Both the eupneic and the sigh rhythms involve activation of the majority of neurons. However, each type of rhythmic activity is dependent on distinct mechanisms. Fictive sigh depends critically on synaptic mechanisms that involve P/Q type calcium channels, suggestive of a subset of neurons with specialized synapses for this type of rhythm generation, since only a very small number of respiratory neurons receive glutamatergic inputs that depend on P/Q type calcium currents. The fictive sigh rhythmic type also depends on mGluR8 receptor activation. Eupneic rhythmic activity relies on NMDA-dependent mechanisms.