REPORT ON GIANT NERVE FIBRES IN NANOMIA

Abstract

The siphonophore Nanomia has a pair of giant fibres running along the stem, the larger of which conducts at velocities up to 3 metres/second. Histologically the two giant fibres appear to be composite, partially syncytial structures representing specializations within the general ectodermal nerve plexus. Electrical events are through-conducted independently in the two fibres. Both systems excite the stem musculature. Conduction velocities are proportional to fibre diameters. Conduction occurs in both directions in each giant fibre. An absolute refractory period of 5-7 milliseconds is observed. The two systems are spontaneously active, showing distinctive pulse patterns. Intracellular recordings with glass microelectrodes from the larger giant fibre show conventional action potentials of about 60 millivolts. Addition of tetrodotoxin to the outside medium at 10[-6]g/ml does not affect spike production. Palpon tentacles have two chains of small neurones. Two conduction systems, one faster than the other, can be shown physiologically in the tentacles. This may be taken as a simplified model for the stem. The existence of two synergistically acting fast conducting systems is interpreted in relation to gradation of response intensity with distance from the site of stimulation.