NEUROBIOLOGY OF ECHINODERMATA
Authors: PENTREATH V. W. , COBB J. L. S. 
Published: January 2008

During the past ten years much information has been added to our knowledge of nerve and muscle systems of echinoderms.

1. Electron‐microscopy has shown that all the main nerve trunks consist of large numbers of small, parallel‐running unmyelinated axons which are packed tightly together. Glial cells are generally absent. Discrete regions of neuropile are recognizable by the interweaving of axons, and the presence of vesicles. It has not yet been found possible to locate synapses with certainty in the nervous system, but it appears that they are chiefly confined to neuropile. The obvious nerve cords are massive accumulations of neurons which do not appear to interact locally.

2. Peripheral axons are difficult to distinguish because both interstitial and muscle cells have processes which often resemble axons. Ultrastructural analysis of this problem is aggravated by difficulties in fixation. However, the electron‐microscope has shown that much of the echinoderm body wall contains a thick subepithelial plexus of processes from epithelial cells. Epithelial cells may thus act as sensory cells and supply axons to the plexus.

3. With the exception of striated muscles in some pedicellariae, all echinoderm muscles so far examined are of the smooth type. These muscles characteristically contain large filaments, and in this way do not resemble vertebrate smooth muscle. Some muscles are innervated by simple axonal contact, in others the muscles themselves send processes towards the nervous tissue.

4. Physiological studies of electrical activity in nerve and muscle systems have not added significantly to our knowledge of function. Several authors have demonstrated that massed electrical activity is conducted decrementally along the radial nerve cords, but this does not explain any known aspect of coordination. The only records of electrical activity from single neurons (Takahashi, 1964) have not been repeated.

5. There is strong evidence for two types of neurons in the central nervous system of echinoderms. One of these contains acetylcholine, the other dopamine and/or noradrenaline. Electron‐microscopical histochemistry has given good indication that catecholamines are bound in echinoderm nerve tissue to particles similar to those reported in other invertebrate nervous tissues, and there is good evidence that acetylcholine is bound to synaptic vesicles which are morphologically identical to those present in the mammalian brain. The available data further indicate that acetylcholine is a transmitter in sensory and motor neurons, while dopamine and/or noradrenaline are transmitters in interneurons. Such interneurons may be involved in the coordination of the movement of the tube‐feet. Other substances which have been implicated in neuro‐effector mechanisms in other animal groups have not been found or are present in very small quantities.

6. Studies on the reproductive physiology of starfish have shown that several substances in the radial cords play important roles in its control. Such substances cannot at present be called neurosecretions because it is not known if they are derived from neurons.

7. Pharmacological studies on isolated muscle tissues have not added significantly to our knowledge of their control. The potency of ACh in causing contraction is well documented, and anticholinesterases are similar in effect. Catecholamines, although clearly very important in the nervous system, do not produce clear‐cut effects. The published reports of relaxation to noradrenaline may well be due to direct effects on the muscle. No definite information has been obtained on the role of the adrenergic parts of the nervous systems of echinoderms, other than showing that they are not involved in motor responses. Extensive studies with a wide variety of drugs have produced inconsistent and largely negative results.