Figure 2.
Models of the pontine network responsible for paradoxical (REM) sleep generation. This figure aims to compare our model (B) and that of Lu et al.4 (A). In both models, the SLD contains PS-on glutamatergic neurons generating PS. Their activation at the onset of PS is gated by the removal of a tonic GABAergic input activated during waking and SWS and arising from vlPAG/dDpMe neurons. Lu et al. further hypothesized that the SLD also contains a population of GABAergic PS-on neurons inhibiting at the onset of and during PS the vlPAG/dDpMe PS-off neurons. They proposed that a flip-flop switch between these 2 populations of GABAergic neurons control PS occurrence. In our model, PS-on GABAergic neurons also control PS occurrence by means of their projections to the vlPAG/dDpMe PS-off GABAergic neurons, but they are localized in the vlPAG/dDpMe and a medullary nucleus named the dorsal paragigantocellular reticular nucleus (DPGi).2,3 We propose that their activation at the onset of PS is due to an intrinsic “clock like” mechanism.
Another major difference between the two models concerns the pathways by which SLD triggers the muscle atonia during PS. We propose that SLD PS-on glutamatergic neurons activate glycinergic/GABAergic neurons localized in the GiV, which in turn, hyperpolarize cranial and spinal motoneurons, leading to muscle atonia. Lu et al. proposed that SLD PS-on glutamatergic neurons inhibit spinal motoneurons via direct projections to spinal interneurons co-containing GABA and glycine. One could argue that these two hypotheses are not mutually exclusive. We finally propose that SLD PS-on neurons activate the cortex by means of their projection to intralaminar thalamic nuclei whereas Lu et al. proposed that a small nucleus close to the SLD named precoeruleus (PC) generate theta oscillations during PS via projections to the medial septum.
DPGi, dorsal paragigantocellular reticular nucleus; dDpMe, dorsal part of the deep mesencephalic nucleus; GiV, ventral gigantocellular reticular nucleus; PC, precoeruleus nucleus; SLD, sublaterodorsal tegmental nucleus; vlPAG, ventrolateral periaqueductal gray.