Functional multimodality of axonal tree in invertebrate neurons

This review, based on invertebrate neuron examples, aims at highlighting th e functional consequences of axonal tree organization. The axonal organizat ion of invertebrate neurons is very complex both morphologically and physio logically. The first parr shows how the transfer of information along senso ry axons is modified by presynaptic inhibition mechanisms. In primary affer ents, presynaptic inhibition is involved in: 1) increasing the dynamic rang e of the sensory response; 2) processing the sensory information such as in creasing spatial and/or temporal selectivity; 3) discriminating environment al information from sensory activities generated by the animal's own moveme nt; and 4) modulating the gain of negative feedback (resistance reflex) dur ing active rhythmic movements such as locomotion. In a second part, the who le organization of other types of neurons is considered, and evidence is gi ven that a neuron may not work as a unit, but rather as a mosaic of disconn ected 'integrate-and-fire' units. Examples of invertebrate neurons are pres ented in which several spike initiating zones exist, such as in some stomat ogastric neurons. The separation of a neuron into two functionally distinct entities may be almost total with distinct arborizations existing in diffe rent ganglia. However, this functional separation is not definitive and dep ends on the state of the neuron. In conclusion, the classical integrate-and -fire representation of the neuron, with its dendritic arborization, its sp ike initiating zone, its axon and axonal tree seems to be no more applicabl e to invertebrate neurons. A better knowledge of the function of vertebrate neurons would probably demonstrate that it is the case for a large number of them, as suggested by the complex architecture of some reticular interne urons in vertebrates, (C) 1999 Editions scientifiques et medicales Elsevier SAS.