Summary
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1.
In rabbits and cats anaesthetized by urethane-chloralose or pentobarbital sodium, stimulation of the commissural afferent pathway produced a negative field potential with maximal amplitude in the CA3 basal dendritic layer, and with a latency indicative of monosynaptic activation of excitatory synapses on the basal dendrites.
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2.
Mossy fibre stimulation resulted in a similar field potential restricted to the mossy fibre layer. Comparable negative field potentials were found in the layer of apical dendrites in CA1 in response to commissural and Schaffer collateral stimulation, suggesting a dendritic location of these synapses.
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3.
All negative field potentials grew in amplitude on tetanic stimulation, to produce large extracellular spikes, indicating their association with excitatory synaptic activity.
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4.
Usually, all pathways employed failed to produce EPSPs on single shock stimulation, in spite of their capability of discharging the cells, suggesting that the synaptic depolarization takes place at some distance from the soma.
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5.
Electron microscopy of degenerated commissural afferent fibres showed them to make contact with spines or the smooth surface of thin dendrites. The indentification of the postsynaptic element as pyramidal cell dendrite was ascertained. The mossy fibres end on ramified dendritic spines in CA3.
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6.
By comparison with normal electron micrographs, all the pathways, shown physiologically to be excitatory, terminate on thin dendrites, the contacts being of type 1.
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This investigation was supported by Public Health Service Research Grants NB 04764 and NB 02215, from the National Institute of Neurological Diseases and Blindness, which are gratefully acknowledged.
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Andersen, P., Blackstad, T.W. & Lömo, T. Location and identification of excitatory synapses on hippoeampal pyramidal cells. Exp Brain Res 1, 236–248 (1966). https://doi.org/10.1007/BF00234344
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DOI: https://doi.org/10.1007/BF00234344