Phase portraits of the integrate-and-fire model obtained from the Connor-Stevens model with standard parameters (Connor et al., 1977). The $V$- and $Vs$-nullclines are drawn as full and dashed lines, respectively. (Left) The $V$-nullclines are drawn for different values of $Iapp$⁠: low (light blue), $Iapp$ at the transcritical bifurcation in fast subsystem (medium blue), and high (dark blue). (Right) Detailed portion of the phase portrait (for the high value of $Iapp$⁠), together with the corresponding trace of the limit cycle oscillation in orange.