  The paper highlights the role of a positive feedback gating mechanism at the
cellular level in the robust- ness and modulation properties of rhythmic
activities at the circuit level. The results are presented in the context of
half-center oscillators, which are simple rhythmic circuits composed of two
reciprocally connected inhibitory neuronal populations. Specifically, we focus
on rhythms that rely on a particu- lar excitability property, the
post-inhibitory rebound, an intrinsic cellular property that elicits transient
membrane depolarization when released from hyperpolarization. Two distinct
ionic currents can evoke this transient depolarization: a
hyperpolarization-activated cation current and a low-threshold T-type calcium
current. The presence of a slow activation is specific to the T-type calcium
current and provides a slow-positive feedback at the cellular level that is
absent in the cation current. We show that this slow- positive feedback is
necessary and sufficient to endow the network rhythm with physiological
modulation and robustness properties. This study thereby identifies an
essential cellular property to be retained at the network level in modeling
network robustness and modulation.
