| Signal flow in the brain is not unidirectional; feedback represents a key element in neural signal processing. To address the question on how do neural feedback loops work in terms of synapses, microcircuitry, and systems dynamics, we developed a chick midbrain slice preparation to study and characterize one important feedback loop within the avian visual system: isthmotectal feedbackloop. The isthmotectal feedback loop consists of the optic tectum (OT) and three nucleus isthmi: Imc, Ipc and SLu. The tectal layer 10 neurons project to ipsilateral Imc, Ipc and SLu in a topographic way. In turn Ipc and SLu send back topographical (local) cholinergic terminals to the OT, whereas Imc sends non-topographical (global) GABAergic projections to the OT, and also to the Ipc and the SLu. We first study the cellular properties of Ipc neurons and found that almost all Ipc cells exhibited spontaneous activity characterized with a barrage of EPSPs and occasional spikes. Further experiments reveal the involvement of GABA in mediating the spontaneous synaptic inputs to the Ipc neurons. Next we investigate the mechanisms of oscillatory bursting in Ipc, which is observed in vivo, by building a model network based on the in vitro experimental results. Our simulation results conclude that strong feedforward excitation and spike-rate adaptation can generate oscillatory bursting in Ipc neuron in response to a constant input. Then we consider the effect of distributed synaptic delays measured within the isthmotectal feedback loop and elucidate that distributed delays can stabilize the system and lead to an increased range of parameters for which the system converges to a stable fixed point. Next we explore the functional features of GABAergic projection from Imc to Ipc and find that Imc has a regulatory role on actions of Ipc neurons in that stimulating Imc can evoke action potentials in Ipc neurons while it also can suppress the firing in Ipc neurons which is generated by somatic current injection. The mechanism of regulatory action is further studied by a two-compartment neuron model. Last, we lay out several open questions in this area which may worth further investigation. |
