Fig I - uploaded by Ulrike Spörhase
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a-e. Lateral giant fibers (LGF) of midbody segments (segments 50 and 51) in a whole-mount preparation; ventral views, anterior at top. a Photomontage of different focus levels showing the bilateral LGFs with contralateral somata and three of their collaterals (CL2-CL4) in segment 50; both CL3s give rise to a subcollateral CLY and, like the two CL4s, contact each other, x 360. b Camera lucida drawing of both segments with the complete set of LGF collaterals (CL1-CL4) and their relation to the segmental nerves (SN1, SN2/3); note variations in LGF diameter and in number and size of dendrites at CL3s and somata, x 81. e Drawing of area marked in a showing the contact region at the septal membrane (S) of two consecutive LGF elements (left LGF of segment 50 filled, left LGF of segment 51 stippled, right third collateral open). One large (D) and two small dendrites emerge at the soma and numerous short extensions at CL3; the arrow points to the contact between both CL3s. Scale bar: a = 50 ~tm, b = 220 I~m, c =40 p.m
Source publication
The anatomical organization of the two dorsal giant fiber systems of the earthworm Lumbricus terrestris is demonstrated in whole mounts and serial-section reconstructions based on backfillings of the ventral nerve cord with cobalt chloride. Both the medial and lateral fiber systems can be labeled selectively over more than ten body segments. They s...
Citations
... the lobster Giant fiber systems typically consist of intersegmentally projecting neurons, which are often coupled electrically to each other and which are designed for rapid information transfer in the central nervous system (CNS) (Eaton, 1984). Like the welldescribed giant fiber systems in the crayfish (see Wine and Krasne, 1982) (Günther and Walther, 1971;Dorsett, 1978;Gras et al. 1988), the giant fiber system of the lobster includes paired medial and lateral elements (Ma, 1994). The data presented here demonstrate that the morphological features of the lobster LG and MG neurons closely resemble those of their putative crayfish homologs. ...
... After Lucifer Yellow CH injection, dye-coupling is regularly observed between the ipsilateral but not the contralateral LGs, suggesting the existence of gap junctions along the large contact zones ( Fig. 2; Remler et al. 1968;Wine and Krasne, 1982;Viancour et al. 1987). The transfer of fluorescent dye molecules or cobalt between neurons has been observed in other invertebrate giant fiber systems in which the cells are electrically coupled to each other (Kensler et al. 1979;Strausfeld and Bassemir, 1983;Gras et al. 1988). Here we observed that ortho-and antidromic action potentials can be faithfully propagated over several segments in ipsilateral LGs stimulated at frequencies of up to 100 Hz. ...
... was not directly demonstrated. The LGs in crayfish (Watanabe and Grundfest, 1961) and in earthworms (Günther and Walther, 1971;Gras et al. 1988) are electrically coupled both between ganglia, via the extended contact zones of the anteriorly projecting axons, and within ganglia, via neuritic cross bridges to the contralateral LG neuron. In lobsters, LG axon action potentials never evoke firing of the LG on the contralateral side of the same or adjacent ganglia. ...
Serotonin-containing neurosecretory neurons in the first abdominal ganglion (A1 5-HT cells) of the lobster (Homarus americanus) ventral nerve cord have been shown previously to function as 'gain setters' in postural, slow muscle, command neuron circuitries. Here we show that these same amine neurons receive excitatory input from lateral (LG) and medial (MG) giant axons, which are major interneurons in phasic, fast muscle systems. Activation of either LG or MG axons elicits short-latency, non-fatiguing, long-lasting excitatory postsynaptic potentials (EPSPs) in A1 5-HT cells which follow stimulus frequencies of up to 100 Hz in a 1:1 fashion. Single spikes triggered in either giant axon can produce EPSPs in the A1 5-HT cells of sufficient magnitude to cause the cells to spike and to fire additional action potentials after variable latencies; action potentials elicited in this way reset the endogenous spontaneous spiking rhythm of the A1 5-HT neurons. The giant-axon-evoked EPSP amplitudes show substantial variation from animal to animal. In individual preparations, the variation of EPSP size from stimulus to stimulus was small over the first 25 ms of the response, but increased considerably in the later, plateau phase of each response. When tested in the same preparation, EPSPs in A1 5-HT cells evoked by firing the LG axons were larger, longer-lasting and more variable than those triggered by firing the MGs. Firing A1 5-HT cells through an intracellular electrode, prior to activation of the giant fiber pathway, significantly reduced the size of LG-evoked EPSPs in A1 5-HT cells. Finally, morphological and physiological results suggest that similarities exist between giant fiber pathways in lobsters and crayfish. The possible functional significance of an involvement of these large amine-containing neurosecretory neurons in both tonic and phasic muscle circuitries will be discussed.
Multisegmental methylene blue filling of the three dorsal giant fibres in the nervous system of the earthworm was obtained by the use of electrophoresis. There was no difference in aldehyde fixed and unfixed tissue; due to its larger diameter, the median giant fibre could be stained over longer distances than the lateral ones. Accumulations of the dye were detected in the median giant fibre in the dorsal openings of the myelin sheath and at the origins of the ventral branches descending from both fibre systems. Since no significant axonal filling was observed by using several other dyes similar to methylene blue in diffusion and dialysis properties, the staining mechanism obviously depends on the specific binding sites of methylene blue in combination with the current. Therefore, the revealed phenomena provides evidence of a direct correlation between methyelne blue coupling and electrical coupling in the dorsal giant fibres of the earthworm.
