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The putative location of the MLR in zebrafish inferred from previous studies in other vertebrates. a, Schematic illustration of the MLR location in vertebrates (dashed line) in relation to the mesopontine cholinergic cells (yellow). The structures classically defined as part of the MLR are the PPN, CnF and LDT. The characteristics of the structures were taken from the Allen Mouse Brain Atlas (https://atlas.brain-map.org). b, Schematic of the larval zebrafish brain depicting the probable location of the MLR (dashed white line, dorsal view) in relation to the LC and the mesopontine cholinergic cells labeled in the Tg(chata:GAL4) transgenic line. The hindbrain RSNs and the nMLF were labeled using spinal backfills. Scale bar, 50 µm. c, Scheme comparing the nomenclature and boundaries of the brain stem areas in different vertebrate classes. d, Applied nomenclature in the larval zebrafish brain stem. The anatomical landmarks were defined based on the projecting neurons labeled using spinal backfills (not shown) and the expression patterns of the Tg(chata:GAL4) (yellow) and Tg(vmat2:GFP) (cyan) transgenic lines. The PP region contains the isthmus and r1. The trochlear nucleus (4N) lies in the rostral border of the isthmus; among other structures, r1 contains the LC. The P region is defined rostrally by r2, containing the rostral trigeminal motor nucleus (5Nr), the caudal part of the trigeminal motor nucleus (5Nc) and caudally by rhombomere 4 (r4), including the Mauthner cell (M cell). The RP region consists of rhombomere 5 (r5) and rhombomere 6 (r6) containing the abducens (6N) and facial nucleus (7N), respectively. The medulla is delimited rostrally by rhombomere 7 (r7) and caudally by the rostral spinal cord. Under this nomenclature, the oculomotor nucleus (3N) is a midbrain structure, but the nMLF should be considered as a diencephalic structure. All images were taken from the Web interface of mapzebrain (https://fishatlas.neuro.mpg.de/). A, anterior; P, posterior.
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