Organisation of inputs to spinal interneurone populations

Abstract

In order to use electrical stimulation or biological repair to attempt to alleviate spinal cord dysfunction, a key problem will be how to target the intervention. Since the majority of inputs to spinal motoneurones originate from intrinsic spinal premotor interneurones, these are key targets for interventions that may help restore function. Information on the organisation of these neurones could thus be crucially important to determine how to proceed. Understanding the organisation of spinal interneurones is no easy task. In this article I review evidence of the connectivity of some of the groups of spinal premotor interneurones that have been studied, focusing particularly on whether they form subgroups and how these can be identified.

Figure 1. Schematic diagram of some of the inputs to non-reciprocal inhibitory interneurones and midlumbar interneurones

These diagrams summarise some of the inputs established for group I non-reciprocal inhibitory interneurones (based on Harrison & Jankowska, 1985a), Harrison & Jankowska, 1985b and midlumbar interneurones with group II input (based on Edgley & Jankowska (1987), Davies & Edgley (1994) and other studies (see Jankowska, 1992)). Excitation is represented by lines with a forked end, inhibition by the line ending with a filled circle. Although these diagrams are complex enough, they do not show all connections and do not represent the specific origins of the afferents (specific muscles for proprioceptors, specific skin areas for cutaneous afferents).

Figure 2. Distributions of inputs to midlumbar interneurones

The figure compares the observed patterns of input from group I afferents of different nerves to midlumbar interneurones (open bars) with the patterns expected based on the frequency of occurrence of each individual input if the inputs were distributed randomly among the interneurones (grey bars). Each bar chart compares 2 sources of input and the 3 sets of columns shown represent the proportion of the population (percentage of neurones) with both inputs, the first alone and the second alone. The combinations of inputs compared in each bar chart are shown by the labels at the top and side of the figure. The figure takes the same format as Fig. 2 of Harrison & Jankowska (1985b) and reveals the same pattern: for all possible combinations of inputs the frequency of occurrence of each possible convergence pattern matches closely the pattern expected if the inputs were distributed independently. Abbreviations: Qe, quadriceps group I EPSPs; Qi, quadriceps group I IPSPs; Ge, gastrocnemius-soleus group I EPSPs; Gi, gastrocnemius-soleus group I IPSPs; He, hamstring group I EPSPs; Hi, hamstring group I IPSPs.