Spinal Cord Plasticity - History And Concepts

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SPINAL CORD PLASTICITY - HISTORY AND CONCEPTS

Howard Vernon, DC, FCCS(C)

Canadian Memorial Chiropractic College

1900 Bayview Avenue, Toronto

The phenomenon of referred pain from spinal origins has been described since the later 1800's with the work of McKenzie and Head. In the early 1900's, Goldthwait, as well as early chiropractic authors expanded on the concept. One of the prevailing theories advanced for this phenomenon involved "hyperactivity of the spinal cord". This theory culminated in the work of Korr et al. in the 1940's-50's, in the form of the concept of "central facilitation". Both osteopathic and chiropractic experts have adopted this as the model of "subluxation -- induced" physiopathology.

Notwithstanding these developments, modern neuroscientists have only just begun to study the phenomenon of deep somatic pain (as distinguished from cutaneous pain), and only very recently, as with the work of Wall and Woolf in 1984, have the central effects of deep noxious pain on spinal dorsal horn neurones begun to be elucidated. Several findings of the recent decade compel us to consider a much greater degree of complexity of spinal cord processing of deep pain transmission. For instance, areas of experimentally-induced hyperalgesia remain resistant to peripheral analgesics for significant periods of time. Coderre et al. have found that mustard oil-induced pain injury creates signs of contralateral hyperalgesia (reduced latency for withdrawal reflex) which persists even when peripheral nerves on the injured side are transected. Wall and Woolf showed that dorsal horn neurones remained excited for much longer periods after C-fibre stimulation from muscle and joint nerves as compared to cutaneous nerves.

Woolf has proposed a theoretical model for these effects known as "central sensitization". These changes in the spinal dorsal horn are characteristic of neuroplastic changes in the presence of severe and/or prolonged noxious input from deep pain sources. This model is highly reminiscent of Korr's original concept.

The consensus of studies reporting this phenomenon includes the following phenomena: Wind-up and sensitization of dorsal horn neurones; lowering of their thresholds for activation; unmasking of latent, "silent" synaptic connections; facilitation of convergent inputs; expansion of their receptive fields; and long-lasting discharge activity (ectopic, oscillating) post-stimulus.

We are compelled to ask if this model, as an outgrowth of the earlier one proposed by Korr, is not applicable in explaining subluxation-induced effects? If so, is this model comprehensive in it's inclusion of motor and autonomic effects as was the earlier model? The answers to these and other questions pose the great challenges to our current researchers.

The Canadian Chiropractic Association.

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By Howard Vernon

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