MORPHOLOGICAL AND ELECTROPHYSIOLOGICAL PROPERTIES OF NEURONS IN AN AREA OF THE CHICK BRAIN INVOLVED IN LEARNING

The intermediate, medial part of the hyperstriatum ventrale (IMHV) is an area of the avian forebrain which is essential for two forms of ear ly learning in the domestic chick. We have developed an in vitro slice preparation which contains part of the IMHV and have found that the e lectrophysiological properties of the area show a considerable degree of plasticity. In particular, age and prior learning appear to modify the properties of single neurons recorded intracellularly. We have use d the in vitro slice preparation to make intracellular recordings from 38 single neurons in the IMHV and have then dye-injected each cell to find out whether there is any relationship between electrophysiologic al and morphological characteristics. The basic membrane properties of each neuron were measured. Responses to standard electrical stimuli, delivered extracellularly, were also recorded, and each neuron was cla ssified on this basis. Finally, the presence or absence of spontaneous ly occurring bursts of EPSPs was noted. At the end of recording biocyt in was injected into the cell. After the tissue had been processed, ea ch cell was drawn. The area of the cell body was measured, the number of dendrites was counted, and dendritic extent and branching were esti mated. Each cell was also classified as 'spiny' or 'non-spiny'. We fou nd that neurons displaying one particular type of response to external stimulation possessed a well defined set of morphological and electri cal properties. In addition, three parameters - electrical resistance, somatic area, and the presence or absence of dendritic spines - were related to specific subsets of anatomical and physiological characteri stics. The possible relevance of these findings to the plasticity of t he IMHV is discussed.