The Synaptic Linkage of ‘Direct’ Inhibition.

The establishment, of the monosynaptic EPSP and the direct IPSP in relation to the I a volley has been compared with intra-cellular recording from motoneurones. The EPSP's appear at threshold stimulation of the I a afferents whereas an appreciable fraction of the I a afferents must be activated to evoke minimal observable direct IPSP's. In some motoneurones a maximal I a volley from the nerve to an antagonist muscle may fail to produce a direct IPSP, but do so after post-tetanic activation. With a nerve divided into many branches a maximal group I volley in any of them may not produce a direct, IPSP in the moto-neurone to an antagonist muscle whereas a synchronous volley in all of them evokes a large IPSP. Occasionally evidence has been found suggesting that occlusion my also occur in the inhibitory pathway.