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.