Video-enhanced contrast-differential interference contrast microscopy has revealed new features of axonal transport in the giant axon of the squid, where no movement had been detected previously by conventional microscopy. The newly discovered dominant feature is vast numbers of "submicroscopic" particles, probably 30- to 50-nanometer vesicles and other tubulovesicular elements, moving parallel to linear elements, primarily in the orthograde direction but also in a retrograde direction, at a range of steady velocities up to +/- 5 micrometers per second. Medium (0.2 to 0.6 micrometer) and large (0.8 micrometer) particles move more slowly and more intermittently with a tendency at times to exhibit elastic recoil. The behavior of the smallest particles and the larger particles during actual translocation suggests that the fundamental processes in the mechanisms of organelle movement in axonal transport are not saltatory but continuous.

The giant nerve fiber of the squid (Loligo pealii L .) has been investigated in situ, and in fresh and fixed preparations, by differential interference microscopy and electron microscopy . A continuous, three-dimensional network, composed of threadlike elements, was disclosed in the axoplasm . The threadlike elements in the axoplasm are twisted as a whole into a steep, right-handed helix . In a peripheral ectoplasmic region, the elements are more parallel to one another and more densely packed than in a central endoplasmic core . The threadlike elements can be resolved into a hierarchy of decreasing order of size . Successive levels of the hierarchy are formed by the association of smaller elements into larger ones . The following levels in the hierarchy of network elements have been distinguished : 1-3-µ-wide threads, 0 .1-0 .35-µ-wide strands, and 70-250-A-wide unit-filament strands . The differential interference microscope selects, from the network, threads oriented at a specific angle to the long axis of the axon . The specific angle depends upon the orientation of the long axis of the axon relative to the direction of shear . It is postulated that the network configuration is expressed in the solid-state properties of the axoplasm essential for the normal functioning of the nerve fiber .

The assembly of filamentous elements and their relations to the plasma membrane and to the nuclear pores have been studied in Deiters' neurons of rabbit brain . Electron microscopy of thin sections and of ectoplasm spread preparations have been integrated with physicochemical experiments and differential interference microscopy of freshly isolated cells . A neurofilamentous network extends as a continuous, three-dimensional, semilattice structure throughout the ectoplasm, the "plasma roads," and the perinuclear zone of the perikaryon . This space network consists of -90-A wide neurofilaments arranged in fascicles which are interconnected by an exchange of neurofilaments . The neurofilaments consist of intercoiled -20-A wide unit-filaments and are associated through cross-associating filaments with other neurofilaments of the fascicle and with microfilaments . The -20-50-A wide microfilaments display intimate associations with the plasma membrane and with the nuclear pores . Electron microscopy of thin sections from glycerinated and heavy meromyosin-treated Deiters' neurons shows that actin-like filaments are present in the pre-and postsynaptic regions of synapses terminating on these neurons .It is proposed that the neurofilamentous space network serves a transducing function by linking plasma membrane activities with the genetic machinery of the neuron . INTRODUCTIONtion of structures and mechanisms responsible for such a transduction is essential for an understandAt present one of the most important problems in ing of the functioning of the neuron . However, neurobiology is to determine how the activities of concrete data concerning the morphological and the neuronal surface are linked to the genetic physicochemical organization of a plasma memmachinery in the nucleus of the neuron . A descrip-brane-DNA axis in eukaryotic cells is scarce .