Structural features of crayfish phasic and tonic neuromuscular terminals

Abstract

We examined the fine structure of terminals of the phasic and tonic excitatory axon to the crayfish limb extensor muscle. The phasic terminals are known to release 50-100 times more transmitter for a small length of terminal for a single impulse. Phasic terminals labeled with horseradish peroxidase (HRP) were relatively thin and contained a single unbranched mitochondrion; tonic terminals were much thicker, and their varicosities contained several multibranched mitochondria. Tonic terminals devoted a larger proportion of their total volume to mitochondria. The percentage volume of clear synaptic vesicles was slightly higher in phasic axon terminals, but as the tonic axon terminals were fivefold larger in volume, the total synaptic volume is much greater in tonic than phasic terminals. The number of synapses per length of terminal, and the total number of active zones per length of terminal, were greater for tonic terminals, and individual synapses were, on average, slightly larger in surface contact area for tonic terminals. In contrast, individual active zones were, on average, longer in phasic synapses. A higher proportion (50%) of phasic synapses had multiple active zones than was the case for tonic synapses (16%), and pairs of closely spaced active zones were more frequently found on phasic synapses. These findings clearly rule out synapse and active zone number as a factor contributing to higher transmitter output, but suggest that active zone size and synaptic complexity, as evidenced by multiple closely spaced active zones in a single synapse, are likely to play a causal role in the greater transmitter release of the phasic terminal. Even synapse complexity would not be enough to account fully for the large difference in terminal transmitter output, and additional factors may include electrical and biochemical 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

“…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.…”

“…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.…”

“…Since the tonic terminals show many more synapses structurally per length of terminal than the phasic counterpart but display a lower synaptic output (King et al, 1996;Bradacs et al, 1997), we initially postulated that the tonic terminals would show a greater effect to the excitatory neuromodulator serotonin (5-HT) than phasic terminals by recruiting silent synaptic sites during neuromodulation. To examine differences in the action of 5-HT on the two types of nerve terminals, measures of EPSPs and excitatory postsynaptic currents (EPSCs), from discrete regions of the terminals, were used for comparisons of altered synaptic function.…”