Fig. 6.
Proposed mechanism of phrenic long-term facilitation. Intermittent hypoxia increases the release of serotonin in the vicinity of the synapse between descending bulbospinal respiratory neurons and phrenic dendrites. The activation of 5-HT2A receptors initiates a signal transduction cascade that leads to new protein synthesis. These newly synthesized proteins may act both presynaptically and postsynaptically to increase synaptic efficacy between bulbospinal respiratory neurons and phrenic motoneurons and hence may give rise to a facilitated phrenic motor output.