Differential physiology and morphology of motor axons to ventral longitudinal muscles in larval Drosophila

Abstract

Morphological and physiological characteristics of the two major motor axons supplying the commonly studied ventral longitudinal muscle fibers (6 and 7) of third-instar Drosophila melanogaster larvae were investigated. The innervating terminals of the two motor axons differ in the size of their synapse-bearing varicosities. The terminal with the larger varicosities also fluoresces more brightly when stained with the vital fluorescent dye 4-(4-diethylaminostyryl)-N-methylpyridinium iodide (4-Di-2-Asp) and occupies a larger total contact area on the muscle fiber. Through selective simultaneous recording of synaptic currents from identified boutons in living preparations during elicitation of synaptic potentials, it was shown that the axon with the smaller varicosities generates a large excitatory junction potential (EJP) in muscle 6 and that the axon with the larger varicosities generates a smaller EJP. Short-term facilitation is more pronounced for the smaller EJP. In preparations treated with 4-Di-2-Asp, the fluorescence of smaller varicosities increases with stimulation that elicits the large EJPs, indicating an activity-dependent entry of calcium that enhances mitochondrial fluorescence. The differences in morphology and physiology of the two axons are similar to, though less pronounced than, those observed in "phasic" and "tonic" motor axons of crustaceans.

Assessing ultrastructure of crustacean and insect neuromuscular junctions

“…Recent studies using the combined approach have enhanced the view that many terminals possess silent synapses, and that the number of active synapses can be adjusted during short-term and long-term facilitation to Ž . meet physiological requirements Wojtowicz et al, 1994 . In addition, there is reason to believe that the number of synapses on a terminal is only one of the factors responsible for synaptic efficacy in both Drosophila and crus-Ž taceans Kurdyak et al, 1994;. Cooper et al, 1995c . As shown here, such studies may be subject to errors affecting the estimates of the sizes of small structures, but the general features of synaptic proliferation and enlargement are not qualitatively altered by these considerations.…”

Actions of MDMA at glutamatergic neuromuscular junctions

“…The innervation and synaptic properties of the Is and Ib motor nerve terminals that innervate muscle 6 of the third instar Drosophila larva have been thoroughly studied (Atwood et al, 1993;Kurdyak et al, 1994;Li et al, 2002). The Ib and Is terminals have different morphology and physiology, though they can be recruited separately or in unison.…”

“…The Ib and Is terminals have different morphology and physiology, though they can be recruited separately or in unison. The terminals of the Is axon contain small varicosities along its length and give rise to large EPSPs in the muscle, while the Ib axon has large varicosities and produces smaller EPSPs (Atwood et al, 1993;Kurdyak et al, 1994;Stewart et al, 1994) (Fig. 4A and B).…”

“…Selective stimulation to the axons that give rise to the Ib and the Is terminals was produced by altering the stimulus intensity and duration while monitoring the EPSP amplitudes in m6. Since the EPSP amplitudes are reflective of the type of terminal responding this measure was used (Harrison and Cooper, 2003;Kurdyak et al, 1994). The physiological solution, HL3, is the same as previously described .…”

Untitled

“…Chiba [29] boutons, and have been likened to the "phasic" or "fast" motor axons found in 174 crustaceans [48,49]. The multiple innervation provided by Type Is MNs could also be 175 important for coordinating the activity of muscle groups [28].…”