Woolsey & Chang (1947) introduced a new technique into the study of the origin of the pyramidal tract when they recorded the arrival of antidromic pyramidal activity, initiated from the medulla oblongata, at the surface of the cortex. The present investigation has largely employed this useful method which has been extended by the insertion of microelectrodes into the cell layers of the cortex. By this means the electrical activity of single cells or groups of cells may be recorded as the cell bodies are depolarized by impulses passing antidromically up their axons. Accurate localization of cells giving rise to the pyramidal tract is thus possible, and the conduction velocities oftheir respective axons may be estimated. Results obtained by this method have been checked by the electrical stimulation of small areas of cortex and recording the consequent orthodromic pyramidal activity in medulla or cord.
METHODSA series of twenty-five adult cats were used for these experiments. The animals were anaesthetized by Dial (Ciba) 0-5 ml./kg intraperitoneally, an additional 0-2-04 ml. of pentobarbitone solution (gr. 1/ml., 'Sagatal', May and Baker) being administered if necessary. After tracheal cannulation, the pyramidal tract was exposed in the medulla or cord by one of the following methods.(1) Ventral approach to the medulla, by removal of larynx, part of pharynx and oesophagus. The longi capitis muscles were then reflected and a window made in the basiocciput with a dental burr. This approach enabled stimulating or recording electrodes to be placed directly in the pyramid with the animal in the lateral position.(2) Dorsal approach to the fourth ventricle by removal of part of the occipital bone and cerebellum. Electrodes could then be inserted into the pyramids through the medulla, the animal being in the anatomical position.(3) Laminectomy, exposing the spinal cord at cervical or lumbar segments, so that electrodes could be inserted into the lateral corticospinal tract.