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"Strong" and "weak" synaptic differentiation in the crayfish opener muscle: structural correlates
C K Govind1, J Pearce, J M Wojtowicz
1Life Sciences Division, Scarborough College, University of Toronto, Ontario, Canada.
Synapse (New York, N.Y.)
|January 1, 1994
Summary
In crayfish, proximal neuromuscular synapses are stronger than central ones due to more and longer presynaptic dense bars, not innervation density. This structural difference enhances transmitter output.
Area of Science:
- Neuroscience
- Cell Biology
- Animal Physiology
Background:
- The crayfish Procambarus clarkii's limb opener muscle receives multiterminal innervation from a single excitor motoneuron.
- Neuromuscular synapses exhibit regional differences in synaptic strength, with proximal regions showing greater junctional potentials at low frequencies.
Purpose of the Study:
- To investigate the structural basis for the observed physiological differences in synaptic strength between proximal and central regions of the crayfish limb opener muscle.
- To identify presynaptic correlates of synaptic strength in this model system.
Main Methods:
- Focal extracellular recording to measure quantal release.
- Fluorescence microscopy with 4-Di-2-Asp vital dye to assess innervation density.
- Freeze fracture electron microscopy to examine synapse intramembrane organization.
- Serial section electron microscopy to analyze presynaptic structures.
Main Results:
- Proximal neuromuscular synapses exhibited higher quantal release and larger junctional potentials compared to central synapses.
- Innervation density and synapse number/size were similar across regions.
- Proximal synapses showed longer and more numerous presynaptic dense bars at active zones.
Conclusions:
- Presynaptic dense bar number and length are significant correlates of synaptic strength at the crayfish neuromuscular junction.
- Co-operativity among multiple dense bars in proximal synapses may enhance transmitter output.
- Synaptic strength differences are attributed to presynaptic structural variations rather than innervation patterns.