Does the right side know what the left is doing?

Following peripheral-nerve lesions there are well-documented events that af fect the contralateral nonlesioned structures. These contralateral effects are qualitatively similar to those occurring at the ipsilateral side, but a re usually smaller in magnitude and have a briefer time course. It is uncle ar whether the findings are an epiphenomenon or serve a biological purpose, but in either case the existence of these effects implies the presence of unrecognized signalling mechanisms that: link the two sides of the body. St rong circumstantial evidence argues against a peripheral mechanism (for exa mple, via circulating factors) and in favour of a central mechanism, in par ticular signalling via the system of commissural interneurons that is prese nt in spinal cord and brainstem. While an altered pattern of activity in th is system might underlie the phenomenon, there are several reasons for prop osing that the changes depend upon chemical signals, possibly growth factor s. Because of its relative easy access for experimental manipulation, the s pinal cord could serve as a model system to study these transmedian signall ing systems.