Figure 7. A model summary for motor sequence generation.

(A) Schematics of neuronal circuit of C. elegans avoidance behavior. Feedforward excitation between local interneurons AIB and RIV triggers turning behavior. Glutamatergic inhibition between AIB and RIB flexibly controls the motor state transitions. Feedback inhibition from the turning module helps terminate backward behavior. (B) Illustration of the biophysical meaning of the type-II transition. The sub-threshold membrane potential of RIV fluctuates around the fixed point x₀, just like a particle (red) in an energy well. When RIV membrane potential crosses the threshold x_th, RIV along with other neurons in the turning module become fully activated and a turn starts. (C) A simple stochastic model without short-term synaptic plasticity cannot account for the experimentally observed transition rate. (D) Schematics of a three-module model based on the animal connectome and experimentally identified functional motifs. Short-term synaptic depression (STD) was introduced in the feedforward inhibition. (E) Type-II transition rate during ALM/AVM triggered escape responses. Pink line is a theoretical fit using Equation 4. ${\tau }_g$ in Equation 4 is given by the glutamate decay constant in Figure 4E.