Fig. 8

Models summarizing stimulated and intrinsic activities of RIS and comparison of sleep and stop neurons across model systems. a Fraction of behaviors found in animals before, during, and following optogenetic RIS stimulation (blue bar), categorized as forward (green), stop (gray and white), and reversal (red) movement. Also indicated is the contribution of RIS neurotransmitters to fast and sustained phases of induced stops. b Ca²⁺ activities of nerve ring and branch regions of the RIS axon, accompanying locomotion behaviors. The nerve ring region is active to induce slowing, while the branch region is additionally active when reversals are induced. c Comparison of stop cells¹³ and sleep neurons/systems in various model organisms. Hierarchy indicates complexity of the respective brains but implies no phylogenetic relationships. Left: Cells that stop or slow down locomotion when activated, in C. elegans (RIS), D. melanogaster larvae (PDM-DNs posterior dorso-medial brain lobe descending neurons¹¹), X. tropicalis tadpoles (GABAergic MHRs, mid-hindbrain reticulospinal neurons), Tr2 cells⁷⁰ in the anterior brain of the leech H. medicinalis, glutamatergic neurons in the MLR (mesencephalic locomotion region) and RS stop cells of the cMRRN (reticulospinal cells of caudal middle rhombencephalic reticular nucleus) of the lamprey P. marinus^10,71, and several types of mammalian stop cells: V2a reticulospinal interneurons in rostral medulla or caudal pons⁸, GABAergic neurons in the MLR, GABAergic and glycinergic neurons in the gigantocellular nucleus (GiA), and glycinergic neurons in the lateral paragigantocellular nucleus (LPGi)⁷². No stop cells were identified in zebrafish. Right: Sleep promoting neurons/systems (green: directly, and blue, indirectly promoting sleep, reviewed in ref. ⁷³) are peptidergic and GABAergic RIS and ALA cells in C. elegans (thus combining functions of stop and sleep neurons in RIS); in zebrafish larvae RFamide neuropeptide VF (NPVF), similar to C. elegans flp-11 derived peptides, inhibits serotonergic neurons in the ventral raphe nucleus; and in the mouse, GABAergic/peptidergic neurons in the preoptic area (POA), inhibitory neurons of the parafacial zone (PZ) of the brain stem, and GABAergic neurons of the ventral medulla (vM) are involved in sleep control. Transmitters used by each cell type are indicated. No sleep systems are known yet for fly larvae, tadpoles, leech, or lamprey. Animal silhouettes were acquired from http://phylopic.org/