Fig. 9.

Transient response of CPG in closed-loop system can lead to “autoresuscitation” after hypoxic perturbations. The open-loop system was simulated with g_tonic = 0.3800 nS (red curve) and g_tonic = 0.3791 nS (blue curve) until it reached steady state. At t = 0, we “closed the loop” and allowed g_tonic to vary as a function of $P{a}_O{}_{{}_2}$ throughout the remainder of the simulation. From these initial conditions, the blue trajectory approaches eupnea, whereas the red trajectory approaches tachypnea. Dashed line indicates that initial conditions determined from steady states of open-loop simulations with g_tonic values corresponding to $P{a}_O{}_{{}_2}$ levels above (below) this line will approach eupnea (tachypnea). At t = 180 s, $P{a}_O{}_{{}_2}$ was set to 40 mmHg momentarily and then immediately went back to being determined by the system dynamics. This hypoxic perturbation takes the trajectory to $P{a}_O{}_{{}_2}$ levels below the steady-state dividing line, but the transient response allows the system to recover to eupnea. At t = 360 s, $P{a}_O{}_{{}_2}$ was set to 30 mmHg momentarily and then immediately went back to being determined by the system dynamics. The transient response again leads to an abrupt initial increase in $P{a}_O{}_{{}_2}$ following the perturbation, but it is not enough to get over the dividing line and the trajectory ultimately approaches tachypnea.