The structure of nerve fibres in cephalopods and crustacea

Abstract

In Sepia the peripheral nerves consist of 6000-8000 fibres, varying in diam. from 1-200 [mu] (to nearly 1 mm. in Loligo), each axon filling a sheath formed of nucleated sheets of tissue which histologically and in its staining reactions resembles collagenous connective tissue. The viscous-fluid axoplasm of a giant fibre, although a syncytium of processes from several cells, shows no definite continuous neurofibrils. The faint longitudinal striation visible in fresh axons suggests longitudinally orientated micelles which by coagulation produce the more definite fibrils apparent in damaged or stained axons. By cutting out and weighing the appropriate parts of microphotographs of the nerve the axoplasm was found to occupy 65-70% of the cross-sectional area. In Mala the axons, making up 60-70% of the cross-sectional area, are surrounded with a cellular collagenous sheath, and grouped into bundles bounded by a perineurium. Their diam. varies from 1-20 [mu]. Examination of serial sections showed no cell bodies along the length of the nerve. In the collagenous sheath of both Sepia and Maia there is a layer of orientated fatty molecules, but no true myelin sheath is present.