Hippocampal polymorph neurons are the cells of origin for ipsilateral association and commissural afferents to the dentate gyrus.

Neurons in the polymorph layer of the hippocampal dentate hilus were found to be the cells of origin for ipsilateral association and commissural projections to the dentate gyrus. Both afferent systems terminate in the proximal one-third of the stratum moleculare. The polymorph layer is the primary target for noradrenergic, serotonergic and cholinergic input to the hippocampal dentate region. It is hypothesized that one or all of these afferent groups affect hippocampal function through synaptic modification of the defined polymorph afferent system.