Figure 10.

The major synaptic features of the HVC microcircuit revealed in the present study by paired recordings and antidromic stimulation of HVC_RA neurons are shown. HVC_RA (gray circles) and HVC_X (white circles) neurons form excitatory synaptic connections (arrows) on interneurons (black circles), which provide divergent inhibitory input (t-endings) on PNs of both types. Fast excitation is mediated by ionotropic glutamate receptors, whereas fast inhibition is mediated by GABA_A receptors. Additional polysynaptic and possibly monosynaptic excitatory pathways and polysynaptic inhibitory pathways also provide a synaptic linkage from HVC_RA to HVC_X neurons (dashed lines). These monosynaptic and polysynaptic pathways are dependent on ionotropic glutamate receptors, presumably involving direct synapses between HVC_RA axon collaterals and HVC_X neurons and intervening synapses between HVC_RA axon collaterals and other HVC interneurons.