The spinal cord and its brain: representations and models. To what extent do forebrain mechanisms appear at brainstem...

This review takes up and updates some major issues dealt with in the book "How brain-like is the spinal cord?" (Windhorst, U. (1988) How Brain-like is the Spinal Cord? Interacting Cell Assemblies in the Nervous System. Springer: Berlin). 1. The main subject dealt with in the book was on the code used by the central nervous system (CNS) to represent and convey information. Specifically, it was proposed that, apart from the generally accepted frequency code (rate of cell firing) and others, information is also encoded in the synchronized or otherwise correlated firing of assemblies of neurons. (ii) Since the temporary establishment of cell assemblies requires mechanisms of flexible neuronal interactions, the second important topic was neuronal plasticity and memory. In fact, plasticity is at the heart of the "population code" used for representation by neuronal assemblies. 2. The third topic was loosely related to a specific representation in the CNS, namely that by so-called "internal models" that represent aspects of the CNS' peripheral environment. 3. It was argued that the above mechanisms of representation and operation are used not only by higher, e.g. cortical, CNS structures, but also by the spinal cord. These topics are here dealt with in turn, always with regard to the special question as to what extent forebrain mechanisms appear at brainstem and spinal cord levels. There is accumulating evidence that in fact the spinal cord uses ensemble processing, synaptic plasticity and internal models to represent and handle its external environment.