Nerve-ending specializations in the central ganglia of Planorbis corneus.

The neuropile in the central ganglia of Planorbis corneus was studied by electron microscopy, using different fixatives. Several types of specialization were observed in the membranes of adjacent axonal processes, varying from only slight membrane thickenings to structures resembling desmosomes. Specialized structures similar to the 'spinule complexes' in mammalian brain are widely distributed in certain areas of neuropile. They consist of a small projection which invaginates a neighbouring neurone; the membranes are closely apposed and there is associated electron-dense material. The problem of identifying chemical synapses in this tissue is discussed. Many axonal processes are varicose, and it is suggested that varicosities may also be sites of transmitter release.