Organization of input to the interneurones mediating group I non-reciprocal inhibition of motoneurones in the cat.

Patterns of convergence of different presynaptic fibre types onto interneurones mediating non-reciprocal inhibition of motoneurones have been studied in order to investigate to what extent the population of these interneurones is homogeneous or can be divided into subgroups on the basis of their input. In a sample of interneurones, all of which were interposed in pathways from the group I afferents of one group of muscles (triceps surae and plantaris), individual interneurones exhibited a wide variety of convergence patterns. Some interneurones were influenced by only a few types of afferent or descending fibre systems whereas others were influenced by many. Furthermore, various fibre systems excited and/or inhibited individual interneurones in different combinations. While there appeared to be too many patterns of convergence to allow any simple classification into a few distinct groups of interneurones, two possibilities were considered. One was that certain presynaptic fibre types influence individual interneurones in preferred combinations. The other was that they converge entirely at random. To investigate this, the frequencies of convergence of various pairs of fibre types were predicted assuming that each of them influences a proportion of the interneurones independently of other sources. Generally, there was close correspondence between such predicted and observed frequencies of occurrence of tested combinations of input. These findings are thus compatible with an organization whereby individual presynaptic fibres innervate a random sample of the population of interneurones. Deviations from the predicted incidence of convergence patterns were found primarily for synaptic actions mediated di- or oligosynaptically and are attributed to a consequence of convergence at the pre-interneuronal level. A particular consequence of such an organization is that interneurones in pathways of non-reciprocal inhibition are shared by afferents of different muscles in a continuum of combinations. The functional implications of this arrangement are discussed.