Synaptic Structure and Transmitter Release in Crustacean Phasic and Tonic Motor Neurons

Abstract

The paired phasic and tonic motor neurons supplying the extensor muscle in the crayfish leg were investigated to establish whether differences in synaptic structure could account for large differences in transmitter release at the neuromuscular junctions. Nerve terminals with transmitter release that had been assessed from recordings made with a focal "macro-patch" electrode were subsequently labeled, processed for electron microscopy, and reconstructed from serial sections. At a frequency of 1 Hz, quantal contents of phasic terminals were 90-1300 times greater than those of tonic terminals when both were recorded at the same location. At higher frequencies, facilitation was pronounced at tonic, but not phasic, terminals. Reconstructions of recording sites showed that both phasic and tonic terminals possessed many small synapses, usually with one or more structurally defined active zones. Mean synaptic contact area was larger for tonic terminals, and the number of individual synapses per length of nerve terminal was also larger. Active zones were not different in size for the two terminals. At low frequencies, quantal emission per synapse is much greater for phasic terminals. The higher quantal content of phasic terminals and their synapses cannot reasonably be accounted for by more or larger synapses or active zones at the recording sites. Because structural features alone are not likely to produce the very large differences in quantal content of phasic and tonic terminals observed at low stimulation frequencies, it is likely that other properties of the nerve terminal are largely responsible for these differences.

The Neuromuscular Junctions of the Slow and the Fast Excitatory Axon in the Closer of the CrabEriphia spinifronsAre Endowed with Different Ca2+Channel Types and Allow Neuron-Specific Modulation of Transmitter Release by Two Neuropeptides

“…Terminals of slow and fast neurons innervating crustacean muscles differ in morphological and physiological parameters such as number of release sites, quantal content, and facilitation or depression of transmitter release (Hoyle and Wiersma, 1958;Bittner, 1968;Rathmayer and Hammelsbeck, 1985;Atwood and Wojtowicz, 1986;King et al, 1996;Bradacs et al, 1997;Nguyen et al, 1997;Lnenicka et al, 1998;Msghina et al, 1998Msghina et al, , 1999. While studying peptidergic modulation of release by the FMRFamide-like DF 2 (DRNFLRFamide) and proctolin, we noted that DF 2 affected the slow and fast axons differentially.…”

Intrinsic differences in sensitivity to 5-HT between high- and low-output terminals innervating the same target

“…In fact, it was shown that phasic terminals, in the same type of NMJ preparations as used in our study, have a 2Á/3 times larger amount of calcium entering during evoked stimulation as compared to tonic terminals . This difference appears to be related to the differences in the calcium channels of the two terminals as well as synaptic structure in the degree of spacing between active zones on synapses (King et al, 1996;Cooper et al, 1996b;Msghina et al, 1998).…”

“…swellings of the terminal). Previous work in serial sectioning of these regions had shown that the majority of synapses occur within the varicosities and not in the bottleneck regions (Florey and Cahill, 1982;Cooper et al, 1996a;King et al, 1996;Msghina et al, 1998). From photos, the innervation profiles on the fibers are able to be traced as shown (Fig.…”

“…Although the varicosities of tonic terminals are much larger, they have a significantly lower transmitter output than phasic terminals at low frequencies of nerve impulse activity (Bradacs et al, 1997;King et al, 1996;Msghina et al, 1998). The two excitatory motor neurons are morphologically different.…”

Developmental morphological changes in protrusions in Artemia intestinal circular muscles

“…Schrehardt (1987) previously reported that midgut innervation exclusively involves its posterior part and is stimulated by multipolar neurons. While chemical synapses occur through vesicles on the side of the nerve, a synapse-like structure has been reported in Crustacea at the neuromuscular junction (Govind & Derosa, 1983;Msghina et al, 1998;Johnstone et al, 2011). Tisdale & Nakajima (1976) reported that synaptic vesicles vary in size.…”