title: Modular organisation of the rodent cerebral cortex
Creators: name: Wallace, Mark Nelson
Publisher: name: University of Glasgow
Year: 1982

description: Postmortem phosphorylase a activity has been used in muscle as a histochemical marker of metabolic changes produced by electrical stimulation. Therefore it seemed worth investigating the possibility that, in the brain, postmortem levels of phosphorylase a might be linked to functional activity. In the present study, it was found that in the fore-brains of mice, within 10 sec. of decapitation, the phosphorylase a levels increased to a maximum. Thereafter the levels of phosphorylase a fell gradually and at varying rates in different parts of the brain. When 5 min was allowed to elapse between decapitation and freezing the brain, the phosphorylase a levels were relatively stable. It was such steady-state levels of phosphorylase a that were studied for the remainder of the project. The first section of the Results presents evidence that consistent differences in the postmortem levels of phosphorylase a could be produced by altering the functional state of the brain prior to sacrifice. To do this, sections were taken from the brains of mice which had been transiently anaesthetised with ethyl chloride vapour immediately before decapitation ("unanaesthetised" mice) and from mice which had been anaesthetised with sodium pentobarbitone for at least 10 min prior to death. In sections from the "unanaesthetised" mice there were relatively high levels of phosphorylase a in layers I, IV and Vb of the somatosensory cortex. In surface parallel sections through layer IV the staining was patchy; the hollows of the whisker barrels were strongly stained whereas the barrel walls were relatively pale. By contrast, the staining in sections from the anaesthetised group was more variable and in most cases the levels of phosphorylase a were higher in the outer three layers of the somatosensory cortex than in the inner three. In sections from three of these mice the phosphorylase a activity in coronal sections was organised into bands of high staining that were oriented at right anples to the surface of the cortex. The bands extended from layer I to layer V. These represent cylinders of high phosphorylase a activity each of which has a diameter of about 250,4711. These cylinders were not in register with the whisker barrels but are arranged at random with respect to them. This provides evidence for the existence cf two separate overlapping systems of modules in the mouse neocortex. Evidence of a columnar organisation was also observed in the primary and secondary visual and auditory areas and in the motor cortex. There was also evidence of a modular organisation in the superior colliculus and striatum and in many parts of the hippocampal region, including parts where there has not been any previous description of modules. In studying a penicillin induced epileptiform focus no evidence was found of any significant changes in phosphorylase a activity associated with either the primary or the secondary foci. This finding was in marked contrast to the results obtained in an experiment where an epileptiform focus was studied with the 14C-2-deoxyglucose technique. Phosphorylase a levels were found to be related to the architectonic organisation of the brain. Thus when surface parallel sections are cut through layer IV of the neocortex, the three primary sensory areas can be distinguished because of their relatively high levels of phosphorylase a activity. As the postmortem phosphorylase a levels are altered by tissue damage, such as that produced by inserting a glass microelectrode, the phosphorylase method allows microelectrode tracks to be studied in relation to the known architectonic organisation. An upper limit to the number of modules in the mouse neocortex and hippocampal formation was estimated at 3000 columns in the neocortex and 800 modules in the hippocampal formation.