Spinal Fixation: Long-Term Alterations in Spinal Reflex Excitability with Altered or Sustained Sensory Inputs

Abstract

The mammalian spinal cord has long been known to contain a rich variety of reflex pathways, in addition to providing the pathways for transmission of neural signals to and from the body and the higher nervous system. For many years, these reflexes were assumed to be essentially unchanging, hard-wired entities that provided the underpinnings for response to stimuli. Sherrington (1905)) noted that reflex activity could be increased or decreased with activation, but that these effects were transient. In the 1960s and 1970s work by investigators such as Thompson (e.g., Thompson and Spencer, 1966) began to study the ability of sensory inputs to the reflex pathways to alter reflex activity. Soon, the work of Kandel (e.g., Carew, Pinsker and Kandel, 1972) and his associates showed in Aplysia that reflex alterations could have a longer-lasting component. This led to the realization that in the mammalian cord there could be longer lasting aspect of reflex excitability alterations. However, this fact had been shown to occur with learning paradigms much earlier. Shurrager and his colleagues (e.g., Shurrager and Culler, 1940) had demonstrated that spinal reflexes responded with increased activity to a classical conditioning paradigm and that the increased responsiveness lasted for hours in spinalized animals. While controversial, these data set the stage for increased interest in the possibility that spinal reflexes were not simply relatively unchanging neural networks left over from the distant past and superceded by increasing encephalization of function in higher animals.