Signalling from neurones to glial cells in invertebrates.

Recent work shows that glial cells in species throughout the animal kingdom appear to contribute to the functioning of the neurones and are equipped to receive signals from them. However, the detailed mechanisms of the signalling and its role in vivo are generally unclear. Parts of some invertebrate nervous systems are particularly favourable for addressing these problems, and the four preparations that have been studied most intensively are the subject of this review. Between the giant axons and their Schwann glial cells in squid and crayfish, within snail brain, and in leech ganglion, there appear to be multiple, and in some cases very complex, signalling pathways, whose precise functions remain to be elucidated. In bee retina only a single signal to the glia has been demonstrated, and its function appears to be to activate transfer of metabolic substrates to the photoreceptor neurones.