SPINAL SENSORIMOTOR SYSTEMS AND THEIR SUPRASPINAL CONTROL

Spinal motor systems receive a convergent input from descending tracts and peripheral afferents. As a rule, there is multiform descending convergence from different supraspinal centers and wide multisensorial convergence from different peripheral afferents. The balance between the descending and segmental input may vary between different systems and different cells within a system. The different spinal motor systems do not consist of homogeneous interneuronal populations, but of different interneuronal subgroups with different patterns of descending and peripheral afferent convergence and with different patterns of projection to different target motoneurons. The basic function of spinal motor systems in motor control is to initiate and organize the course of movements. Three components of this function may be discerned: (a) Transmission and coordination of descending information for the performance of centrally induced movements and their adaptation to peripheral conditions. Different subgroups of interneurons may be selected for different motor tasks. (b) Transformation of afferent sensory information to motor patterns. These 'spinal reflexes' are not stereotyped responses to a given sensory input. Their amplitude and direction depend on the descending and segmental conditions and, accordingly, the utilization of different available pathways. (c) Endogenous generation and/or participation in the performance of complex movement patterns (e.g. spinal locomotion, scratching and possibly breathing). The three functions are given in the order of their presumed importance for motor control in higher vertebrates. Under natural conditions they are interwoven and complement each other. © 1990.