Central versus peripheral aspects of neuromuscular regionalization

the secondary spindle afferents. It has been shown (Binder et al. 1976; Binder & Stuart 1980a) that not only group Iaafferents but also spindle group 11 derents are responsive to a distinctive set of motor units in a muscle. Windhorst and Meyer-Lohmann (1977) suggest that synchrony in neighboring spindles is caused not only by activity in specific motor units, but perhaps also by shared gamma input. It is worth noting that spindle group 11 derents seem to have powerful projections, particularly to dynamic y-motoneurones (see Appelberg et al. 1983; 1986; Johansson 1981; 1985; 1988) and to homonymous as well as heteronymous muscles. I t still remains to be investigated whether the central projections of the group I1 spindle afferents are strongest to the y-motoneurones that innervate the regions of the muscles accommodating the receptors. The group I1 projection to y-motoneurones may constitute a powerful positive feedback circuit with the capacity to enhance the partitioning. The role of the fusimotor system in the partitioning hypothesis needs to be thoroughly examined, particularly since the control of the fusimotor system can hardly be disregarded in a discussion of the function of the Ia svstem and of localized stretch reflexes. It seems natural to view the y-motor spindle system, with its sophisticated descending and reflex control, as a functional unit.