DIVERGENCE OF HIPPOCAMPAL MOSSY FIBERS

By connecting the fascia dentata with the hippocampus proper, the axon s of the granule cells, the mossy fibers, represent an important eleme nt of the main excitatory, trisynaptic pathway of the hippocampal form ation. In this review the various synaptic connections of the mossy fi bers are discussed. It turns out that the mossy fibers do not only est ablish synapses with the pyramidal neurons of regio inferior as tradit ionally assumed, but a variety of local circuit neurons as well as pro jection cells are also contacted by the mossy fibers. Thus there is an underestimated divergence of the impulse flow within the ''trisynapti c'' pathway at the level of the mossy fibers. Similarly, the pattern o f afferent input to the granule cells, especially that of GABAergic ne urons, is more complex than previously assumed. In this respect the co ncept of a unidirectional ''trisynaptic'' pathway certainly is an over simplification. In particular, the hilus of the fascia dentata, that t he mossy fibers traverse on their way to regio inferior, is often negl ected in this concept. The hilar region comprises a large variety of m orphologically and functionally distinct neuronal types that, to a lar ge extent, are targets of hilar mossy fiber collaterals. By focusing o n the mossy fiber system, an attempt is made in this review to summari ze new data on hippocampal circuitries that have been accumulated sinc e the original description of the trisynaptic pathway. This concept, w hich originally comprised the synapses of the perforant path fibers on dentate granule cells, the mossy fiber synapses on CA3 pyramidal neur ons, and the synapses of the Schaffer collaterals on CA1 pyramidal cel ls, has been of great heuristic value but needs to be modified in view of recent morphological and physiological data.