Fig. 7. (  Left ) Simplified diagram of the pontomedullary pattern generator that transforms tonic excitatory drive into rhythmic neuronal discharge patterns. (From  top to  bottom ) Excitatory drive to the respiratory system is mostly tonic and arises from central and peripheral chemoreceptors and also from other central networks,  e.g. , the reticular formation. The drive activates neurons in the pre-Bötzinger region, which through reciprocal connections develop two respiratory phases. Additional neuronal connections are required to generate a distinct functional discharge pattern. Tonic excitatory drive and phasic excitation reach the premotor neurons of the caudal ventral respiratory group. Inspiratory premotor neurons reciprocally excite each other (“self-reexcitation,”  dotted line ). Phasic excitation is relayed by the premotor neurons to the motoneurons. The neurons of timing and pattern generation as well as the premotor neurons are also subject to tonic γ-aminobutyric acid–mediated inhibition that has a gain modulatory effect on the neuronal discharge frequency patterns. In addition, there is phasic inhibition of premotor neurons, whereas the premotor neurons themselves inhibit the motoneurons of the opposite phase, possibly through interneurons. (  Right ) The effects of sevoflurane on excitatory and inhibitory neurotransmission to inspiratory premotor neurons.  Triangular symbol = excitatory input;  round symbol = inhibitory input;  arrows = effects of sevoflurane;  downward arrow = decrease;  upward arrow = increase; ø= no effect. AMPA =α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor; E = expiratory; Fⁿ= neuronal discharge frequency; GABA =γ-amino-butyric acid–mediated inhibitory input; GABA^A=γ-aminobutyric acid type A receptor; GLU = glutamatergic excitatory input; I = inspiratory; NMDA =  N -methyl-d-aspartate receptor.