On the Functional Significance of Long Monosynaptic Descending Pathways to Spinal Motoneurones

Abstract

Studies in behaving animals show that the activities of neurones with direct long descending connections to motoneurones, such as corticomotoneuronal cells to hand muscles in the primate, often do not co-vary with the activities of the target muscles. Explanations for this include the operation of populations of neurones involving many input cells with different patterns affecting many motoneurones of different muscles, estimated via network analysis (Fetz, 1992), or the effects of other pathways setting recruitment patterns at motoneuronal level (Bennett & Lemon, 1994). The pathways that include these long monosynaptic connections are accorded a great deal of importance in controlling the movements concerned and it seems to be most often assumed that it is the excitation derived via this direct link that is the most important part of the control. In contrast we present here results from another system which shows an extreme case of lack of co-variance, and we argue from these that rather more importance should be given to events at spinal level in producing the patterns of motoneurone output.