Contribution to the knowledge of the nervous system in the tentacles of some coelenterates (Anemonia sulcata, Metridium senile, Cerianthus membranaceus, Tealia felina and Hydra vulgaris)
Bijdragen tot de dierkunde , Volume 46 - Issue 2 p. 219- 260
The nervous system in the tentacles of the sea anemones Tealia felina, Anemonia sulcata, Metridium senile and Cerianthus membranaceus was studied using light microscopic and electron microscopic techniques. Because of the small dimensions of the nerve cells (6—7 µm) and of the neurites (diameter < 1 µm) satisfactory information could not be obtained using conventional histological techniques. Electron microscopic investigation showed that the nervous system can be divided into three parts: the plexus round the mesogloea, a nervous system between the muscles (obviously connected with the plexus) and sensory cells in the outer layer of the tentacle connected to the plexus by nerve fibres. The latter nerve cells with their fibres are arranged radially in the tentacle ectodermis. These are the only sensory cells discovered in the tentacles of the sea anemones. In these radial neurites and in a number of neurites in the plexus, dense core granules are found. In the nervous system between the muscles and in a number of neurites in the plexus, opaque granules are found. Neurites containing dense core as well as opaque granules were never observed. Only in the radial neurites and in the plexus a yellow F(ormol) I(nduced) F(luorescence) was observed. Analysis of the emission spectrum showed that the F.I.F. had developed from a catecholamine (most probably noradrenalin). Therefore the dense core granules contain a catecholamine. On the basis of morphological similarity, the neurites containing opaque granules may be identified as purinergic as proposed by Burnstock (1972), since the innervation of the muscles in the sea anemones very much resembles the innervation of smooth muscles in vertebrates. Synapses as described by a.o. Westfall (1973a) could not be demonstrated. However, desmosome-like structures were found between the epithelial cells and between the muscles, so that a non-neural conduction (c.q. myoepithelial conduction) is probable. This myoepithelial conduction may explain the presence of a “second nervous system” postulated by Bullock & Horridge (1965) which is supposed to be a slowconductive system. A morphological indication for a “second nervous system” has never been found. The two transmitter substances mentioned (no indication was found for the presence of GABA and acetylcholine), i.e. a catecholamine, most probably noradrenalin, and a purine derivate, both have an excitatory function. The possible role of glutamate as an inhibitor has been discussed. Glutamate acts as a possible inhibitor, since it is released from contracting muscles and inhibits the contraction via an unknown mechanism. Hydra was investigated and the findings were discussed in relation to the existing literature. Only the existence of synapses was discrepant, since these structures could not be demonstrated. Regarding the possible transmitters a catecholamine could be demonstrated with the F.I.F. method. A purinergic muscle innervation is possible in view of our experience with sea anemones.
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van Marle, J. (1977). Contribution to the knowledge of the nervous system in the tentacles of some coelenterates (Anemonia sulcata, Metridium senile, Cerianthus membranaceus, Tealia felina and Hydra vulgaris). Bijdragen tot de dierkunde, 46(2), 219–260.