Can sense be made of spinal interneuron circuits?

Abstract

It is increasingly clear that spinal reflex systems cannot be described in terms of simple and constant reflex actions. The extensive convergence of segmental and descending systems onto spinal interneurons suggests that spinal interneurons are not relay systems but rather form a crucial component in determining which muscles are activated during voluntary and reflex movements. The notion that descending systems simply modulate the gain of spinal interneuronal pathways has been tempered by the observation that spinal interneurons gate and distribute descending control to specific motoneurons. Spinal reflex systems are complex but current approaches will continue to provide insight into motor systems. During movement, several neural mechanisms act to reduce the functional complexity of motor systems by inhibiting some of the parallel reflex pathways available to segmental afferents and descending systems. The flexion reflex system is discussed as an example of the flexibility of spinal interneuron systems and as a useful conceptual construct. Examples are provided of the kinds of experiments that can be developed using current approaches to spinal interneuronal systems.

Energy, information, detection, and action

“…Stoffregen & Bardy (S&B) could suggest that adequate position perception (kinesthesia) results from some integration (a "global array") of spindle signals with those of other sensory systems. This would be consistent with the evidence that perception of position is based on signals from muscle, joint, and skin afferents (Gandevia & Burke 1992) and that these signals are integrated at the level of neurons of ascending tracts (Arshavsky et al 1985;Lundberg 1975;McCrea 1992). However, it has been shown that neurons integrating these afferent signals also receive independent signals from central pattern generators (e.g., Arshavsky et al 1985).…”

Organization of mammalian locomotor rhythm and pattern generation

“…There is general consensus that descending control and proprioceptive regulation of many motor behaviors involves activation of groups of muscles (synergies) performing elementary movements rather than activation of individual muscles (Bernstein, 1967;Lundberg et al, 1987;McCrea, 1992;Bizzi et al 2000). This suggests that the spinal cord should contain networks of interneurons each of which activates select motoneuron populations to produce a particular muscle synergy.…”

“…It would make eminent sense for such interneurons to be major targets for descending control and proprioceptive regulation and to be part of the substrate for muscle synergies. Local inhibitory interactions among these interneurons may allow descending signals to dynamically select the most appropriate behavior from the available repertoire (McCrea, 1992). We suggest that these networks can be engaged in motor tasks other than locomotion.…”

The control of multi-joint movements relies on detailed internal representations

“…This view has not received much empirical support. On the contrary: the spinal cord appears to contain a rich assembly of interneuron circuits, crucial not only for reflex organisation but also used in the organisation of undisturbed movements (see McCrea, 1992, for review and references). Secondly, though we support his argument that human motor behaviour is considerably richer than a collection of condi tioned reflexes, we feel that his statement that skill acquisition has nothing to do with " beating" neural connections in the brain cannot hold.…”