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This paper unifies and outlines some important points made by recent experimental work, dealing with MS ion channels and growth cone motility. The considerable work carried out in cell biology, neurochemistry and membrane biophysics in the past decade allows us to imagine the neurite growth in term of known molecules and chains of simple mechanisms. On the other hand, it is becoming increasingly clear the real complexity of the growth cone behavior. There remain important gaps in our understanding of the role of MS channels in growth cone motility. At present, a major limitation for physiological work is the lack of specific blockers. An encouraging development of molecular genetic studies of Drosophila SAK channels (60) will probably provide valuable tools for straightforward experiments.

A single neuron, located in the center of each segmental ganglion of H. medicinalis is antidromically activated by electrical stimulation of the ventral cord anteriorly and posteriorly to the ganglion, at the same threshold as the fast conducting system (FCS) and with a latency equal to the FCS conduction time. This neuron is activated trans-synaptically by tactile and photic stimulation of the skin and by stimulation of high-threshold fibres running along the cord. A spike evoked by intracellular stimulation of this neuron propagates along the FCS. Intracellular staining shows that this neuron sends two axonal branches in the anterior and posterior median connectives. Direct electrical stimulation of touch cells (T cells), as well as mechanical stimulation of the skin, lowers the threshold of and may eventually fire, the FCS neurons, not only at the level of the ganglion to which they belong, but also at the level of the neighbouring ganglia. This effect is mediated by bilateral pathwasy located in the lateral connectives. It is concluded that the FCS consists of a chain of single neurons, located in each ganglion and electrotonically coupled to each other. Touch cells project with excitatory synapses on the FCS neurons.

The mechanisms underlying the diurnal rhythm of flashing in L. lusitanica were investigated. It was shown that the diurnal rhythm of flashing was present also in animals kept in continuous darkness, showing that it was not directly determined by the periodic variations of the ambient light intensity which take place through the nicthemeron. During the hours in which the animals did not flash, photogenic volleys of normal size and contour could still be recorded, although at a lower frequency; however, they were not followed by light emission. Statistical controls ruled out the possibility that the absence of flashing during daytime was due to the reduced repetition rate of the photogenic volleys. The ablation of the gonads performed during day-time was followed by the reappearence of a flash in the wake of each photogenic volley. It is concluded that two mechanism control the diurnal rhythm of flashing, namely; a central mechanism, responsible for the decrease in volley frequency; and a peripheral mechanism, of gonadal origin, which is responsible for the inhibition of the photogenic organ.