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. Subsets of somato-sensory neurons are specified by the POU domain factors brn3a/b/c, contributing to their development, axonal projection and survival. The vertebrate somatic-visceral ‘duality’ is reflected by the differential usage of specific homeodomain transcription factors that drive cell-type specification. These viscero-sensory neurons are located in the blood vessels, airways or digestive tract monitoring the internal partial pressures of oxygen (pO2) and carbon dioxide (pCO2), blood pH, organ stretch and, last but not least, taste, and produce cardiovascular, respiratory and digestive responses. The visceral nervous system, in turn, is composed of sensors for internal conditions and effectors controlling body homeostasis. The vertebrate somatic nervous system involves somato-sensory neurons, located mostly in the dorsal root ganglia, ear and nose, perceiving environmental stimuli (primarily mechanical and olfactory, but also electromagnetic and thermal) and somato-motor neurons governing the locomotor response.
have now used molecular fingerprinting to tackle a long-standing question in comparative neuroanatomy and neurophysiology, which is the evolutionary origin of the somatic versus visceral nervous system duality. For example, it has been proposed that somato-motor neurons in vertebrates, insects, nematodes and annelids and branchio-motor neurons in ascidians and vertebrates are homologous cell types. In recent years, the comparison of molecular fingerprints has been used for tracing the evolution of animal cell types. Accordingly, somatic versus visceral nervous systems would mediate sensory-motor responses to the external and internal environment, respectively.
Starting with the work of the French physiologist Claude Bernard in the 19th century, vertebrates have been considered ‘dual entities’, composed of a ‘somatic’ and a ‘visceral’ animal responding to different environments: a milieu extérieur in which the organism is situated, and a milieu intérieur in which the tissue elements live.
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