessing changes in presynaptic inhibition of Ia afferents during movement in humans

Different methods, based on different principles, have been proposed to estimate changes in presynaptic inhibition of Ia terminals (accompanied by primary afferent depolarization, (PAD)) during voluntary contraction in humans. (i) A discrepancy between the H-reflex amplitude, at an equal level of EMG activity, in two situations (e.g., walking and standing) may be taken as suggesting a different control of PAD interneurones in the two cases. (ii) A conditioning stimulation (vibration or electrical stimulation) is used to activate PAD interneurones and to evoke presynaptic inhibition of the afferent volley of the test reflex. The resulting long-lasting depression of the reflex depends on the excitability of PAD interneurones, but can be contaminated by long-lasting post-synaptic effects. (iii) The amount of reflex facilitation evoked by a purely monosynaptic Ia volley varies inversely with the on-going presynaptic inhibition of Ia afferents mediating the conditioning volley, and can be used to assess this on-going presynaptic inhibition. None of these methods can provide by itself unequivocal evidence for a change in presynaptic inhibition of Ia terminals, but reasonably reliable interpretations may be proposed when congruent results are obtained with different methods. Thus it has been shown that, during selective voluntary contraction, presynaptic inhibition is decreased on Ia afferents projecting on motoneurones of the contracting muscle and increased on Ia afferents projecting on motor nuclei not involved in the contraction.