Excitatory synapses on hippocampal apical dendrites activated by entorhinal stimulation.

In rabbits anesthetized with urethane-chloralose, the responses produced in the field CA1 by stimulation of the entorhinal area were studied with intracellular and extracellular recording. The most important pathway involved was found to be the three-synaptic path: Entorhinal area — dentate granule cells— CA3 pyramids— CA1 pyramids. This conclusion is based upon the following experimental data: The main CA1 activity occurred later than expected for a monosynaptic path, it was always secondary to dentate granule cell discharges, it disappeared after a section severing the Schaffer collaterals at the CA2/CA1 border, it showed longer latency the longer the recording electrode was placed from the fimbria. The final link in this three-synaptic pathway is the Schaffer collaterals. Their synapses produce a large negative field potential at the level of the profuse branching of the apical dendrites. The negativity is most likely due to the inward current flowing across the activated excitatory synapses. Extracellular recording of the population spike, produced by the Schaffer collateral synaptic activity, showed the spike to be initiated in that part of the dendritic membrane which borders the territory of the Schaffer synapses. The spike was conducted towards the soma with a speed of about 0.4 m/sec. Intracellular recording from cat CA1 pyramids gave as a rule no sign of soma epolarization in spite of the appearance of synaptically initiated action potentials. The results indicate that synapses located to the dendrites can initiate a local spike that may be conducted towards the soma with a low conduction velocity, and probably, with a low safety factor.