Somatic and visceral neurons are distinguished by a small set of transcription factors in vertebrates. Schematic of the main neuronal types that form the sensorimotor circuits in vertebrates, and their molecular code. An exemplar of each category is shown with its target organ. Magenta: somatic sensory neurons (with a history of Brn3 expression), including somesthetic neurons (mechanoreceptors, proprioceptors, thermoreceptors and nociceptors) of the trigeminal and dorsal root ganglia, and at least some of their post-synaptic partners in the nuclei of the trigeminal nerve and dorsal horn of the spinal cord, respectively, retinal ganglionic cells and vestibulo-cochlear neurons, and the latters’ targets, the hair cells of the inner ear. Blue: somatic motoneurons (with a history of Mnx1 and Lhx3/4 expression) that control bodily motions. Green: neurons of the visceral sensory pathways and motor outflow (with a history of Phox2b expression): first-order sensory neurons in epibranchial ganglia and second-order sensory neurons of the nTS; the carotid body; sympathetic, parasympathetic and enteric ganglionic neurons, and preganglionic neurons to the latter two; and branchiomotor neurons (respiratory in aquatic vertebrates). V, trigeminal nerve; Vm, motor nucleus of the trigeminal nerve; VIII, vestibulo-cochlear nerve; IX, glossopharyngeal nerve; X, vagal nerve; cb, carotid body; cg, cochlear ganglion; cn, cochlear nuclei; dh, dorsal horn; dmnX, dorsal motor nucleus of the vagus nerve; drg, dorsal root ganglion; hc, cochlear and vestibular hair cells; ens, enteric nervous system; mesV, mesencephalic nucleus of the trigeminal nerve; na, nucleus ambiguus; ng, nodose ganglion; nTS, nucleus of the solitary tract; pg, petrosal ganglion; prV, principal nucleus of the trigeminal nerve; psg, parasympathetic ganglion; rgc, retinal ganglion cells; sg, sympathetic ganglia; spV, spinal nucleus of the trigeminal nerve; tg, trigeminal ganglion; vg, vestibular ganglion; vh, ventral horn; vn, vestibular nuclei.