Neurobiology of the Echinodermata

Abstract

There has been little progress since the classical anatomical studies in understanding the form and function of the nervous system of echinoderms until relatively recently. There is, admittedly, a substantial behavioural literature but much of this can be confusing and even contradictory. There were early attempts to record electrical activity with extracellular electrodes but the compound potentials recorded to gross stimulation were clearly highly artifactual. Brehm (1977) showed that it was possible to record single unit activity extracellularly and further that this was possible because the brittlestar preparation that he used contained neurones much larger than average. Since then the large neurones of brittlestars have been exploited to produce a growing amount of information about function in echinoderm nervous systems at the cellular level. It is possible to use multiple recording sites from intact animals and monitor the activity that co-ordinates behaviour. There is data on the sensory perception of a range of environmental parameters. These direct electrophysiological measurements of response to stimuli are invariably consistent and are thus much more valuable than the inconsistent behavioural criteria previously used. It is also now possible to record intracellularly from both ectoneural neurones and hyponeural neurones. Lucifer yellow can be injected iontophoretically into both classes of neurones and preliminary data has now been obtained on the general morphology of individual cells within the nervous system (Cobb 1985). The radial nerve cords consist of connected segmental ganglia. The layout of the large neurones in each segmental ganglia is similar whatever the position of the ganglia within the radial nerve cords. Longitudinally running large neurones pass through at least 4 or 5 segments and show a fine plexus of varicose terminals at each end. These neurones are multimodal in the information they transmit about changes in the environment. The circumoral ring does not show complex structure but appears to act as a connection between the radial nerve cords and does not appear to contain organizing centres. The present evidence suggests that any part of the radial nerve cords when receiving significant local sensory input can act to coordinate whole animal behaviour and thus the echinoderms can be considered “brainless”.