The spatial organization of the efferent projections of CA1 and CA3 hippocampal pyramids has been studied using recordings of fibre volleys, orthodromic and antidromic population spikes and synaptic field potentials, following microelectrode stimulation of the fimbria, CA1 alveus, or subiculum.
Only CA3 pyramidal cells were found to send their axons into the fimbria. In the septal two thirds of the hippocampus the CA1 pyramidal cells project in a caudal direction to the pyramidal part of the subiculum. The temporal third was not explored for technical reasons.
Fimbrial fibres are arranged in a strictly parallel fashion, the rostro-medial CA3 cells distributing their axons near to the hippocampus, while those located at the temporal extreme distribute their axons to the outer edge of the fimbria. The organization of the Schaffer collaterals and the projections of the CA1 cells consisted of parallel lamellae, oriented nearly transversely to the longitudinal axis of the hippocampus in rabbits (more obliquely in cats). The findings indicate that CA3 cell discharge via the Schaffer collaterals represents a major input driving the CA1 cells.
The dichotomy with regard to hippocampal output suggests that the CA3 and CA1 regions of the hippocampus may subserve different functions, thus probably participating differentially in various behavioural situations.
This organization makes it possible to study the behaviour of animals with selective and regional de-efferentation of the CA3 or of the CA1 regions by making discrete lesions in the fimbria and alveus, respectively. Alternatively, recording the fibre volley from the fimbria may provide a useful monitor of the output of the CA3 region during different behaviours.
Key wordsHippocampus Output fibres Cat and rabbit
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