THE NEUROBIOLOGY OF THE ECTONEURAL/HYPONEURAL SYNAPTIC CONNECTION IN AN ECHINODERM

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Abstract
The nervous system of echinoderms consists of 2 parts, the ectoneural and the hyponeural. The latter is purely motor in function and of mesodermal origin. It is separated from the main ectoneural nervous tissue by a true basement membrane of collagenous connective tissue. Previous anatomical work has suggested that a chemical synapse occurs across the basement membrane with the hyponeural neurons being post-synaptic. The brittlestar Ophiura ophiura contains very large ectoneural interneurons and hyponeural motor neurons. The use of intracellular recording electrodes to dye-fill the cells and monitor action potentials and synaptic potentials are described. The synaptic morphology of both ectoneural and hyponeural neurons is described from dye-filled cells and shows that the large axons break up into a fine varicose plexus at the terminal regions. Preliminary recordings have been made from ectoneuronal neurons. Further work is required before function at the cellular level in this nervous system is understood. The report describes in detail, using simultaneous recording from 2 electrodes, how activity within the ectoneural system drives the hyponeural motor system and thus produces muscular contractions in the intervertebral muscles.

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