Plastic Spinal Motor Circuits in Health and Disease

In former times, the spinal cord was considered a hard-wired network for spinal reflexes and a conduit for long-range connections. This view has changed dramatically over the past few decades. It is now recognized as a plastic device whose structures and functions adapt to changing circumstances. While such changes also occur under physiological conditions, the most dramatic alterations take place during or after various pathological events. It is astonishing what mechanisms the musculo-skeletal system has evolved to come to grips with the damages. Many of these changes are maladaptive, but some appear to help adapt to the new conditions. Although myriads of studies, using manifold methods, have been devoted to elucidating the underlying mechanisms, in humans and animal models, the etiology and pathophysiology of various diseases are still little understood, due to a number of reasons. We will here try to summarize some results and remaining problems in a selection of diseases, in particular spinal muscular atrophy (SMA), amyotrophic laterals sclerosis (ALS), and predominantly spinal cord injury (SCI) with occasional relations to stroke. Especially the changes in SCI (and stroke) depend on the cause, site and extent of the afflicted damage and are therefore multifarious. At the end, we will briefly summarize results indicating that operant, classical and instrumental conditioning can be used to produce plastic changes in healthy people, with potentials for applications to patients with spinal cord injury. In order not to overload the article, we will not delve deeply into sub-cellular processes.