Motor primitives in the spinal cord as a basis for motor learning and action

Recent work in frog and rat spinal cord has revealed the spinal interneuron systems can be thought of as comprising biomechanical ’primitives’. These primitives could be similar to primitives of computer science in that complex programs of behavior repertoires could be built from them. The biological primitives could thus form a bootstrap or basis set for learned motor behaviors. The concept of primitives used here is closely allied to other ideas in neurophysiology, notably the compounding of reflexes suggested by Sherrington, the building blocks of Getting, and the flexor reflex afferent ideas of Lundberg, Jankowska and colleagues. The biomechanical cohesion of the spinal cord responses that allow them to be viewed as primitives has been revealed by microstimulation of spinal grey matter and by examination of reflex behaviors (Bizzi et al. 1991, Giszter et al. 1993). It has long been known that the spinal frog (i.e. transected so as to be tetraplegic) shows a remarkable capacity for motor behavior despite the absence of descending controls from the brain. The wipe or scratch responses of this preparation show adjustment to body posture, selection and adjustment of response type based on skin location and motor equivalence. This panoply of behavior demonstrates a local capacity of the isolated spinal cord for organizing adjusted action sequences. The basic element of these sequences and adjustments is likely to be a set of motor primitives.