Neurociliary synapses in Pleurobrachia (Ctenophora)

Abstract

Summary Synapses of a typical form, with synaptic vesicles of 30 to 45 mµ diameter, on one side of a synaptic cleft of 10 to 12 mfx, are found near the bases of the long ciliated cells which make up the comb plates. The side containing the vesicles is interpreted as a section of a nerve fibre terminal in contact with the ciliated cell. Occasionally nerve-cell bodies, identified by similar vesicles, are encountered. These nerves are probably ramifications of the ectodermal nerve net which spreads over the whole animal and which transmits the inhibition of the ciliary waves when the animal is touched. Distinctly different nerve fibres, which run from the apical organ along the ciliated grooves to the end of the comb rows, are able to initiate a ciliary wave along the comb row. The excitatory nerves appear as rather empty axons which mingle with the bases of the ciliated cells. The question of the propagation of excitation between these axons, between the ciliated cells of a comb plate, and between comb plates, remains open, although regions where excitatory axons and ciliated cells make contacts between themselves and with each other have some resemblance to electrically transmitting synapses.