now show that brn3 is expressed in clusters of mechano- and nociceptive sensory neurons (as identified by coexpression of the neuropeptide Sensorin A in the opisthobranch gastropod Aplysia californica (the California sea slug) and in the pulmonate snail Lymnaea stagnalis. While the pedal cord innervates the body and foot and mediates locomotion and escape behaviors, the visceral cord innervates the pharyngeal walls, salivary glands, mantle, kidneys, gonads, gills, liver, heart and gut, among others. The mollusk nervous system comprises ventral somatic (pedal) and lateral visceral chains of ganglia, connected by commissures and longitudinal connectives (Figure 1). Mollusks are an excellent choice, given that a huge body of electrophysiological, behavioral and anatomical studies revealed a rich diversity of visceral and somatic functions. To elucidate the evolutionary origin of the viscero-somatic duality, Nomaksteinsky and co-workers have investigated several species of mollusks, which, together with annelids and other invertebrates, belong to the superphylum of Lophotrochozoa. phox2, on the other hand, has been found in the motor neurons of the cerebral ganglion of the post-metamorphic ascidian Ciona, an invertebrate chordate, considered viscero-motor as they innervate the muscles of the branchial basket. Drosophila hb9 specifies subsets of somato-motor neurons and interneurons and is required for the development of their projections. In Caenorhabditis and Drosophila, the brn3 orthologs unc86 and acj6 are essential for specification and axonal targeting of the primary touch and olfactory somatic-sensory neurons, respectively. Previous studies indicated that the brn3-hb9- phox2-based differential specification of the somatic and visceral neurons is conserved outside vertebrates, at least to some degree. Finally, in almost all sensory and motor neurons of the vertebrate viscero-reflex-circuits the paired-like homeodomain transcription factor phox2b is present, likewise controlling specification, differentiation and axonal connectivity. In somato-motor neurons the EGH homeodomain transcription factor hb9/mnx is active, controlling specification, migration and axonal projection.
![the somatic nervous system is the somatic nervous system is](https://4.bp.blogspot.com/__bX62K386ZI/TSxhFEe7bNI/AAAAAAAAAHw/y12cJqSGTVE/s1600/Somatic+Nervous+System+%2528SNS%25297.jpg)
Subsets of somato-sensory neurons are specified by the POU domain factors brn3a/b/c, contributing to their development, axonal projection and survival.
![the somatic nervous system is the somatic nervous system is](https://prod-images-static.radiopaedia.org/images/43188210/c7c16c337e19f6775ba51cc2a0a88e_gallery.jpeg)
The vertebrate somatic-visceral ‘duality’ is reflected by the differential usage of specific homeodomain transcription factors that drive cell-type specification.