Inhibitory V1 and V2b subtypes in zebrafish and mice. (A) In zebrafish, V1 (yellow) and V2b (brown) divide into fast (light gray, outline) and slow (dark gray, outline) subtypes. V1 neurons: The fast V1 subgroup (yellow, light gray outline) inhibits both slow (green, dark gray outline) and fast (green, light gray outline) motor neurons in addition to slow-type V2a neurons (pink, dark gray outline). The slow V1 subgroup (yellow, dark gray outline) inhibits slow motor neurons (green, dark gray outline). V1 neurons also project to dorsal CoPA neurons (red) which receive sensory input, V2a neurons (pink), V2b neurons (brown), and commissural neurons (gray). V2b neurons: Slow V2b neurons (brown, dark gray outline) inhibit fast motor and other V2b neurons. Fast V2b neurons (brown, light gray outline) inhibit slow motor and other V2b neurons. V2b neurons in zebrafish also project to V2a, V1 and commissural neurons. (B) In mice, V1 neurons and subdivide into Ia-interneurons (light blue, outline), which receive sensory input and inhibit motor output; Renshaw cells (orange outline) which form recurrent connections with motor neurons; and four clades: Sp8 (purple outline), FoxP2 (pink-red outline) and Pou6f2 (pink outline). V1 neurons also receive input from V3 neurons (brown). V2b neurons include Ia- and Ib- (dark blue, outline) interneurons. V2b-derived Ia-interneurons inhibit motor and other V2b neurons. V2b neurons also inhibit V0c neurons. An additional V2c class is present in mice (red-pink) with an unknown projection pattern.
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