Physiological Recordings of High and Low Output NMJs on the Crayfish Leg Extensor Muscle

Abstract

We explain in detail how to expose and conduct electrophysiological recordings of synaptic responses for high (phasic) and low (tonic) output motor neurons innervating the extensor muscle in the walking leg of a crayfish. Distinct differences are present in the physiology and morphology of the phasic and tonic nerve terminals. The tonic axon contains many more mitochondria, enabling it to take a vital stain more intensely than the phasic axon. The tonic terminals have varicosities, and the phasic terminal is filiform. The tonic terminals are low in synaptic efficacy but show dramatic facilitated responses. In contrast, the phasic terminals are high in quantal efficacy but show synaptic depression with high frequency stimulation. The quantal output is measured with a focal macropatch electrode placed directly over the visualized nerve terminals. Both phasic and tonic terminals innervate the same muscle fibers, which suggests that inherent differences in the neurons, rather than differential retrograde feedback from the muscle, account for the morphological and physiological differentiation.

Active zone structure‐function relationships at the neuromuscular junction

“…This muscle receives both excitatory and inhibitory inputs, with multiple synapses onto each individual muscle fiber. This review will focus on the excitatory inputs, of which there can be several types, including phasic (stronger synapses that fire in short bursts of activity) and tonic (generally weaker and fire more consistently), which will be discussed in more detail below (Bradacs, Cooper, Msghina, & Atwood, ; Wu & Cooper, ). Unlike the frog or mouse NMJ, which will be discussed in detail below, at the crayfish NMJ the presynaptic motoneuron makes multiple small contacts with the postsynaptic muscle fiber, rather than one large contact or endplate (Cooper & Cooper, ; Jahromi & Atwood, , see Figure ).…”

“…These data are in line with previous characterizations of synapses that require high temporal fidelity in neurotransmitter release (“fast” synapses) in that release is largely gated by a single open VGCC (“single domain gating,” see Stanley, for a comprehensive review). Note that this is a different definition of “fast” synapses than the “fast” and “slow” descriptors used to describe phasic and tonic synapses at this preparation (Bradacs et al, ; Wu & Cooper, ). In contrast, Kennedy et al () suggested that individual VGCC openings during an action potential at this synapse likely do not contribute productively towards release, but instead add to the intracellular calcium concentration.…”

Focal Macropatch Recordings of Synaptic Currents from the <em>Drosophila</em> Larval Neuromuscular Junction

“…5. Pick a wandering third-instar larvae and dissect it as described in 27,28,29,30 . NOTE: To visualize boutons, use the fly strain CD8-GFP (see Table of Materials) 1.…”

Serotonin and Synaptic Transmission at Invertebrate Neuromuscular Junctions

“…Potentially approaches with RNAi might be practical to address more species-specific manipulations in synaptic function in a variety of crustaceans [ 72 - 75 ]. The same physiological and anatomical advantages of the Drosophila NMJs apply for the crustaceans, but in addition, the wide range in known diversity in synapses within crustaceans makes them attractable for comparative studies in commonalities of mechanisms in low- and high-output synapses or ones that facilitate or depress rapidly [ 31 , 32 , 76 ]. The parallels to vertebrate central synaptic physiology of phenomenon described at crustacean NMJs are likely one reason of continual interest to a wide variety of researchers investigating synaptic transmission.…”