ELECTROPHYSIOLOGICAL PROPERTIES OF AN ARCHICORTICAL NEURON

The detailed understanding of the electrophysiology of central neurons derives primarily from the extensive intracellular microelectrode studies of the spinal motoneuron.2*13*14.19~22 ,39,66 The important model derived from these investigations has already proved applicable to the neurons of a variety of submammalian species: the supramedullary cell of the p ~ f f e r f i s h , ~ . ~ the giant ganglion cell of A f i l y ~ i a , ~ ~ ~ ~ and the sensoryneuron of the crayfish.'* By contrast, the test of the motoneuron model in mammals has been largely limited to other nerve cells of the spinal cord (for example, Clarke's column and interneurons) .'I .26 In particular there is a paucity of information about cortical neurons. It is indeed difficult to obtain intracellular recordings in the mammalian cortex due, in large part, to the small size of most cortical cells and to brain pulsation. The present series of investigations received their impetus from Phillips' success in impaling the neocortical Betz By activating these cells antidromically, Phillips was able to show that the steps in the production of the antidromic spike were similar to those in the spinal motoneuron. He demonstrated further that excitation occurred with depolarization and inhibition with hyperpolarization. Since the Betz cell is an unusually large-sized and fairly specialized neocortical motor cell, it seemed advantageous to test the motoneuron model on another cell that is both morphologically and functionally different. The hippocampal pyramidal cell was selected for study because it is a more typically correlational neuron, that is, it does not have specific motor or sensory functions and it is part of the archirather than the neocortex. The hippocampus is a particularly favorable cortex for single unit analysis. I t is directly accessible following suction removal of the overlying part of the hemisphere. Therefore gross stimulating and recording electrodes can be placed under direct vision on various portions of the exposed archicortex. The absence of pial covering permits multiple electrode penetrations to be made rapidly and easily. The relative simplicity of the cellular architecture and its constancy throughout the mammalian class have traditionally attracted the attention of morphologists. Consequently the anatomy and hodology of the hippocampus are particularly well described.'*3s~40~45-46 The somata of the primary neurons, the pyramidal cells, are clustered in a tightly packed layer (FIGURE la). These neurons send their axons into alveus, fimbria, and fornix, an outflow tract that is readily accessible for stimulation (FIGURE l b ) . The pyramidal cell can therefore be activated antidromically and identified electrophysiologically. There are also available two orthodromic pathways to the pyramidal cells and an extensive system of recurrent collaterals.