In vivo conduction properties of regenerating giant nerve fibers in earthworms

Axonal conduction and electrical coupling in regenerating earthworm giant axons

Severed halves of medial giant axons (MGAs) and lateral giant axons (LGAs) in earthworms survive and are functionally reconnected as early as the first postoperative week. During the first 150 postoperative days, there is an increase in conduction velocity of action potentials and strength of electrotonic coupling between the severed axonal stumps across the lesion site. Electrophysiological analyses suggest that this functional reconnection occurs by transmission of action potentials through the lesion site by active propagation along neurites which make electrotonic connections rather than chemical synapses. The regenerated connections restore the original connectivity pattern for conduction of action potentials or spread of electrotonic potentials; i.e., MGA stumps reconnect with MGA stumps, and LGA stumps with LGA stumps. These and other data suggest that the mechanisms responsible for establishing appropriate functional reconnection of severed earthworm giant axons requires cell-specific matching of axons and neurites, rather than a competition between appropriate and inappropriate functional connections.

Analysis of neuritic outgrowth from severed giant axons in Lumbricus terrestris

This study analyzes the detailed morphometric pattern at various postoperative times of neuritic outgrowths from the proximal and distal stumps of two uniquely identifiable axons. Morphological patterns of neuritic outgrowths from stumps of severed axons were compared for medial and lateral giant axons in the central nervous system of the earthworm Lumbricus terrestris. Outgrowths from proximal and distal stumps were labeled by injection of fluorescent dye into axonal stumps and assessed according to morphometric parameters. Outgrowths from axonal stumps of severed giant axons were statistically indistinguishable for most morphometric measures of neuritic quantity, shape, direction, and location. There were two exceptions to this general rule: 1) proximal stumps of medial giant axons produced significantly more neurites than distal stumps of medial giant axons, and 2) proximal stumps of lateral giant axons produced significantly longer neurites than proximal stumps of medial giant axons. No measure of neuritic outgrowth showed a significant change from the second through seventh postoperative week, suggesting that most outgrowth occurred in the first two postoperative weeks and that neuritic morphology remained stable through the seventh postoperative week. Neurites grew across the lesion site in relatively straight trajectories parallel to the longitudinal axis of the ventral nerve cord and often grew alongside the appropriate axonal stump across the lesion site. The length of neurites growing in close apposition to appropriate axonal stumps or giant axons was much greater than expected, had outgrowth been randomly directed. These data provide a basis for future investigations of the mechanisms that regulate neuritic outgrowth.

The emergence of electroreceptor organs in regenerating fish skin and concurrent changes in their transduction properties

The process of regeneration of skin patch denervated empullary electroreceptor organs of the African catfish Clarias gariepinus has been investigated at an ambient temperature of 28 degrees C with both electrophysiological and histological methods. At day 1 after denervation none of the receptor organs on the skin patch showed afferent activity. At this stage none of the ampullary organs previously recorded showed a normal appearance. Degenerative changes consisted of a decreased number of receptor cells and an often invisible lumen. At day 7 regeneration seems to start with a high density of primordial ampullary organs, more than a seven-fold increase compared to controls. In these units, the level of spontaneous activity is very low: compared to controls, more than a two-fold increase in mean interspike interval. At this stage, the sensitivity to electrical stimuli is already at the level of untreated control organs. At day 15 there is a lower, i.e. approximately normal, density of ampullary organs with a normal morphology. In these units both spontaneous firing and sensitivity returned to normal. It can be concluded that the functional dichotomy between spontaneous firing and sensitivity that was found in degenerating ampullary electroreceptor organs is also found during the process of their regeneration, although the underlying cellular changes may be totally different. The speed of recovery suggests that only regeneration of the distal part of the sectioned nerve fibers takes place.

Donor-recipient interconnections between giant nerve fibers in transplanted ventral nerve cords of earthworms

Twelve segments of ventral nerve cord (VNC) from donor earthworms, Eisenia foetida, were transplanted into recipient worms from which a comparable length of VNC had been removed. Within the first few days after transplantation, bud-like formations, containing outgrowths of the giant nerve fibers, were evident at the ends of transplanted and recipient VNC. Morphological and electrophysiological evidence indicated that by 4-10 days after transplantation, medial (MGF) and lateral (LGF) giant fibers within the transplanted VNC formed cell-specific connections with their counterparts in the recipient VNC. Although the diameters of the giant fiber connections in the transplant-recipient junctions were often larger than normal, spike conduction across the junction was initially slow (approximately 1.0 m/s) but gradually increased over the next 2-3 weeks. Within the transplant, giant fibers were initially normal in appearance, but spike conduction was slow (1-2 m/s). During the next few weeks velocities increased by as much as fourfold and then stabilized for the next several months. However, by 4-5 weeks after transplantation, giant fiber morphology within the transplant was altered significantly, as indicated by the formation of numerous branch-like extensions along the length of each giant fiber. By 9-10 months there were further morphological changes in the transplant, as indicated by decreased branching of the giant fibers and altered neuropile. Despite these morphological changes, through-conduction of giant fiber spikes remained reliable.

Electrophysiological correlates of rapid escape reflexes in intact earthworms, Eisenia foetida. I. Functional development of giant nerve fibers during embryonic and postembryonic periods

Grids of recording electrodes etched onto printed circuit boards were used for noninvasive recording of medial (MGF) and lateral (LGF) giant nerve fiber spikes in developing earthworms, Eisenia foetida. Stereotyped patterns of through-conducted giant fiber spikes, evoked by light tactile stimulation, were first detectable in the normal crawling embryonic stage and continued to be detectable throughout postembryonic development. Giant fiber spiking activity in normal crawling embryos was accompanied by stereotyped muscle activity and rapid escape withdrawal, suggesting that giant fiber reflex pathways are functionally intact before the worm hatches. For both the MGF and LFG, several age-dependent changes were noted, including the following: increases in spike conduction velocity, increases in giant fiber diameter, and decreases in spike duration. The MGF conduction velocity in normal crawling embryos was 1.1-1.6 m s-1 (6-7 micrometers diameter) and increased to 7.0-8.5 m s-1 (20-25 micrograms diameter) by 60 days after hatching. The LGF conduction velocity in normal crawling embryos was 0.7-1.1 m s-1 (2.5-4.0 micrometers diameter) and increased to 4.0-5.5 m s-1 (8-14 micrometers diameter) by 60 days after hatching. During postembryonic development MGF and LGF conduction velocities were linearly related to fiber diameter.