The paraneuronal nature of neurons: nonspiking communication in the crayfish central nervous system.

We have found a large portion of the neurons in the abdominal ganglia of the crayfish, Procambarus clarkii (Girard) to be local nonspiking neurons. These neurons are anaxonic, have extensive arborizations of neurites and have their entire structure confined within a single ganglion. They show relatively low membrane potentials of 40-50 mV and under no circumstance generate action potentials, yet their membrane potential changes can effectively modulate the activity of postsynaptic neurons which are generally motoneurons. The transmission is chemical and transmitter release in many, if not all, of them is continuous even at their "resting" level. Electron microscope examination showed an extensive intermingled distribution of both input and output synaptic structures. This synaptic distribution together with the passive electric property suggests that individual neurites of this type of neurons function rather independently, constituting numerous local circuits. Similar nonspiking communication is also found among the ordinary "spiking" motoneurons, indicating that this mode of communication is far more widely used in the crayfish central nervous system than generally believed. These findings appear to necessitate a radical revision of our understanding of how the central nervous system operates in arthropods.