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The Drosophila Giant Fibre System |
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J.A. Drummond, M.J. Allen and K.G. Moffat
Dept. Biological Sciences, University of Warwick, Coventry, CV4 7AL, U.K.
Introduction
P[GAL4]-307 enhancer-trap pattern.
Giant fibre dendritic field
Giant intercommisure
Cervical connective
Ventral nerve cord
Giant fibre - TTMN synapse
Chordatonal neurons
References
The giant fiber circuit mediates the light-off escape response by relaying excitation from the eyes to the muscles of the thorax.
The giant fibers are a pair of large bilaterally symetrical interneurons (2) with cell bodies about 20mm in diameter. The cell bodies are located close to the posterior border of the brain, adjacent to the neuropile of the protocerebrum, about 30mm dorsal to the oesophageal canal. The main process of the neuron is connected to the cell body via a neurite, some 60mm long. The dendritic field, composed of dorsomedial, ventrolateral and posterolateral processes, branch from this point, as does the axon. The axon projects dorsally and posteriomedially towards the ventral midline, before ascending past the oesophageal canal and through the dorsal region of the cervical connective to the mesothoracic neuromere (3).
Each giant fiber axon has a lateral bend in the mesothoracic neuromere.
Just before the lateral bend each giant fiber electrically synapses, near the inframedial bridge at a tuft of collaterals, with the peripherally synapsing interneuron (PSI). The PSI projects across the ganglion and chemically synapses onto the five motorneurons (DLMNs) of the contralateral dorsal longitudinal flight muscles (dlms), fibrillar indirect flight muscles (4). At the end of the lateral bend the giant fiber synapses again, this time with the large tergotrochanteral motorneuron (TTMN) - innervating the tergotrochanteral muscle (also known as the jump muscle or flight starter muscle) (5). The ttm is the largest tubular muscle in the thorax, spanning the thorax dorsoventrally. It originates on the scutum and inserts on an apodeme in the proximal femur of the mesothoracic leg. This muscle acts as a leg extensor during escape take off. In addition to this there is good evidence that the GFs also activate other neurons during the escape behaviour. These include the motorneurons for the bilateral anterior pleural number 3 muscles, controlling abduction of the wings to their flight position; the tibial levator motorneuron (TLMN), responsible for femur-tibia joint extension; and the dorsal ventral muscle motorneurons, innervating the antagonistic muscles to the DLMs to generate wing movements necessary for initiation and maintenance of flight during escape behaviour (6)
P[GAL4]A307, picked out in an enhancer trap screen, marks the giant fibers throughout their development (7) and also appears to mark other neurons of the circuit, including the PSI and the TTMN.
P[GAL4] enhancer trap line A307 crossed to UAS-lacZ (a cytoplasmic LacZ provided by Andrea Brand) on the X chromosome. Beta-galactosidase was detected as described, with the exception of a biotinylated anti-rabbit IgG secondary and visualised using Vector labs Elite ABC kit (PK-6101). A number of distinctive cell bodies can be discriminated, including the GFs, the TTMN and the PSI. Other cells are also marked but have not been identified
The pattern reveals the characteristic morphology of the giant fiber projections, the cell bodies (gfcb), present just below the dorsal protocerebrum. These project a neurite posteriorly and ventrally to the dendritic field and the axons. The axons project ventrally and posteriorly towards the ventral midline subsequently projecting dorsally to the connective (Conn). The axons extend unbranched into the ventral nerve cord (VNC) as far as the mesothoracic neuromere (T2), where they project ventrally and bend laterally (arrowhead). An anterior dorsal cell body (ADCB) is also a characteristic feature of the pattern, with an axon projecting medially toward the deuterocerebrum, ending in a mesh of fine projections.
This image gives an overview of the pattern using a Zeiss axioskop with attached JVC TK-1280E colour video camera . Images were processed on a PowerMac. Greater detail is achieved using higher magnifications and heavier staining (prolonging the colour reaction, using higher concentrations of antibodies and growing the developing flies at 29 C all contribute). Adult CNS, dorsal view. Scale bar is 100µm.
The following images present the data in greater detail.
The accompanying series of images more clearly illustrate divisions and extent of the dendritic field in the brain. From the giant fiber cell body (GFCB) a neurite projects ventrally and posteriorly, to the site of the dendritic arborisations. These are the dorsomedial dendrites (DMD), ventrolateral dendrites (VLD) and the posterolateral dendrites (PLD). Two large cell bodies in the posterior suboesphageal ganglion can also be seen sending axons dorsally towards the foramen.
Adult CNS of which the brain is shown here. View from dorsal. Focused dorsal to ventral, A - D. Scale bar is 50µm
On further staining, processes from the giant intercommisure neurons can be seen crossing the giant intercommisure (GIC). Although these neurons are known to dye couple they have not been shown to participate in the functioning of the GF pathway. At a medial level (compared with the GFs at a dorsal level) are neurons, labelled with red arrowheads, also seen in accompanying images of the connective. Their cell bodies have not been identified
Adult CNS showing brain region . Dorsal to ventral A - D. Scale bar is 50µm
The connective between brain and vnc show gfs and two other pairs of axons. As the giant fiber is at the dorsal edge of the connective it is amongst many descending neurons (8). It is difficult to ascertain whether the other neurons are ascending or descending as they are located medially (red and green arrowheads).
Adult CNS, connective visible here. Scale bar is 50µm
The GFs enter the vnc, passing through T1 into T2 bending and electrically synapsing with the TTMN (synapse shown by red arrowhead). The cell body marked (PSICB) has a position and projection (PSI ) consistent with it being the peripharally synapsing interneuron. This feeds into the bundle of afferent projections, passing by two large cell bodies (black arrowheads, A). The inframedial bridge (IB) is also visible (C).
Adult CNS of which vnc, largely T1 and T2, are visible. Anterior is left. Dorsal to ventral A - E. Scale bar is 50µm
Other neurons (arrowheads) enter the neuromeres via nerve roots at the ventral surface of the vnc (ventral prothoracic nerves and mesothoracic accessory nerves). These neurons project into the neuromeres T1, T2 and possibly T3. Based upon previous descriptions they have been putatively identified as a subset of projections from chordotonal organs (9).
Dorsal view of the ventral surface. Anterior is to left. Adult CNS, T1 and T2 visible here. Scale bar is 50µm
Other neurons (arrowheads) enter the neuromeres via nerve roots at the ventral surface of the vnc (ventral prothoracic nerves and mesothoracic accessory nerves). These neurons project into the neuromeres T1, T2 and possibly T3. Based upon previous descriptions they have been putatively identified as a subset of projections from chordotonal organs (9).
Dorsal view of the ventral surface. Anterior is to left. Adult CNS, T1 and T2 visible here. Scale bar is 50µm
Power,M.E. J.Comp.Neurol. 88: 347-409 (1948)
Bacon,J.P. & Strausfeld,N.J. J. Comp. Physiol. A 158: 529-548 (1986)
Tanouye, M.A. & Wyman,R.J. J. Neurophysiol. 44: 405-421 (1980)
Thomas,J.B. & Wyman,R.J. Nature 298: 12 (1982)
Trimarchi,J.R. and A.M. Schneiderman J. Exp. Biol. 177: 149-167 (1993)
Phelan,P. et .al. J. Neuroscience 116: 1101-1113 (1996)
Strausfeld,N.J. Atlas of an Insect Brain (1976) Springer-Verlag
Phillis,R. et al. Development 122 (10): 2955-2963 (1996)
For complementary data, click here
Flybrain
Genetic Dissection
Enhancer Trap Lines
List
Giant Fiber (Line 307)
Giant Fiber Circuit
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