The Giant Fiber Pathway

The Giant Fiber Pathway

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The giant fiber pathway in normal and shaking-B mutant Drosophila. Images and narrative pertaining to ß-galactosidase and dye filled neurons provided by P.Phelan and J.P.Bacon, Sussex University (J.P.Bacon@sussex.ac.uk). Cobalt images by N. J. Strausfeld, University of Arizona, Tucson (flybrain@neurobio.arizona.edu).

This module shows elements of the giant fiber (GF) pathway: the paired giant fibers; connections between them in the brain by the giant commissural interneurons (GCI); the connections in the mesothoracic ganglion between the GF endings and the peripherally synapsing interneurons (PSI) and tergotrochanteral motor neurons (TTMN), both of which are crucial elements mediating the jump response. The module is predominantly compiled from enhancer trap studies and dye-coupled neurons revealed by intracellular injection of lucifer yellow, and antero- or retrograde filling into giant fibers using cobalt and silver intensification.

Fig 1A. Enhancer-trap line A307 reveals GF anatomy. Dorsal view of an adult nervous system stained for ß-galactosidase activity. The GF are distinguished by their characteristic morphology. The cell bodies (arrowhead on the right) and dendrites are out of focus; the distal tip of the descending axon bends laterally in the mesothoracic neuromere (arrow). The axons of the giant commissural interneurons (GCI) and small clusters of cells in the thoracic and abdominal neuromeres are also labeled. Fig. 1B Lucifer yellow injection into the adult GF reveals a functional assembly of coupled neurons. Images were constructed by stacking optical sections taken at 2 ľm steps in whole-mount nervous systems. This low magnification reconstruction of the whole nervous system shows some dye-coupled elements that match reoprted gene expression shown in Fig. 1A. Note that the contralateral GF and several other neurons in the brain and thoracic ganglion are labeled by dye transfer. These can be viewed at higher magnification by clicking on the boxed areas.

Fig. 2A. The left GF was backfilled with cobalt from its axon in the mesothoracic ganglion. Shown here are the ventral and lateral dendrites that receive mechanosensory and visual inputs, respectively. The dorsal dendrites are cobalt-coupled to the giant commissural interneurons (also termed in the literature as "Giant Interneurons - GIN). The contralateral GF is shown by cobalt migration into it from the GCI.

Fig. 2B. Detail of Lucifer yellow dye-coupled GF (right) and three GCIs that originate in the contralateral protocerebrum. This confocal stack shows the relatively small cell bodies of the GCIs compared to the large cell body of the GF which has been omitted from the optical section stack in Fig. 2A. The dye-coupled GCIs cross the central commissure. Their dendrites arborize in the same region as the dendritic tree of the contralateral GF which, in this example, has not been filled.

Fig.3. Comparison of dye-coupling in mutant and wild-type flies. In the shak-B2 mutant (A), intracellularly injected Lucifer Yellow is confined to the GF. This is in marked contrast to wild-type flies where this low molecular weight dye transfers to the GCIs in the brain, the TTMN, PSIs and several other neurons in the thoracic ganglion (B). The GF shown in (C) was co-injected with a 10,000 MW rhodamine dextran dye, which does not move through gap-junctions. The pattern of labeling (C) is identical to that seen when Lucifer Yellow is injected into the mutant GF (A). Scale: 100 ľm

Fig. 4A, B. details of the terminal of the Lucifer yellow-filled GF shown in Fig.2B. In A, the GF axon shows two regions of dye coupling. Medially, before its characteristic bend towards the margin of the mesothoracic ganglion, it is coupled to the ipsi- and contralateral PSIs and weakly coupled to a cluster of other interneurons, the cell bodies of which show up pale. Also medial, is the site of coupling between the filled GF and the contralateral GF which is here visible as the pale mirror image of the filled neuron. The peripherally directed terminal of the GF is dye-coupled only to the ipsilateral TTMN. B shows a more ventral level of the ganglion, revealing dendrites of the PSI that, presumably, are conventionally postsynaptic to other neurons.

Fig. 5A, B. Two levels showing stacked optical sections of the pro- (T1) and mesothoracic (T2) ganglia. Several descending axons, amongst them the right GF, have been filled transsynaptically with cobalt introduced into the mechanosensory strand of the antennal nerve. Note the characteristic bend of the GF. This fill was of short duration and only the contralateral PSI has received cobalt across the gap junction. Details of the connection between the GF and PSI are shown in Fig. 6.

Fig. 6. Two consecutive optical stacks showing the specializations from the GF at its area of contact with the axon of the PSI.

Fig. 7A-D. The GF dye couples sequentially to other neurons during metamorphosis. These fluorescence micrographs show dorsal views of pupal nervous systems in which Lucifer Yellow was injected intracellularly into the right GF. (A) At 45 hours APF (after puparium formation), the GF is dye-coupled to the TTMN (arrowhead) at the lateral margin of the mesothoracic neuromere. TTMN processes are not visible. (B) At 48 hours APF, the GF axon, now clearly bends laterally in the mesothoracic neuromere (arrow). The bilaterally symmetrical coupled neurons are the PSIs. Posterior to these, part of the medial neurite of the TTMN is shown (lower arrowhead) contacting the distal region of the GF axon. The TTMN cell body is faintly labeled and out of focus (right arrowhead). (C) At 65 hours APF, the GF and GCIs are dye-coupled in the brain. Their clustered cell bodies are out of focus. (D) At 95 hours APF, the pattern of GF dye coupling in the brain and thoracic ganglion is as observed at 65 hours APF, is essentialy adult-like, and shows no further increase in GF or GCI axonal diameters. Scale bars: A, B, 50 ľm; C, D, 100 ľm

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Flybrain Genetic Dissection Enhancer Trap Lines List Giant Fiber (Line 307) Giant Fiber Pathway

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