The glia-neuronal interaction: Some observations

Abstract

UNTIL some 10 years ago, the functions performed by glial cells in the brain and peripheral nerves lay almost entirely in the realm of speculation. As an example, CAJAL, in an early paper entitled “Anatomical Mechanisms of Thought, Association and Attention”l attributed two different roles to the cortical glia, each dependent upon the motility of glial processes which his extensive microscopic examinations of stained material seemed to him to support. He supposed, on the one hand, that glial processes might insinuate themselves between pre- and post-synaptic elements and by impeding the flow of ‘nervous currents’ there produce mental relaxation and natural or induced sleep. Retraction of the neuroglial pseudopods would allow reestablishment of the synapse and “the brain would pass from the relaxed to the active stage.” These glial contractions, which might occur “automatically or by an act of will” in his scheme, could direct associative processes in specific directions. “The unpredictable turns that associations sometimes take-the fading away of ideas and words, the momentary halting of speech, the obsessive persistence of memory, the repression of an idea or experience as well as all types of erroneous motor reactions and other psychological phenomena can be understood . . . by supposing the neuroglia of the gray matter serve as an insulating and switching mechanism for nervous currents, permitting connections when they are active, and acting as insulators during repose.” His second hypothesis holds that when the perivascular astrocytes contracted, their endfeet attached to the walls of brain capillaries would pull on the wall of the vessel, enlarge its lumen, and thus increase local blood flow. To explain attention, he hypothesized that such contractions were voluntarily initiated in the cortical zone appropriate for the perceptions or recollections in order to produce and sustain the increased metabolism there. Apparently these ideas soon lost their appeal for CAJAL since he does not mention them in the two volumes on neuroanatomy that insure for him a permanent place in the history of science.2 Though SCHLEICH~ restates the glial switchboard idea in a fanciful way, neither of Cajal’s ideas seems to have received much currency nor, to my knowledge has either of them been directly tested experimentally. This despite Cajal’s statement about them at the end of his paper: “Needless to point out, a hypothesis represents a new path, opened up by experiments and observations, and even if it does not immediately reveal the truth, it