Tuesday, August 29, 2017

Journal Club: Fast Silencing Reveals a Lost Role for Reciprocal Inhibition in Locomotion




By Peter R. Moult, Glen A. Cottrell, and Wen-Chang Li
Neuron Volume 77, Issue 1, 9 January 2013, Pages 129-140

Reciprocal inhibition is considered as a fundamental building block in neural circuits for rhythmic motor pattern generation. Despite commonality in many rhythmic systems, however, the reciprocal inhibition is often found to be not a necessary component for rhythmogenesis. This is because a network is often capable of exhibiting rhythmic activity even when the reciprocal inhibition was removed mechanically or pharmacologically. The excitatory synaptic components within each half of the network are more crucial.  In this study, Wen-Chang Li's group challenged this idea by using optogenetics to hyperpolarize the entire population of neurons in one side of the spinal cord in a tadpole.

When the entire half of the spinal cord was suppressed by flashing yellow light, the other half also stopped bursting. This indicates that, in this reciprocally inhibitory network, the excitation in one side is necessary for the excitation of the other. Firing in one side would cause a bombardment of IPSPs, which induce post-inhibitory rebound. The authors also get a similar result by giving a strong hyperpolarizing current into one of the reciprocally inhibitory neurons. They concluded that the reciprocal inhibition is necessary in the tadpole swim circuit; it induces rebound excitation in the contralateral neurons.

In this paper, it is noteworthy that the author also mentioned about the "functional homeostasis" of the neural circuit. They said the rhythmic activity recovers after half an hour by itself without the reciprocal inhibition. The mechanism of the recovery still unknown.




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