Model for regulation of male tail posture. Proper tail posture is maintained through a balance of ventral and dorsal curling of the male tail. (a) Ventral tail curling. Acetylcholine acts through male-specific and likely ventral body wall muscles to curl the tail ventrally. Acetylcholine acts together with serotonin, likely in part by acting directly on male-specific muscles to promote ventral tail curling. Acetylcholine also acts through GABAergic neurons to promote ventral tail curling by activating ventral muscles and/or by inhibiting dorsal muscles. (b) Dorsal tail curling. Acetylcholine acts through dorsal body wall muscles to promote dorsal tail curling. Acetylcholine also acts through GABAergic neurons to promote dorsal tail curling in part by inhibiting ventral tail curling via the unc-49 receptor. Dorsal tail curling, seen when males have their spicules inserted into the vulva, is inhibited by gamma-aminobutyric acid through the unc-49 receptor.