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Journal of Experimental Biology 32,59-69 (1955)
Published by Company of Biologists 1955

Studies on the Myoneural Physiology of Echinodermata : II. Circumoral Conduction in Cucumaria

WALTER POPLE ¹ and D. W. EWER ²

¹ Department of Zoology, University of Natal, Pietermaritzburg
² Department of Zoology, University of Natal, Pietermaritzburg; Rhodes University, Grahamstown, South Africa

1. The general characteristics of circumoral nervous conduction in Cucumaria have been studied by the use of preparations consisting of the retractor muscles and radial nerves of two adjoining radii joined by a sector of circumoral nerve ring and by the use of similar preparations of all five retractor muscles and the complete circumoral nerve ring.

2. The characteristics of the responses of muscles stimulated by way of circumoral nerve tracts are as follows: the muscles respond with a quick and a delayed response; the magnitude of these responses depends upon the intensity of stimulation applied to an adjoining radial nerve, but is unaffected by frequency of stimulation up to a rate of 10 s./sec.; at high frequencies of stimulation both quick and delayed responses are depressed; the conduction velocity of impulses releasing quick and delayed responses is of the same order; the delayed response may show a prolonged facilitation previously analogized with post-tetanic potentiation. In these characteristics the muscular responses to impulses conducted in the circumoral nerve tracts are similar to those found to impulses conducted in the radial nerve tracts alone.

3. When, in a preparation of the five-retractor muscles, a radial nerve is stimulated, the muscles of radii nearer the stimulated nerve contract more strongly than those of radii further away.

4. Evidence is presented in favour of the view that this ‘decremental’ effect is dependent upon the geometrical arrangement of the fibre tracts in the circumoral nerve. The effect is not dependent upon frequency-sensitive synaptic junctions nor upon proprioceptive relays.