Fig. 6.

Coexistence of 2 stable periodic orbits (bistability) in the closed-loop respiratory control model. A and B: simulations with identical parameter values but different initial conditions. From top to bottom: CPG voltage (mV), lung volume (liters), arterial oxygen (mmHg), chemosensory-dependent input to CPG (nS). x-Axis is time (s). A: “eupneic” bursting. The central BRS circuit responds to time-varying chemosensory input by producing a regular breathing rhythm at ~10 breaths/min. Lung volume varies between 2 and 3 liters. Blood oxygen ($P{a}_O{}_{{}_2}$) varies between 90 and 110 mmHg. B: different initial conditions lead to pathological “tachypneic” spiking. The CPG receives elevated tonic input causing sustained spiking at several hertz, leading to ineffective motor output. Lung volume fluctuates by <0.1 liters, and blood oxygen is approximately constant at a pathologically reduced level (25 mmHg).