Neural network topology. Structure of the network model. (A) The actual spatial location of neurons in their corresponding two-dimensional layers is shown. Mossy fibers (MFs) are shown in green, Golgi cells (GoCs) in red and granule cells (GCs) and parallel fibers (PFs) in blue. Only 1% of the GCs and PFs are displayed for better visualization. (B) Schematic diagram illustrating the connectivity between the layers. MFs (green) excite both GCs (blue) and GoCs (red). The axons of GCs form ascending fibers, which bifurcate in the PF fiber layer and spread in each direction of the x axis. These PFs excite the GoCs along the way. By contrast, GoCs inhibit the GCs in their vicinities. Two inhibitory circuits driven by the excitatory MF inputs emerge from this synaptic organization. One is a feedforward (FF) inhibitory circuit and the other is a feedback (FB) inhibitory circuit. The FF circuit works through the MF-GoC-GC pathway, whereas the FB circuit works through the MF-GC-PF-GoC-GC pathway. (C) Inset showing the neurotransmitters and synaptic receptors used by each modeled synaptic connection. The GoC model has α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR), which are activated by MF or PF glutamatergic (Glu) terminals. The GC model has AMPAR and N-methyl-D-aspartic acid receptors (NMDAR; activated by MF Glu terminals), and GABAa receptors (GABAaR) (activated by GABAergic terminals (GABA) coming from the nearby GoCs).