Figure 1. Hypothetical relationships between the dorsal and intermediate zone group II interneurones and GABAergic interneurones mediating presynaptic inhibition of transmission from group II afferents to these interneurones.

The proposal that all the indicated actions of group II afferents are mediated by axon collaterals of the same fibres is based on results of morphological studies that revealed that the same fibres project to the dorsal horn and to the intermediate zone (Fyffe, 1979; Hongo, 1992). Two distinct subpopulations of GABAergic interneurones modulating transmission to dorsal horn and intermediate zone interneurones, respectively, are proposed to account for the differences in the effectiveness and in the time course of presynaptic inhibition in the dorsal horn and in the intermediate zone (Hammar et al. 2002). Dark lines to and from intermediate zone interneurones indicate the disynaptic pathway and lighter lines to and from the dorsal horn and intermediate zone interneurones, the trisynaptic one. Ipsi, ipsilateral motoneurones. For further explanations see text.