Lamellar organization of hippocampal excitatory pathways

Summary

• 1. Entorhinal activation of the hippocampal cortex involves the sequential activation of a four-membered pathway: the perforant path from the entorhinal area — the mossy fibres from the dentate granule cells — the Schaffer collaterals of the CA3 pyramidal cells and finally, the CA1 pyramidal cell axons in the alveus.

• 2. The spatial orientation of these four fibre bundles has been studied by recording the extracellular field potentials (population spike), signalling the discharge of neurones in response to orthodromic or antidromic impulses. The height of the population spike was taken as an indicator of the number of cells discharged (see the previous paper).

• 3. The perforant path fibres from neighbouring parts of the entorhinal area run in a parallel fashion in a direction nearly transversely to the longitudinal axis of the hippocampus. In the dorsal part of the hippocampus, this direction was nearly sagittal, confirming Lømo (1971 a). The mossy fibres as well as the Schaffer collaterals and the alvear fibres were all found to run in the same direction. Thus, a point source of entorhinal activity projects its impulses through the four-membered pathway along a slice, or lamella, of hippocampal tissue oriented normally to the alvear surface and nearly sagittally in the dorsal part of the hippocampal formation. Also with more temporal locations of the stimulating and recording electrodes, the lamellar organization was maintained, but with a different orientation, matching the curving of the hippocampus so that the angle between the plane of the lamella and the longitudinal axis remained the same.

• 4. By injection of a quick-setting solution of vinyl acetate, the direction of the arteries and veins in the hippocampal formation was displayed. The branches from the artery running in the hippocampal fissure are nearly straight and are oriented in a direction similar to that of the lamellae.

• 5. The hippocampal cortex seems to be organized in parallel lamellae, both with regard to the neuronal and the vascular system. By means of this lamellar organization, small strips of the hippocampal cortex may operate as independent functional units, although excitatory and inhibitory transverse connections may modify the behaviour of the neighbouring lamellae.