[Morphologic and functional peculiarities of the visceral protoneuron reexamined in light of recent data]

The recent works showing the richness and complexity of the sensory innervation of viscera raise various questions which are of general interest for modern physiology. 1 - The primary afferent neurons are much more numerous and heterogeneous than it was usually expected. Moreover, another category of neurones, namely the sensory intrinsic neurons, must be taken in consideration. These neurones end either in the intrinsic plexuses of viscera or in the prevertebral ganglia. 2 - Another major question concerns the mechanisms responsible for the functional diversity of interoceptors including the chemoreceptors. Several hypothesis have been proposed involving the presence of membrane receptors, the release of neurotransmitters by chromaffin cells and the deformation of tissue. In any way, the functional characteristics of interoceptors do not result in morphological differentiation since free endings are mainly found in the visceral area. 3 - Does the functional properties of primary afferent neurons are related to their morphological (diameter of fibres, size of cell bodies) or their biochemical (type of neurotransmitters) characteristics? So far, it is not possible to answer this question. Nevertheless the available data suggest that a such relationship partly exists. 4 - The visceral afferents are largely implicated in various physiological mechanisms involving visceral motility, homeostasis and behaviour. Therefore, their role in pain appear relatively less important than previously expected. Conversely, the nervous mechanisms induced by the interoceptor activation play a major part in that they prepare and potentialize the effects produced by the other mechanisms.