Tapping in to the intelligent cord

The intelligent cord has circuits that independently coordinate and monitor information coming in from the body and generate appropriate outgoing signals to muscles, particularly during actions such as walking. The concept of a so-called “central pattern generator”, or CPGs, has been around for a long time and was demonstrated in animals very convincingly many years ago. It is now accepted that humans have CPGs in regions of the cord involved in walking and rhythmic generators are proposed for other functions as well (see presentation 74.18, for example].

The brain obviously has executive control but this is often lost after SCI resulting in paralysis. But more recent evidence in the lab and clinic suggests with a little local priming these rhythmic functions can be restored to some extent.

One demonstrated way of priming CPGs is with electrical stimulation. This can be achieved in a number of ways but each has its own benefits and drawbacks. Inserting fine electrodes into the cord tissue itself (intraspinal microstimulation – ISMS) offers the potential to more precisely stimulate groups of neurons controlling muscles, the drawback being it is invasive to do so and often requires more accurate placement of multiple stimulating electrodes.  Epidural stimulation, where the surface of the cord is stimulated, is less invasive and entirely feasible due to the fact that epidural stimulation is used routinely for example in pain management. The stimulation is less precise and one might suspect that this would cause uncoordinated stimulation of all muscles but it doesn’t if done at a level just enough to prime but not activate. And an increasing number of labs are looking at stimulation with electrodes placed on the skin – transcutaneous stim – which is obviously the least invasive option but has the real potential to stimulate both incoming (sensory) and out-going (motor) signals.

As you move from ISMS to epidural to transcutaneous stimulation the stimulating energy needed greatly increases. A number of posters I saw today examined these techniques and a couple are worth a mention. PJ Grahn [presentation 74.02] compared results from ISMS and epidural stimulation in a pig model. M Krenn [presentation 74.03] looked at how selective one can be using transcutaneous stimulation as they tried to stimulate the sensory fibres coming up from muscles in the leg to enhance reflexes in humans.