An axon pacemaker: diversity in the mechanism of generation and conduction of action potentials in snail neurons.

One of the main principles in neuroscience is that in vertebrate motoneurons and certain interneurons the decision to initiate an action potential is made at the initial segment of the axon, the axon hillock [Kandel E. R. and Schwartz J. H. (1991) In Principles of Neural Science (eds Kandel E. R., Schwartz J. H. and Jessell T. M. ), pp. 166-167. Elsevier, New York]. The situation in invertebrate motoneurons is different. The axon has many arborizations near its soma, in the nearby neuropil, and many branches in the target region. The action potentials are generated in the region of the axon in the neuropil and in some cases the trigger zone can be found more than 1mm apart from the soma [Tauc C. (1962) Aplysia. J. gen. Physiol. 45, 1077-1097]. Thus, it is obvious that, in a neuron, the removal of the trigger zone would cease the spiking activity in the axon. The purpose of this work is to demonstrate that, in snails, there are axons of certain neurons, like neuron B2, which are able to maintain their firing activity after the removal of their cell body and the so-called trigger zone.